Has overweight become the new normal?
Evidence of a generational shift in body weight norms
August 27, 2008
Mary A. Burke
Frank Heiland†
Carl Nadler‡
Abstract
We test for differences across the two most recent NHANES survey periods (1988-1994 and
1999-2004) in self-perception of weight status by observing different birth cohorts at the
same age. We find that the probability of self-classifying as overweight is significantly
lower, and the probability of self-classifying as underweight significantly higher, in the more
recent survey, controlling for objective weight status and numerous socio-demographic
factors. The shift in overweight self-classification appears most pronounced between the
earlier and later cohorts of young adults (ages 17-35), while the shift in underweight selfclassification occurred largely across cohorts of adults ages 36-55. The evidence suggests
that the weight range perceived as “normal” shifted to the right between the earlier and later
surveys, perhaps as a response to the increase in average body mass index (BMI) in the
population. Examining the data further, we show that the findings support a theoretical
model (by two of the current authors) in which body weight norms vary directly with mean
BMI in an individual’s social reference group. In particular, the younger age groups
experienced the largest increases in mean BMI and obesity rates, and also had the sharpest
declines in the tendency to self-classify as overweight. The differences in self-classification
are not explained by differences across surveys in body fatness conditional on BMI, nor by
the change that occurred between the survey periods in the CDC’s threshold for overweight.
The welfare implications of the change in weight standards are ambiguous: if norms
influence outcomes, obese individuals may now face weaker social incentives to lose weight;
on the other hand, overweight and obese individuals may also benefit from greater socialand self-acceptance.

Burke: Research Department, Federal Reserve Bank of Boston, 600 Atlantic Avenue, Boston, MA 02210,
phone: 617-973-3066, e-mail: mary.burke@bos.frb.org.
†
Heiland: Deartment of Economics and Center for Demography and Population Health, Florida State
University, Tallahassee, FL 32306-2180, e-mail: fheiland@fsu.edu.
‡
Nadler: Research Department, Federal Reserve Bank of Boston, 600 Atlantic Avenue, Boston, MA 02210,
phone: 617-973-3096, e-mail: carl.nadler@bos.frb.org.
1
1. Introduction
A large, multidisciplinary literature has investigated self-perceptions of body size and body
shape, studying the determinants of these as well as their relationship to various aspects of
physical and mental health. The bulk of the existing studies have focused on the (crosssectional) relationship between socio-economic characteristics and the self-perception of
body weight appropriateness.1 Consequently, little is known about the evolution of body
weight norms and ideals, either longitudinally within individuals or across different
generations. Several recent articles have reported that significant percentages of individuals
who would be classified by the CDC as either overweight or obese (on the basis of their BMI
values) perceive their weight to be “normal”, “appropriate,” or “acceptable.” (See Rand and
Resnick 2000; Chang and Christakis 2003; Neighbors and Sobal 2007; Howard et al. 2008).
Neighbors and Sobal (2007, p.437), looking at a recent cohort of university students,
conclude that “overweight females may not be seeking to attain the very thin sociocultural
‘ideal’ female form.” They speculate that body size ideals may be shifting, or at least that
larger body sizes have become more acceptable as the population has become heavier.
Recent work on the spread of obesity in social networks suggests, but cannot confirm, that
weight gain may be “contagious” among friends because normative judgments of body size
depend on—and so may change over time with—the size of our friends.2
In a previous theoretical paper (Burke and Heiland 2007), we posited that the social norm for
body weight—defined as a reference point against which individuals judge the
appropriateness of their own weight—varies directly with mean body weight in one’s social
reference group. This framework predicts that the social weight norm in the United States
would have increased over the past 20-30 years in response to the increase in actual mean
body weight over the same period—between NHANES III (conducted over 1988-1994) and
NHANES 1999-2004, mean BMI among individuals ages 17-74 increased by just over 6
percent, from 26.3 to 27.9, and the obesity rate jumped from 22 percent to 32 percent.3 If the
weight norm in fact shifted across these periods, we might expect that the average individual
observed today is (weakly) less likely to classify herself as overweight, and more likely to
classify herself as underweight, than was a similar individual (at the same BMI value)
observed 20 years ago
The NHANES III and NHANES 1999-2004 surveys provide indirect information on weight
norms with which we can test these predictions. In each of these surveys, people ages 17 to
1
Among socio-economic factors, race and ethnicity have been found to have sizable effects, with several
studies reporting black-white differences in self-perceptions of body size and in body satisfaction. AfricanAmerican women tend to point to larger images for “ideal” size than do white (non-Hispanic) American women
and they are more likely to perceive themselves as normal weight and less likely to be dissatisfied with their
body weight and BMI compared with white women. For a survey of this literature, see Flynn and Fitzgibbon
(1998); more recent studies include Lovejoy (2001) and Fitzgibbon, Blackman, and Avellone (2000) and Burke
and Heiland (2008).
2
For details see Christakis and Fowler (2007), Trogdon et al. (2008), Fowler and Christakis (2008), Halliday
and Kwak (2007), and Renna et al. (2008).
3
We argued that these weight gains were initiated by an exogenous decline in the full (money plus time) cost of
food, and that a subsequent increase in the norm set off social multiplier effects that led to further increases in
mean weight. In this paper we focus on the impact of mean BMI on weight norms. We discuss the
identification issues raised by potential reverse causality further below.
2
74 were asked whether they consider themselves (at their current weight) to be either
“underweight,” “about right,” or “overweight.”4 (The choice to self-classify as “obese” was
not included.) Examining these responses, we find that the probability of self-classifying as
overweight is indeed significantly lower, and—within some age ranges—the probability of
self-classifying as underweight significantly higher, in the more recent survey, controlling for
objective weight status and numerous socio-demographic factors. The shift in overweight
self-classification appears most pronounced between the earlier and later cohorts of young
adults (ages 17-35), while the shift in underweight self-classification occurred exclusively
across cohorts of adults ages 36-55.
We argue that our model of socially endogenous weight norms helps to explain the overall
differences in self-classification tendencies between the survey periods and can explain some
of the variation across age groups in the magnitude of such shifts. In particular, we find that
members of groups (defined by age-range, sex, and birth cohort) with higher average BMI
are less likely to classify themselves as overweight, and more likely to classify themselves as
underweight, than are members of groups with lower average BMI, controlling for own BMI
and other factors. In the empirical models, this relationship between peer-group BMI and
self-perception can account for the differences in perception tendencies across surveys. For
example, the more recent cohort of young adults (17-35) is both (a) heavier on average, and
(b) less likely to classify themselves as overweight (conditional on actual weight), than was
the previous cohort at the same age. Across cohorts of 36-45 year olds and 46-55 year olds,
the increases in mean BMI and obesity rates were smaller than they were for the younger age
ranges, as were the decreases in the tendency to classify as overweight.5
However, mean BMI at ages 56-74 was also higher in the later survey than the earlier survey,
while the tendency to classify as overweight stayed roughly constant across the survey
periods for this group. This inconsistency suggests that peers’ average BMI may have
different impacts on norm formation at different ages. The self-perceptions of the young
concerning their weight might be more malleable than the self-perceptions of older people,
and therefore more susceptible to variation in reference-group-mean BMI. If childhood
experience is particularly salient, generational differences in childhood obesity rates might
also be linked to generational differences (in adolescence and into adulthood) in weight
norms and self-reported weight classification. Unfortunately, we observe the relevant
childhood obesity rates only for a subset of the adolescents and adults surveyed in NHANES
III and NHANES 1999-2004, and within this subset the relevant rate is ambiguous in some
cases. Using these limited observations, we find a correlation between childhood obesity
rates and adult self-classification tendencies in the raw data, but multivariate regression
analysis yields no significant relationship.
4
Beginning in 1999, respondents ages 16 to 74 were asked this question. For consistency we will consider only
17-74 year olds in both survey periods.
5
We also consider the impact of the contemporaneous obesity rate among the age-by-sex reference group on
the self-classification of weight status, and find that higher peer obesity rates reduce the tendency to selfclassify as overweight. Due to collinearity, however, we cannot determine whether mean peer-group BMI or
the peer-group obesity rate is the more important social reference point.
