Evolution and Family Life
Sex and reproductive
variance:
Males: bigger gamble;
hi risk/hi reward
Females: better odds,
lower payoff
Trivers-Willard Hypo
•
•
•
•
Invest in the sex with the greater likelihood of reproductive success
Module vs. effect (unconcious, emotional vs. conscious deliberation)
Nursing behavior vs. inheritance
Importance of assessing maternal condition (condition during gestation
sets stage for unconscious differential investment)
• More likely to find evidence of T-W module at work when:
– Maternal condition is properly assessed
– Measuring unconscious rather than deliberative behaviors
Trivers/Willard Hypothesis
• Good conditions: invest in
boys
• Not so good conditions:
invest in girls
• Humans?
– Medieval Portugal church
records – more boys
conceived during years
when rains were good (Jolly,
1999, p. 122)
– High income mothers
breastfeed sons more than
daughters, vice-versa low
income.
– IBI longer for hi income
after son; vice-versa for low
income
– However, other
contemporary studies have
found mixed or negative
results
Primate infanticide
• Sara Hrdy: Langur Monkey infanticide
• Also documented in lions and gorillas.
• New male has limited time as alpha, no time to spend
on non-genetic offspring.
• Adaptive value of female promiscuity: “seeds of
confusion” theory
Risks of step-parenting.
Note: Violent step parents represent less than 1% of step families
•
•
Cross-culturally (60 different societies; Daly & Wilson, 1988) most common reasons for killing
or abandonment of newborns/infants:
Questionable paternity; low reproductive value of infant (weak, sick, abnormal; poor
maternal circumstances (unmarried, little family/social support; poor resources)
©John Wiley