3
Some of the usual pitfalls associated with testing social interactions—such as simultaneity
bias and endogenous peer selection—do not arise in this framework. First, the peer-group
variables we use, including average BMI and the obesity rate in the peer group, are not
aggregates of the dependent variable. Second, the social reference groups are exogenous
because they are based only on age, sex, and birth cohort. While we cannot be certain that
the peer variables do not merely proxy other common influences within social reference
groups, models that substitute fixed effects of reference groups for average BMI (or the
obesity rate) do not perform as well.
We also consider the possibility that apparent changes in self-perception can be accounted
for by changes in body fat percentage conditional on BMI. Self-judgments of weight status
may depart from CDC standards, with good reason in some cases, because the standards are
based on BMI and not body fat. If individuals assess their own weight status on the basis of
fatness instead of, or in addition to, overall size, and if the average person today has a lower
body fat percentage, conditional on BMI, than the average person in previous generations, we
might expect people in the present to be less likely to classify as overweight, even with no
change in social norms. As it happens, however, body fat percentage for the average
individual, conditioned on BMI, has increased over time, and we find that observed
differences in self-perception between the surveys are robust to controls for individual body
fat percentage.
It is also possible that changes in subjective assessments of underweight, normal weight, and
overweight preceded the increases in mean BMI that occurred between the survey periods.
Imagery in the popular media, public health messages, or scholastic content, for example,
may have changed between the survey periods, promoting a larger body size ideal and a
greater level of “fat acceptance” in particular. As far as official government standards go, the
standard for overweight actually became stricter between the survey periods, not looser.6
Evidence on changes in media imagery is mixed. While the number of “plus-sized” female
models has increased, images of women in popular media remain skewed to the left of
average; emphasis on male appearance, and on ideal musculature in particular, seems to be
growing. Educational programs in schools aimed at preventing eating disorders may have
promoted healthier size ideals. If the shift in norms did in fact precede the shifts in the
weight distribution, it remains a mystery why such an exogenous shift would have occurred.
In some cases, the shifts in self-perception resulted in greater consistency with CDC
standards: for example, the tendency of normal-weight individuals to self-classify as
overweight declined across the surveys on average. In other cases, however, the shifts
moved self-perceptions away from CDC standards, as overweight and obese people became
less likely to self-classify as overweight than previously, especially among younger people.
The welfare implications of the change in weight standards are ambiguous: if norms
influence outcomes, obese individuals may now face weaker social incentives to lose weight;
on the other hand, overweight and obese individuals may also benefit from greater socialand self-acceptance.
6
In 1998, on the recommendation of the World Health Organization, the CDC reduced the BMI thresholds for
overweight, from 27.8 for men and 27.3 for women, down to 25 for both sexes. This new standard is applied
retroactively when we measure rates of overweight and obesity in surveys conducted prior to 1998.
4
2. Objective vs. subjective weight status: how self-classification varies with age, sex, and
survey period.
Table 1 gives cross-tabulations of subjective weight status against CDC-defined (“objective”)
weight status, separately by survey period and sex, for the full age range, from 17 to 74
years. The value in a given cell indicates the percentage of individuals in a given objective
category (the row variable) that reported being in a given subjective category (the column
variable). For example, in NHANES III, among women with normal BMI values, 56 percent
considered their weight to be “about right” and 39 percent considered themselves to be
“overweight.” For women in NHANES 1999-2004, the corresponding figures are 64 percent
and 32 percent. Among women qualifying as overweight but not obese, 83 percent reported
feeling overweight as of NHANES III, and only 78 percent said the same in 1999-2004. All
of these differences across surveys are significant. For a given objective weight status and
observation period, men are significantly less likely than women to self-classify as
overweight and significantly more likely to classify as underweight. However, like women,
men’s self-perceptions also shifted significantly across survey periods. For example, among
overweight (but not obese) men, the share that self-classified as overweight fell from 59
percent in NHANES III to 53 percent in NHANES 1999-2004. Among obese men the share
that considered themselves overweight fell from 89 percent to 84 percent.7
The trends in the tendency to self-classify as underweight offer less support for our
theoretical model. Among women, for most of the objective weight categories we observe
no significant increase between survey periods in the share that self-classified as
underweight. Among men, the tendency to self-classify as underweight increased
significantly among normal-weight individuals, but not for any of the other weight
categories.
Tables 2 and 3 replicate Table 1 for two different age groups: 17-25 year olds and 36-45 year
olds. These tables show how the temporal changes in self-classification differ by age and
gender.8 Observe in Table 2 that the difference in responses across the survey periods is
more dramatic among young women ages 17-25 than for the female population as a whole:
for this age group, the share normal-weight women that self-classified as overweight fell
from 37 percent to 25 percent between the surveys, as compared to the decline from 39
percent to 32 percent for women overall. Among overweight young women, the share that
self-classified as overweight fell by an estimated 14 percentage points, from 85 percent to 71
percent, while the decline for women as a whole was just 5 percentage points. Among obese
young women the decline in the percentage classifying as overweight, from 95 percent to 84
percent, was also larger than the corresponding decline for women in general. Of course, the
standard errors on the estimates for young women are greater than those for the female
7
These differences in self-reporting conditional on weight status cannot be explained by decreases in average
BMI conditional on the objective weight category, since the conditional means either remained constant or
increased between the survey periods.
8
Objective weight status for 17-20 year olds is determined using the CDC’s reference distributions, which are
gender and age specific. At a given gender and age, underweight is defined as below the 5 th percentile of the
relevant BMI distribution, between 5th and 85th is defined as normal, 85th to 95th is considered overweight but
not obese, and 95th and above is obese. Cutoff points by age and sex are shown in the data appendix, Table A.2.
5
population as a whole, so these differences in differences may be less than what the point
estimates indicate.
Among normal-weight men ages 17-25, the share that considered themselves overweight did
not differ significantly across the surveys. However, among overweight (not obese) young
men, the share that self-classified as overweight fell significantly, from 51 percent to 42
percent; and among obese young men, the share that classified themselves as overweight fell
dramatically, from 90 percent to 71 percent. Among young people of either sex, we observe
a significant increase in the tendency to self-classify as underweight only among obese
individuals.
For the 36-45 year olds, seen in Table 3, a few different patterns emerge. First, notice that
the decline in the tendency of women to self-classify as overweight is less pronounced than it
was among 17-25 year olds. Among normal-weight women in particular, the share that
considered themselves overweight was 41 percent in both survey periods. For men in this
age group, we see some significant declines in the tendency to self-classify as overweight,
but only among normal-weight and overweight men and not also among obese men. This
last fact stands in stark contrast to the large decrease across the surveys, noted above, in the
percentage of men ages 17-25 that considered themselves overweight.
The trends in underweight self-classification also differed between 17-25 year olds and 36-45
year olds. Among normal-weight men age 36-45, the proportion that self-classified as
underweight increased from 9 percent to 19 percent, a difference that is highly significant.
For underweight women and underweight men alike, the share that self-classified as
underweight increased by potentially large margins between the surveys, although the
estimates of the respective differences are not very precise.
Table 4 juxtaposes the changes in actual BMI, current obesity rates, and childhood obesity
rates across surveys, by age group, against the changes in the probabilities of self-classifying
as overweight, conditional on being overweight (but not obese) and conditional on being
obese. Inspecting the table, we see that the youngest age bracket, for both males and
females, experienced the largest gains in the childhood obesity rate between surveys as well
as the biggest declines in the conditional probabilities of self-classifying as overweight. The
youngest male group also had the biggest increase in mean BMI across the surveys, although
increases in obesity rates across the surveys were greatest for the second-youngest group for
both sexes. Also complicating the picture, we observe that the oldest age bracket of either
sex showed large increases across the surveys in mean BMI and contemporaneous obesity
rates, while at the same time becoming more likely to self-classify as overweight.
3. Multivariate regression analysis
The differences in weight perception across the surveys shown in the tables above do not
control for differences across the survey periods in characteristics that may influence survey
responses, such as educational attainment, race/ethnicity, and household income. In addition,
we would like to test the hypothesis that differences across surveys in the self-classification
of body weight reflect a generational shift in weight norms induced by differences in average
6
BMI across birth cohorts. To address these issues, we conduct a multinomial logit analysis
of the survey responses, adopting various model specifications. The dependent variable is the
response to the NHANES self-perception question, which can take on one of three values:
“underweight”, “about right,” or “overweight.”
In our theoretical model (Burke and Heiland 2007), the weight norm is defined as a specific
point value, and thresholds for overweight and underweight are not defined. (Rather, utility
is decreasing in the squared difference between own weight and the norm, such that
“overweight” is a matter of degree and not an either-or proposition.) In contrast, the
NHANES survey question forces individuals to adopt implicit thresholds for overweight and
underweight. Extending our conceptual framework slightly, we can replace the point-value
norm with a range of weights deemed “about right” and assume that the lower and upper
boundaries of this range depend directly on average weight (BMI) in the reference
population. Under these assumptions, the probability that an individual self-classifies as
overweight should be increasing (weakly) in her own BMI and decreasing (weakly) in the
mean BMI of her social peer group, ceteris paribus. Likewise, the probability that someone
self-classifies as underweight should be weakly decreasing in her own BMI and weakly
increasing in mean peer-group BMI. We will define peer groups quite broadly, as all others
in the same age category and the same sex as of the same survey period.
Empirical setup
We adopt a multinomial logit specification that predicts the probability of each survey
response. The baseline response is “about right” and results are reported as coefficients of
relative risk. To test the significance of differences in responses across survey periods, we
pool the data from the NHANES III and NHANES 1999-2004 surveys, and create a dummy
variable to indicate the survey period from which a given observation is taken.9 To allow for
non-linear age effects, we construct five discrete age categories: 17-25, 26-35, 36-45, 46-55,
and 56-74, which are roughly equal in terms of their weighted share of the combined
NHANES data. (In an alternative specification we let age enter as a continuous variable.) In
what we will call the “baseline model,” we do not test for social effects related to average
BMI or obesity rates in peer groups. In three subsequent models, we add, alternately, one of
three different reference-group variables or “social interaction” terms, where reference
groups are defined on the basis of age range, sex, and survey period. These include (i) the
contemporaneous average BMI in the reference group, (ii) the contemporaneous obesity rate
in the reference group, and (iii) the estimated obesity rate (explained below) that prevailed
among the reference group when they were children ages 6-11.
All models include the following explanatory variables: a dummy variable for female, a
dummy variable for observations taken from NHANES 1999-2004, an interaction term
between the female dummy and the survey dummy, dummies for each age range (where 369
According to Korn and Graubard (1999), it is not necessary to re-weight the data when pooling NHANES
surveys, based on the assumption that the samples are independent across the periods. We control for complex
survey design within each survey using the appropriate strata and primary sampling unit (PSU) variables.
Using Stata’s “svy” commands, the data are weighted correctly and the standard errors are clustered
appropriately.
7
45 is the omitted category), interaction terms between each age range and the survey period,
race/ethnicity dummy variables (white non-Hispanic, African American non-Hispanic,
Mexican-American, and other), individual BMI (treated as a continuous variable),
educational attainment (less than high school, high school graduate, or some college or
better), household income (three discrete categories based on the relationship to poverty-line
income), and marital status (currently married, formerly married, never married). The
construction of these variables is discussed in a data appendix. Sample means of these
variables by sex, pooled across surveys, are shown in Table A.1.
Main results
Table 5 shows the results of the four different models. The baseline model (“Model 1”),
includes just the list of explanatory variables given directly above. The “mean BMI” model
(“Model 2”) includes the baseline variables plus the contemporaneous mean BMI for the
individual’s reference group (defined above). The “obesity rate” model (“Model 3”) includes
the baseline variables plus the contemporaneous obesity rate in the reference group, and the
“childhood obesity” model (“Model 4”) includes the baseline variables and the estimated
childhood obesity rate for the reference group. In the following discussion we focus on the
variation in self-classification tendencies with the survey variable, with interactions between
the survey variable and age, and, from models 2-4, we focus on the impact of the referencegroup variables.
First, observe the results of the baseline model pertaining to the relative risk of selfclassifying as “underweight.” The “NHANES 1999-2004” dummy has a highly significant
coefficient estimated at 2.4. Given the age-survey interaction terms, this result means that,
within the 36-45-year-old age group, the relative risk of self-classifying as underweight
(compared to the risk of self-classifying as “about right”) was about 2.4 times as great on
average among those observed in the later survey period as it was among those ages 36-45
observed in the earlier period.10 This difference across surveys may have been weaker
among women, but the significance level of .085 on the interaction coefficient renders this
statement uncertain. However, there are significant interaction coefficients between the
NHANES 1999-2004 dummy and each of three other age categories (17-25, 26-35, and 5674), each with a value significantly less than one. These results mean that these other age
groups experienced either a significantly less pronounced increase in the tendency to selfclassify as underweight, or even a decrease in this tendency (as may have occurred among
17-25 year olds), between the earlier and later surveys. Notice also that, in line with
objective classifications, the relative risk of self-classifying as underweight is smaller the
higher is an individual’s BMI value. Also as we might expect, women and people with high
household incomes have significantly lower relative risks of classifying as underweight,
respectively, than men and those in the lowest-income group do. African Americans are
significantly more likely than whites to self-classify as underweight.
Now consider the impact of the reference-group variables on the relative risk of selfclassifying as underweight (Table 5, Models 2-4). Most of the coefficient estimates are
10
Fixed survey effects can be misleading in this model, as the survey differences vary considerably across BMI
values. This variation is illustrated in Figures 1-5.
8
quantitatively and qualitatively similar to those in the baseline model. However, in model 2,
the main effect of the NHANES 1999-2004 variable is no longer significant and mean BMI
in the reference group has a coefficient that is significantly greater than one. This latter
result indicates that, the higher is average BMI in the reference group, the greater is the
relative risk that an individual self-classifies as underweight. This agrees with the logic of
the social interactions model: the heavier are your peers, the more likely you are to feel
underweight by comparison. The insignificance of the main survey effect in this model
suggests that social interactions may account for the fact that 36-45 year olds were more
likely to self-classify as underweight in the later survey than in the earlier survey. That is,
average BMI was higher for this age group in the later survey than in the earlier survey (see
Table 6), and as a result the more recent cohort of 36-45 year olds is more likely to feel
underweight. However, including mean BMI in the model does not wipe out the coefficient
on the interaction term, for example, between age 17-25 and NHANES 1999-2004. That is,
despite the fact that mean BMI also increased across survey periods for the age group 17-25
years as shown in Table 6, the more recent crop of 17-25 year olds are not more likely than
their predecessors to self-classify as underweight.
The results of model 3 are similar to those of model 2 in that the main survey effect becomes
insignificant and other coefficients are largely unchanged. The estimated coefficient on the
reference-group’s obesity rate is greater than one, consistent with the notion that having a
greater share of obese peers will make people feel relatively underweight. However, this
estimate is significant only at the 0.07 level, so we cannot have great confidence in the
strength of this effect. In model 4 we include the estimated childhood obesity rate by agesex-survey cohort. The significance of the main survey effect is restored, and we find no
significant influence of the childhood obesity rate on the tendency to self-classify as
underweight. Due to data constraints, the sample size was significantly reduced in model 4
relative to model 3, and we cannot be confident in the accuracy of the survey weights within
the resulting sub-sample. We do not believe that the results of model 4 rule out the
possibility that childhood obesity rates influence self-perceptions into adulthood.
Now observe the results of the baseline model pertaining to the relative risk of selfclassifying as “overweight” as opposed to “about right.” The NHANES 1999-2004 dummy
variable again has a significant coefficient, but now its value is significantly less than one.
This means that, among 36-45 year olds, the relative risk of self-classifying as overweight
was about 30 percent lower on average in the later survey period than in the earlier period, all
else equal. The age-survey interaction terms indicate that, for the two youngest age groups
(17-25 and 26-35), the tendency to self-classify as overweight fell by an even greater
percentage between NHANES III and 1999-2004. The difference in self-classification
tendencies across surveys was roughly the same for 46-55 year olds as for 36-45 year olds,
given that the interaction between “age 46-55” and “NHANES 1999-2004” is not significant.
However, for the oldest group (ages 56-74), the significant interaction coefficient of 1.36
offsets the main survey effect, and the net estimate is that the relative tendency to selfclassify as overweight did not change significantly across the surveys.
The remaining effects in the baseline model are largely in line with expectations. Higher
BMI increases the risk that an individual classifies as overweight. Women are more likely to
9
self-classify as overweight than men, better-educated groups are more likely to feel
overweight than the least educated, and those with middle and high incomes are more likely
to feel overweight than those with the lowest incomes. Those currently married are more
likely to feel overweight than never-married people, and minority groups are significantly
less likely than whites are to consider themselves overweight.
Again we find evidence that average BMI in reference groups may explain differences in
self-classification tendencies across surveys. Looking at the results of Model 2, observe that
mean reference-group BMI has a coefficient that is less than one (0.72), significant at the
.051 level. (The 95 percent confidence interval ranges from 0.52 to 1.00). This means that
those in groups (age-by-gender-by-cohort) with higher average BMI have a lower relative
risk of self-classifying as overweight, all else equal. At the same time, the main survey effect
becomes insignificant, the interaction term between “age 26-35” and “NHANES 1999-2004”
becomes insignificant, and the interaction term between “age 17-25” and the survey dummy
becomes only marginally significant (p-value of .085). These results suggest that higher
mean BMI values among the more recent cohorts in these age ranges may have given rise to
different perceptions, among these same cohorts, of what range of sizes should be considered
normal and at what point one becomes overweight.
In Model 3, the contemporaneous obesity rate among peers is negatively related to the
relative risk of self-classifying as overweight, and the main survey effect is insignificant.
However, unlike the results from Model 2, the significant interaction effects observed in the
baseline model, between the NHANES 1999-2004 dummy and the various age groups,
remain significant in Model 3. Taking these results at face value, we infer that peer obesity
rates explain differences across surveys in the tendency to self-classify as overweight only
for the 36-45 year old group; for the other groups the differences across surveys cannot be
fully explained by differences across cohorts in obesity rates. In Model 4, the cohort-specific
childhood obesity rate wields no significant effect on the relative risk of self-classifying as
overweight, nor does it account for the main survey effect (pertaining to 36-45 year olds)
observed in the baseline model.
We note that the peer variables do not seem to explain some of the fixed differences across
age groups in self-perception tendencies that are indicated by the age-group fixed effects,
Within the NHANES III survey period, those in the youngest group are significantly more
likely (than were those in the reference age group ages 36-45) to consider themselves
underweight, and those ages 56-74 are significantly more likely to consider themselves
overweight, even controlling for average BMI or the obesity rate in the relevant peer group.
Netting out survey effects and age-survey interactions, these age differences are at least
qualitatively similar as of NHANES 1999-2004. We interpret these as life-cycle effects:
young people, and young men in particular, may be more likely to consider themselves
underweight because they are not yet fully grown. Also, people tend to gain weight as they
get older, and may (consciously or not) find it psychologically convenient to adjust their
standard of overweight.
Graphical illustration of results
10
Because we are using a multinomial logit model, it is helpful to illustrate the model results
graphically. Figures 1 through 5 provide such illustrations, using the results of our baseline
model. Figure 1 shows the predicted probability of each outcome as a function of BMI for
women ages 17-25, separately by survey period (top panel), and does the same for men ages
17-25 (bottom panel). Figures 2 and 3 repeat this exercise for 36-45 year olds and 56-74 year
olds, respectively. Figures 4 and 5 compare predicted responses between the youngest group
(17-25) and the middle age group (36-45) within each survey period, separately by sex, to
highlight age differences in self-classification and changes in these age differences over time.
The appropriate values for BMI, gender, and the survey dummy are assigned, and all other
variables are held at their sex-specific sample means, reported in Table A1. Note that these
means are not survey-specific but rather reflect average values across all individuals
observed in both surveys. In these plots, the only source of differences in predicted
probabilities between the survey periods derives from the NHANES 1999-2004 fixed effect
and the interaction term between the given age bracket and the NHANES 1999-2004 dummy.
The vertical lines in each figure indicate the boundaries between the “objective” weight
categories as defined by the CDC. Moving from left to right we have, respectively, the
boundary between underweight and normal weight, that between normal weight and
overweight, and that between overweight and obese. However, in the plots that refer to
people under age 21, we draw two different boundaries for each demarcation. This is
because the boundaries differ between adults (ages 21 and over) and adolescents (ages 17-20,
in our sample). For adolescents, there are in fact different cutoff points at each specific age.
However, we show only the lowest boundary for each category, the purple line pertaining to
17 year olds, together with a black line pertaining to adults. Age-specific cutoffs defining
each category are shown in Table A2.
Comparing figures 1, 2, and 3, we see readily that the differences across surveys in the
predicted probabilities of responding “about right” or “overweight” are greatest for the
youngest age group. For the oldest age group the predicted responses are virtually identical
across the survey periods. However, we see a significant increase across the surveys in the
likelihood of responding “underweight” only among the 36-45 year old group. The pictures
also illustrate the stark differences between men and women in self-perception of weight
status, as well as the significant gaps between subjective and objective classifications.
4. Discussion
Alternative explanations
The CDC’s weight classification system considers only total body mass and not fat mass
specifically, the latter being the stronger predictor of health risks. While BMI and fatness are
correlated, adiposity and related health risks can vary considerably between individuals with
the same BMI. Some people with BMI values that qualify as “overweight” or “obese” have
very low levels of body fat, and some with “normal” BMI values have very high levels of
body fat. This disconnect between BMI and adiposity may be especially acute among
teenage boys (Sardinha et al. 1999). If self-judgements of weight status are based on fatness
rather than on BMI, then the observed discrepancies between subjective and “objective”
weight status may simply reflect the limitations of the BMI-based system, and need not
11
indicate any distorting social influences. In addition, shifts over time in self-classification
could reflect shifts in underlying fatness conditional on BMI. If the average young person
today has a lower body fat percentage, at a given BMI value, than the average young person
in previous generations at the same BMI, we might expect today’s young person to be less
likely to classify as overweight, even with no change in social norms. NHANES
examination data enable us to compute body fat percentage for a significant share of our
sample. When we add controls for individual body fat percentage to our empirical models
(each of Models 1 through 4 described above), we find that our results are largely robust.
Even a casual analysis predicts that this should be the case. Conditional on BMI, the average
body fat percentage increased between the surveys rather than decreased. In light of this
development, self-perceptions based on fatness would have tended toward peoples’ feeling
fatter at a given BMI rather than less overweight.
It is also possible that changes in subjective assessments of underweight, normal weight, and
overweight preceded the increases in mean BMI that occurred between the survey periods.
Imagery in the popular media, public health messages, or scholastic content, for example,
may have changed between the survey periods, promoting a larger body size ideal and a
greater level of “fat acceptance” in particular. Evidence on media imagery is mixed (see
Turner et al. 1997; Holmstrom 2004; Greenberg and Worrell, 2005). While the number of
“plus-sized” female models has increased, images of women in popular media remain
skewed to the left of average; emphasis on male appearance, and on ideal musculature in
particular, seems to be growing in the popular media. As far as official government
standards go, the standard for overweight actually became stricter between the survey
periods, not looser.11 Following the shift in CDC standards, individuals with BMI values
between 25 and 27, for example, should have been more likely to classify as overweight in
the later survey.
In the past decade, however, numerous government programs have emerged that promote the
development of a “healthy” body image and healthy eating behavior. For example, the
National Eating Disorders Association offers the educational program “GO GIRLS!” and the
U.S. Department of Health offers a “Healthy Body Image” book for use in public schools.12
These latter programs, at least, appear to promote more realistic self-assessments of weight,
among girls in particular, as a preventive measure against eating disorders such as anorexia
nervosa and bulimia. Also in recent years, however, several states have come to require that
children and adolescents in public schools be weighed and measured.13 Parents and children
are then informed of the child’s official weight status and, in the case of overweight and
obese children, advised to adopt healthier habits and target a healthier weight. Therefore,
11
In 1998, on the recommendation of the World Health Organization, the CDC reduced the BMI thresholds for
overweight, from 27.8 for men and 27.3 for women, down to 25 for both sexes, and reduced the threshold for
“severe overweight” from 31.1 for men and 32.2 for women to an “obesity” threshold of 30 for both sexes.
These new standards are applied retroactively when we measure rates of overweight and obesity in surveys
conducted prior to 1998. See http://win.niddk.nih.gov/statistics/#whydodiffer
12
The “GO GIRLS!” program “engages high school girls (and boys too!) to advocate for positive body images
of youth in advertising, the media and major retailers.” See http://www.nationaleatingdisorders.org/programsevents/educational-programs.php#go-girls. The “Healthy Body Image” book is part of the “BodyWise”
information kit distributed by The U.S. Department of Health, Office of Women’s Health.
13
See http://www.cdc.gov/HealthyYouth/obesity/BMI/pdf/BMI_execsumm.pdf
12
overweight students may be receiving mixed messages: on the one hand, to lose weight, and
on the other hand, to have a healthy self-image. The net impact of such messages on selfassessments among children and adolescents remains unclear.
Policy and welfare implications
We have identified a number of significant shifts in the self-classification of body weight
between the NHANES III and 1999-2004 survey periods. We have argued that such shifts
reflect socially endogenous differences in body weight norms that arose across birth cohorts
in response to differences in mean BMI and obesity rates across these same cohorts. The
endogeneity of weight norms in general, as well as the specific changes in conceptions of
body weight that we observe, hold implications for policy-makers and for individual wellbeing. While we have not in this paper stressed the influence of body weight norms on
individual weight outcomes, if such an influence is significant, the possibility of selfreinforcing feedback loops arises. For example, as we describe in our earlier work (Burke
and Heiland 2007), a decline in food prices exerts a direct (positive) impact on average body
weight, which leads (with some time lag) to an upward adjustment of weight norms, leading
to a further upward adjustment of average weight, and so on until an equilibrium point is
restored. Policy interventions that aim to reduce obesity may be able to harness the influence
of this ‘social multiplier’ effect. On the other hand, our framework implies that government
messages may hold little sway over de facto weight norms, as evidenced by the considerable
discrepancies between subjective and objective classifications, and also that the influence of
popular media on subjective weight assessments may be overstated.
We observe that women’s notion of overweight (excepting 56-74 year olds) has become less
strict and now shows stronger agreement with CDC standards—see, in particular, Figures 1
and 2. However, a larger share of overweight and obese men and women (defined using
BMI) now classify themselves as “about right,” an indication that such individuals believe
that they do not need to lose weight. Such judgments could be beneficial on net, especially
for individuals who are merely overweight and not obese or severely obese. Against the
health costs of being overweight, one must weigh the costs of dieting and the mental health
costs of negative self-image. Public policy on obesity to date has focused too narrowly on
BMI and not on more fundamental risk factors such as body fat, nutrition, physical activity,
and mental health.
13
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15
Table 1. Weight Perception, Age 17-74
Underweight
BMI Category
Underweight
Normal
Overweight
Obese
Total
Real Rates
BMI Category
Underweight
Women 17-74, NHANES III
About
Right
Overweight
Total
Underweight
43.72
(5.13)
4.57
(0.36)
0.46
(0.10)
0.22
(0.13)
3.93
(0.25)
3.66
(0.36)
56.28
(5.13)
55.99
(1.33)
16.18
(1.19)
3.74
(0.46)
33.53
(0.96)
47.51
(1.11)
0.00
(0.00)
39.44
(1.30)
83.36
(1.16)
96.04
(0.47)
62.54
(1.00)
48.83
(1.11)
100.00
BMI Category
Underweight
100.00
Normal
100.00
Overweight
100.00
Obese
100.00
Total
100.00
Real Rates
Underweight
About
Right
Overweight
Total
Men 17-74, NHANES III
About
Right
Overweight
Total
73.64
(4.99)
15.65
(1.12)
0.70
(0.23)
0.58
(0.36)
7.91
(0.55)
1.40
(0.22)
26.36
(4.99)
73.84
(1.22)
40.72
(1.34)
10.80
(1.48)
48.49
(0.80)
41.49
(1.12)
0.00
(0.00)
10.51
(0.85)
58.58
(1.36)
88.63
(1.60)
43.60
(0.87)
57.11
(1.07)
100.00
Underweight
About
Right
Overweight
Total
100.00
100.00
100.00
100.00
100.00
BMI Category
Underweight
48.97
49.93
1.10
100.00
72.88
27.12
0.00
100.00
(5.25)
(5.38)
(1.08)
(5.51)
(5.51)
(0.00)
Normal
3.80
63.74
32.46
100.00
Normal
18.75
73.72
7.53
100.00
(0.33)
(1.16)
(1.13)
(1.03)
(1.13)
(0.73)
Overweight
0.31
22.04
77.66
100.00
Overweight
1.19
46.05
52.76
100.00
(0.10)
(1.13)
(1.16)
(0.28)
(1.52)
(1.53)
Obese
0.50
6.45
93.05
100.00
Obese
1.03
15.06
83.90
100.00
(0.14)
(0.60)
(0.63)
(0.31)
(1.23)
(1.36)
Total
2.86
32.54
64.60
100.00
Total
7.65
45.24
47.11
100.00
(0.20)
(0.82)
(0.79)
(0.41)
(0.78)
(0.74)
Real Rates
2.49
36.53
60.98
100.00
Real Rates
1.46
31.15
67.39
100.00
(0.24)
(0.93)
(0.93)
(0.17)
(0.75)
(0.76)
Notes: Coefficients represent the share of individuals who perceive themselves as “Underweight,” “About Right,” or “Overweight” by BMI categories. Standard errors are in
parentheses.
16
Table 2. Weight Perception, Age 17-25
Women 17-25, NHANES III
About
Underweight
Right
Overweight
BMI Category
Underweight
Normal
Overweight
Obese
Total
Real Rates
41.55
(6.95)
7.34
(1.25)
0.48
(0.28)
0.00
(0.00)
7.39
(0.80)
6.20
(0.97)
58.45
(6.95)
55.40
(2.43)
14.95
(3.36)
5.34
(2.45)
42.47
(1.85)
64.39
(2.07)
0.00
(0.00)
37.25
(2.63)
84.57
(3.36)
94.66
(2.45)
50.14
(2.09)
29.40
(1.82)
Total
Underweight
100.00
BMI Category
Underweight
100.00
Normal
100.00
Overweight
100.00
Obese
100.00
Total
100.00
Real Rates
Women 17-25, NHANES 1999-2004
About
Underweight
Right
Overweight
Total
BMI Category
Underweight
79.91
(7.11)
25.32
(2.67)
1.13
(0.43)
0.19
(0.19)
19.48
(2.04)
3.65
(0.79)
Men 17-25, NHANES III
About
Right
Overweight
20.09
(7.11)
67.85
(2.82)
47.38
(4.26)
9.48
(2.61)
55.03
(2.11)
64.45
(1.90)
0.00
(0.00)
6.84
(1.30)
51.49
(4.17)
90.33
(2.62)
25.49
(1.85)
31.90
(1.79)
Men 17-25, NHANES 1999-2004
About
Underweight
Right
Overweight
Total
100.00
100.00
100.00
100.00
100.00
100.00
Total
BMI Category
Underweight
32.03
67.97
0.00
100.00
71.74
28.26
0.00
100.00
(8.79)
(8.79)
(0.00)
(6.72)
(6.72)
(0.00)
Normal
5.17
70.00
24.83
100.00
Normal
23.69
71.12
5.19
100.00
(0.71)
(1.74)
(1.75)
(1.83)
(1.75)
(0.90)
Overweight
0.36
28.44
71.21
100.00
Overweight
1.21
56.41
42.39
100.00
(0.35)
(3.51)
(3.53)
(0.60)
(3.53)
(3.39)
Obese
1.01
14.62
84.37
100.00
Obese
2.02
27.44
70.54
100.00
(0.61)
(2.72)
(2.75)
(1.17)
(2.17)
(2.40)
Total
4.39
49.59
46.02
100.00
Total
15.11
56.32
28.57
100.00
(0.56)
(1.73)
(1.65)
(1.10)
(1.45)
(1.33)
Real Rates
3.93
54.81
41.25
100.00
Real Rates
3.71
49.59
46.70
100.00
(0.65)
(1.42)
(1.37)
(0.74)
(1.37)
(1.27)
Notes: Coefficients represent the share of individuals who perceive themselves as “Underweight,” “About Right,” or “Overweight” by BMI categories. Standard errors are in
parentheses.
17
Table 3. Weight Perception, Age 36-45
Women 36-45, NHANES III
About
Underweight
Right
Overweight
BMI Category
Underweight
Normal
Overweight
Obese
Total
Real Rates
34.33
(11.59)
2.32
(0.68)
0.26
(0.21)
0.75
(0.53)
2.43
(0.56)
3.19
(0.62)
65.67
(11.59)
56.66
(2.92)
9.56
(2.31)
2.35
(0.55)
31.37
(2.36)
46.49
(2.49)
0.00
0.00
41.02
(2.79)
90.18
(2.33)
96.90
(0.70)
66.20
(2.34)
50.32
(2.45)
Total
Underweight
100.00
BMI Category
Underweight
100.00
Normal
100.00
Overweight
100.00
Obese
100.00
Total
100.00
Real Rates
Women 36-45, NHANES 1999-2004
About
Underweight
Right
Overweight
Total
BMI Category
Underweight
74.40
(15.10)
9.34
(1.71)
0.42
(0.18)
1.37
(1.31)
4.43
(0.74)
1.06
(0.45)
Men 36-45, NHANES III
About
Right
Overweight
25.60
(15.10)
74.85
(3.01)
38.04
(3.70)
11.99
(3.13)
44.51
(2.22)
33.61
(1.88)
0.00
0.00
15.81
(2.73)
61.53
(3.69)
86.64
(3.40)
51.06
(2.16)
65.33
(1.89)
Men 36-45, NHANES 1999-2004
About
Underweight
Right
Overweight
Total
100.00
100.00
100.00
100.00
100.00
100.00
Total
BMI Category
Underweight
64.68
29.74
5.58
100.00
95.16
4.84
0.00
100.00
(12.72)
(12.85)
(5.06)
(4.27)
(4.27)
0.00
Normal
2.63
56.37
41.01
100.00
Normal
19.15
72.50
8.35
100.00
(0.63)
(3.15)
(2.90)
(2.37)
(2.86)
(1.78)
Overweight
0.37
17.34
82.29
100.00
Overweight
2.56
47.65
49.79
100.00
(0.23)
(2.57)
(2.57)
(0.94)
(3.73)
(3.69)
Obese
0.39
5.40
94.21
100.00
Obese
0.00
12.39
87.61
100.00
(0.23)
(0.93)
(0.92)
0.00
(1.81)
(1.81)
Total
2.62
26.66
70.72
100.00
Total
7.07
42.61
50.31
100.00
(0.51)
(1.58)
(1.46)
(1.02)
(1.86)
(2.03)
Real Rates
2.28
34.34
63.38
100.00
Real Rates
0.87
27.30
71.83
100.00
(0.51)
(1.93)
(2.07)
(0.46)
(1.46)
(1.53)
Notes: Coefficients represent the share of individuals who perceive themselves as “Underweight,” “About Right,” or “Overweight” by BMI categories. Standard errors are in
parentheses.
18
Table 4. Differences in Means and Probabilities, NHANES 1999-2004 - NHANES III
Age Range
Men 17-74
17-25
26-35
36-45
46-55
56-74
BMI
1.600
(0.216)
1.260
(0.268)
1.001
(0.315)
1.199
(0.313)
1.292
(0.214)
Obesity
Rate
* 8.190
(2.062)
* 8.855
(2.290)
* 8.741
(1.970)
* 7.376
(2.404)
* 7.447
(2.116)
Childhood
Obesity Rate
* 5.462
* 2.155
* 0.413
*
*
Feel Overweight
(cond:
Overweight)
Feel
Overweight
(cond: Obese)
Pr. Feel
Overweight
(BMI 28)
Pr. Feel
Overweight
(BMI 33)
-8.870
(4.344)
-6.743
(3.061)
-2.736
(3.820)
1.154
(3.475)
5.299
(2.649)
-14.572
(3.496)
-6.389
(3.786)
3.219
(3.809)
-1.193
(3.923)
2.894
(2.702)
-16.54
-5.97
-16.47
-4.89
-8.69
-2.43
-3.64
-1.01
-1.08
-0.36
*
*
*
*
Women 17-74
17-25
1.826
* 8.619
* 5.211
-8.523
*
-6.222
* -9.37
-1.41
(0.301)
(1.823)
(3.091)
(3.122)
26-35
2.342
* 10.207 * 1.905
-6.325
*
-3.466
* -9.25
-1.39
(0.363)
(2.301)
(2.406)
(1.380)
36-45
1.742
* 8.501
* -0.321
-3.666
*
-0.724
-5.30
-0.78
(0.442)
(2.699)
(1.730)
(1.077)
46-55
0.931
* 3.522
-0.543
0.689
-2.96
-0.43
(0.424)
(2.793)
(1.475)
(0.991)
56-74
1.616
* 9.763
*
1.765
1.597
-2.23
-0.35
(0.263)
(1.944)
(1.598)
(1.082)
Notes: Standard errors in parentheses. * indicates difference is significant at the 5% level. Predictions (columns 6 and 7)
are based on estimates of Model 1 in Table 5. Age, education, and other covariates held at sex-specific global averages.
19
Table 5. Multinomial Logit Estimates
Model 1
Female
NHANES 1999-2004
NHANES 1999-2004 x Female
Model 2
Model 3
Model 4
Underwgt.
Overwgt.
Underwgt.
Overwgt.
Underwgt.
Overwgt.
Underwgt.
Overwgt.
0.23***
5.38***
0.27***
5.10***
0.19***
6.66***
0.18***
5.84***
(0.03)
(0.39)
(0.04)
(0.36)
(0.03)
(0.80)
(0.03)
(0.61)
2.39***
0.72**
0.91
1.03
1.52
1.04
2.28***
0.70**
(0.52)
(0.10)
(0.32)
(0.24)
(0.54)
(0.24)
(0.60)
(0.12)
0.76*
0.88
0.50***
1.04
0.75*
0.88
0.86
0.90
(0.12)
(0.08)
(0.10)
(0.12)
(0.12)
(0.08)
(0.16)
(0.11)
BMI
0.58***
1.56***
0.58***
1.56***
0.58***
1.56***
0.57***
1.55***
(0.01)
(0.02)
(0.01)
(0.02)
(0.01)
(0.02)
(0.02)
(0.02)
Age 17-25
1.66***
1.10
24.00***
0.41*
3.08***
0.64
1.59*
1.05
(0.33)
(0.14)
(18.35)
(0.21)
(1.16)
(0.18)
(0.42)
(0.17)
1.40*
1.13
3.81***
0.77
1.96***
0.86
1.40
1.08
(0.26)
(0.12)
(1.35)
(0.18)
(0.44)
(0.14)
(0.33)
(0.16)
Age 26-35
Age 46-55
Age 56-74
1.06
0.91
0.60
1.14
0.89
1.08
0.69
1.16
(0.26)
(0.11)
(0.19)
(0.20)
(0.23)
(0.16)
(0.17)
(0.19)
1.41*
0.61***
0.97
0.71**
1.19
0.70***
(0.28)
(0.06)
(0.20)
(0.10)
(0.28)
(0.09)
0.34***
0.68**
0.24***
0.74*
0.34***
0.68**
0.29***
0.90
(0.09)
(0.11)
(0.06)
(0.13)
(0.09)
(0.11)
(0.12)
(0.29)
0.61**
0.71**
0.44***
0.82
0.60**
0.74**
0.57*
0.81
(0.15)
(0.10)
(0.12)
(0.14)
(0.15)
(0.11)
(0.18)
(0.17)
NHANES 1999-2004 x Age 46-55
0.5600
1.23
0.67
1.13
0.65
1.08
(0.19)
(0.22)
(0.23)
(0.21)
(0.22)
(0.21)
NHANES 1999-2004 x Age 56-74
0.50**
1.36**
0.46***
1.40**
0.52**
1.36**
(0.14)
(0.19)
(0.13)
(0.20)
(0.15)
(0.19)
1.14
1.60***
1.13
1.61***
1.13
1.60***
1.11
1.68***
(0.13)
(0.12)
(0.13)
(0.12)
(0.12)
(0.12)
(0.14)
(0.18)
1.60***
NHANES 1999-2004 x Age 17-25
NHANES 1999-2004 x Age 26-35
High School Graduate
Some College or Better
Middle Income Group
High Income Group
Formerly Married
Never Married
0.89
1.75***
0.89
1.76***
0.89
1.75***
0.86
(0.09)
(0.11)
(0.10)
(0.10)
(0.10)
(0.10)
(0.11)
(0.13)
0.87
1.38***
0.87
1.38***
0.87
1.38***
0.85
1.42***
(0.09)
(0.08)
(0.09)
(0.08)
(0.09)
(0.08)
(0.09)
(0.11)
0.60***
1.40***
0.60***
1.41***
0.59***
1.41***
0.59***
1.36***
(0.07)
(0.08)
(0.07)
(0.08)
(0.07)
(0.08)
(0.07)
(0.10)
0.90
1.30***
0.91
1.29***
0.90
1.29***
0.92
1.20***
(0.10)
(0.11)
(0.10)
(0.11)
(0.10)
(0.11)
(0.12)
(0.11)
1.11
1.13
1.10
1.13
1.10
1.13
1.26
1.05
(0.20)
(0.11)
(0.20)
(0.11)
(0.20)
(0.11)
(0.28)
(0.13)
2.40***
0.72*
(0.61)
(0.12)
1.05**
0.96**
(0.03)
(0.02)
Mean BMI
Obesity Rate
Childhood Obesity Rate
African American non-Hispanic
Mexican American
Other Race/Ethnicity
1.03
0.96
(0.07)
(0.05)
0.36***
1.48***
0.37***
1.49***
0.36***
1.48***
0.36***
1.48***
(0.14)
(0.02)
(0.14)
(0.02)
(0.14)
(0.02)
(0.17)
(0.02)
1.10
0.73***
1.09
0.74***
1.09
0.73***
1.06
0.77***
(0.15)
(0.05)
(0.15)
(0.05)
(0.15)
(0.05)
(0.17)
(0.06)
1.26
0.69***
1.25
0.69***
1.26
0.69***
1.36*
0.83
(0.19)
(0.07)
(0.19)
(0.07)
(0.19)
(0.07)
(0.23)
(0.11)
N
27060
27060
27060
16639
F-Statistic
87.53
86.03
83.00
86.73
Notes: Coefficients represent relative risks. Standard errors are in parentheses. *** indicates coefficient is significantly different from 1 at the
1% level or better, ** indicates coefficient is significantly different from 1 at the 5% level, * indicates coefficient is significantly different from
1 at the 10% level or better.
20
Table 6. Mean BMI and Obesity Rate by Survey and Sex
Age
17-25
26-35
36-45
46-55
56-74
Total
Age
17-25
26-35
36-45
46-55
56-74
Total
Mean BMI
NHANES III
Men
Women
24.042
23.923
(0.138)
(0.203)
26.134
25.473
(0.151)
(0.198)
27.203
26.767
(0.248)
(0.354)
27.438
28.012
(0.223)
(0.279)
27.356
27.621
(0.143)
(0.177)
26.340
26.256
(0.111)
(0.159)
NHANES 1999-2004
Men
Women
25.664
25.645
(0.147)
(0.194)
27.394
27.815
(0.221)
(0.304)
28.204
28.509
(0.195)
(0.266)
28.637
28.944
(0.221)
(0.319)
28.649
29.237
(0.159)
(0.195)
27.713
28.059
(0.096)
(0.136)
Age
17-25
26-35
36-45
46-55
56-74
Total
Age
17-25
26-35
36-45
46-55
56-74
Total
Obesity Rates
NHANES III
Men
Women
11.76
12.92
(1.25)
(1.02)
14.88
21.35
(1.30)
(1.27)
22.57
25.99
(1.34)
(2.15)
23.80
32.99
(1.81)
(2.03)
25.47
29.99
(1.72)
(1.25)
19.16
24.13
(0.72)
(0.90)
NHANES 1999-2004
Men
Women
22.52
22.50
(1.46)
(1.27)
25.07
33.00
(1.82)
(1.80)
32.45
36.43
(1.43)
(1.61)
32.61
37.52
(1.63)
(1.94)
34.29
41.42
(1.26)
(1.50)
29.42
34.35
(0.68)
(0.91)
Change
Age
17-25
26-35
36-45
46-55
56-74
Total
Men
1.62
6.75%
1.26
4.82%
1.00
3.68%
1.20
4.37%
1.29
4.72%
1.37
5.21%
Women
1.72
7.20%
2.34
9.19%
1.74
6.51%
0.93
3.33%
1.62
5.85%
1.80
6.87%
Age
17-25
26-35
36-45
46-55
56-74
Total
Men
10.76
91.50%
10.19
68.48%
9.88
43.77%
8.81
37.02%
8.82
34.63%
10.26
53.55%
Change
Women
9.58
74.15%
11.65
54.57%
10.44
40.17%
4.53
13.73%
11.43
38.11%
10.22
42.35%
21
Figure 1. Predicted Perception of Weight, Age 17-25 (Panel A:Women, Panel B: Men)
Notes: Predictions are based on estimates of Model 1 in Table 4. The purple lines show the thresholds for
(from left to right) “normal weight”, “overweight”, and “obese” for 17 year olds. The dark lines show the
same thresholds for individuals 21 and over (adults). See Table A.2. for details on these cutoffs.
22
Figure 2. Predicted Perception of Weight, Age 36-45 (Panel A:Women, Panel B: Men)
Notes: Predictions are based on estimates of Model 1 in Table 4. The dark lines show the thresholds for
(from left to right) “normal weight”, “overweight”, and “obese” for individuals 21 and over (adults). See
Table A.2. for details on these cutoffs.
23
Figure 3. Predicted Perception of Weight, Age 56-74 (Panel A:Women, Panel B: Men)
Notes: Predictions are based on estimates of Model 1 in Table 4. The dark lines show the thresholds for
(from left to right) “normal weight”, “overweight”, and “obese” for individuals 21 and over (adults). See
Table A.2. for details on these cutoffs.
24
Figure 4. Predicted Perception of Weight, NHANES III (A:Women, B: Men)
Notes: Predictions are based on estimates of Model 1 in Table 4. The dark lines show the thresholds for
(from left to right) “normal weight”, “overweight”, and “obese” for individuals 21 and over (adults). See
Table A.2. for details on these cutoffs.
25
Figure 5. Predicted Perception of Weight, NHANES 1999-2004 (A:Women, B: Men)
Notes: Predictions are based on estimates of Model 1 in Table 4. The dark lines show the thresholds for
(from left to right) “normal weight”, “overweight”, and “obese” for individuals 21 and over (adults). See
Table A.2. for details on these cutoffs.
26
Data Appendix
Data Sets
The empirical analysis is conducted using data from the National Health and Nutrition
Examination Survey (NHANES), a nationally-representative series of cross-sectional
studies conducted by the Centers for Disease Control. The NHANES data include
observations of weight, height, and weight perception, as well as information about
demographic and socioeconomic characteristics collected via in-person interviews. We
examine data from NHANES III (1988--1994) and NHANES 1999--2004.14 We restrict
the samples to individuals 17 to 74 years of age. We calculate individual BMI values, as
weight in kilograms divided by the square of height in meters, using weight and height
data measured by NHANES surveyors at mobile examination centers around the country.15
Measures
Education
Educational attainment is measured through self-reports of years of education, top-coded at
17+ years in the first three waves, where 16 years is an (imperfect) threshold indicating
college completion. The 1999--2004 data are top-coded at 13, however, such that those
with just some college cannot be distinguished from those with a college degree or better.
In order to compare education effects across NHANES III and 1999--2004, we create
consistent education categories, defined as 0--11 years or “less than high school,’ 12 years
exactly or “high school,” and 13 or more years or “some college (or above).”
Income
NHANES collects self-reported data on household income, rather than individual income,
as a categorical variable. NHANES also includes a related variable based on household
income, the “poverty income ratio,” which is recommended for comparing income effects
across different surveys.16 As recommended in the NHANES analytical guidelines, we
collapse the poverty income ratio into three categories, “low,” “middle,” and “high,”
representing, respectively, individuals with household income up to 1.3 times the poverty
threshold, between 1.3 and 3.5 times the threshold, and more than 3.5 times the
threshold.17
14
Since 1999, the survey has been conducted annually, with statistics reported in two-year increments.
Reported figures for NHANES 1999--2004 refer to the combined data, but figures can be broken out for
1999--2000, 2001--2002 and 2003--2004. Data are available from NHANES 2005-2006, but we limit our
analysis to the 1999-2004 data in order to enable the most comprehensive set of control variables.
15
Individuals were also asked (in an interview session conducted separately from and prior to the
examination) to report their own weight and height. For some individuals, the data contain only these selfreports and not also direct measurements, but we exclude the latter from our analysis. This exclusion
minimizes measurement error and does not affect representativeness, since survey weights are provided that
pertain to use of the examination-only sample.
16
The poverty income ratio is roughly standardized for inflation and takes into account household size,
whereas the raw income categories are not easy to align consistently in real terms across surveys.
17
Individuals with incomes up to 1.3 times the poverty line are eligible for food assistance programs and thus
we might expect categorical differences in outcomes across this divide.
27
Marital Status
We observe information pertaining to marital status and living situation, including whether
individuals are co-habiting with a partner and whether individuals are separated in addition
to the standard legal categories. Using this information we create three categories,
“married,” which includes married people living with a spouse as well as unmarried
individuals co-habiting with a partner, “formerly married,” which includes divorced
individuals as well as separated (or married but estranged) individuals no longer living
with a spouse, and “never married,” which includes those who have never been married
and are not currently co-habiting.
Race and Ethnicity
NHANES III uses four race categories: white (non-Hispanic), black or African-American
(non-Hispanic), Mexican-American, and “other.” In NHANES 1999-2004, an additional
category, “other Hispanic,” was added, to capture non-Mexican Hispanics. To make the
categories comparable across surveys, we merge “other Hispanic” with “other.” In the
analysis, we include dummy variables for each race category, letting whites be the omitted
category.
Childhood obesity rates
We define overweight and obesity for those under age 21 using the CDC’s official BMIfor-age-and-gender reference distributions. We adopt the convention that children and
adolescents with BMI values between the 85th to 95th percentile thresholds in the reference
distribution are “overweight,” and those above the 95th percentile are “obese.”
Underweight children and adolescents are those below the 5th percentile, and the normal
range is between the 5th and 85th percentile values. It is important to note that some sources
adopt the convention that the children and adolescents between the 85th and 95th percentile
BMI values are defined as “at risk of overweight,” rather than “overweight,” and that those
above the 95th percentile are “overweight” rather than “obese.”
For individuals ages 17-42 in NHANES III, and those ages 17-52 in NHANES 1999-2004,
we estimate the childhood obesity rate (among children ages 6-11 of the same sex) that
would have prevailed when they themselves were between the ages of 6 and 11.
Observing the individual’s age and the time period during which she was surveyed, we can
back out the approximate time frame during which she would have been a child. Then we
look up the previously published childhood obesity rates (gender-specific) observed in the
NHES 2 survey, conducted in 1963-1965, and in waves I through III of NHANES,
covering the periods 1971-1974, 1976-1980, and 1988-1994, respectively, (Ogden, et al
2006; Ogden, et al 2002), and assign the rate from the appropriate time period. However,
the year in which a given individual was surveyed is observed only up to a three year
window for NHANES III subjects, and up to a 2-year window for the 1999-2004 subjects.
Therefore, in some cases, we cannot determine with certainty the appropriate survey from
which to draw the childhood obesity rate, and in some cases subjects would have been
children during a time frame for which no survey was conducted. When either of two
surveys might have been correct, we assign the average of the rates from those two
surveys. In some cases, we could not map backward to an appropriate obesity rate. For
example, we would need childhood obesity rates predating 1963 in order to assign
28
childhood obesity rates to individuals ages 43 and older in NHANES III and those 53 and
older in the 1999-2004 survey. Also, 23 year olds observed in NHANES 1999-2000, 26
year olds observed in NHANES 2001-2002, and 28 year olds observed in NHANES 20032004 could not be assigned childhood obesity rates, and instead they were assigned the
rates of 24, 27, and 29 year olds observed during the same survey periods, respectively.
29
Appendix Tables
Table A.1. Measures and Basic Descriptives
Men
Variable
NHANES 1999-2004
BMI
AGE 17-25
AGE 26-35
AGE 46-55
AGE 56-74
NHANES 1999-2004 x AGE 17-25
NHANES 1999-2004 x AGE 26-35
NHANES 1999-2004 x AGE 46-55
NHANES 1999-2004 x AGE 56-74
High School Graduate
Some College or Better
Middle Income Group
High Income Group
Previously Married
Never Married
Obs
14545
14359
14545
14545
14545
14545
14545
14545
14545
14545
14481
14481
13298
13298
14332
14332
Mean
0.530
27.144
0.186
0.223
0.169
0.198
0.095
0.106
0.105
0.103
0.286
0.473
0.297
0.417
0.653
0.098
Std. Dev.
0.499
5.328
0.389
0.416
0.375
0.399
0.294
0.308
0.306
0.304
0.452
0.499
0.457
0.493
0.476
0.297
Min
0
13.8
0
0
0
0
0
0
0
0
0
0
0
0
0
0
Max
1
70.2
1
1
1
1
1
1
1
1
1
1
1
1
1
1
Women
Variable
NHANES 1999-2004
BMI
AGE 17-25
AGE 26-35
AGE 46-55
AGE 56-74
NHANES 1999-2004 x AGE 17-25
NHANES 1999-2004 x AGE 26-35
NHANES 1999-2004 x AGE 46-55
NHANES 1999-2004 x AGE 56-74
High School Graduate
Some College or Better
Middle Income Group
High Income Group
Previously Married
Never Married
Obs
16123
15893
16123
16123
16123
16123
16123
16123
16123
16123
16071
16071
14633
14633
15860
15860
Mean
0.528
27.266
0.180
0.214
0.169
0.217
0.094
0.103
0.105
0.113
0.310
0.464
0.286
0.375
0.604
0.198
Std. Dev.
0.499
6.842
0.384
0.410
0.375
0.412
0.291
0.304
0.306
0.316
0.463
0.499
0.452
0.484
0.489
0.399
Min
0
11.7
0
0
0
0
0
0
0
0
0
0
0
0
0
0
Max
1
79.6
1
1
1
1
1
1
1
1
1
1
1
1
1
1
30
Table A.2. BMI Cutoffs
16
17
18
19
20
Adult
BMI Cutoffs for Women
Under (<) Over (>=) Obese (>=)
16.8
24.6
28.8
17.2
25.2
29.6
17.6
25.6
30.4
17.8
26.2
31.0
17.8
26.4
31.4
18.5
25.0
30.0
16
17
18
19
20
Adult
BMI Cutoffs for Men
Under (<)
Over (>=)
17.2
24.2
17.8
25.0
18.4
25.6
18.8
26.4
19.2
27.0
18.5
25.0
Obese (>=)
27.6
28.2
29.0
29.8
30.6
30.0
Notes: From CDC’s reference distributions. At a given gender and age, underweight is
defined as below the 5th percentile of the relevant BMI distribution, between 5th and 85th is
defined as normal, 85th to 95th is considered overweight but not obese, and 95th and above
is obese.
31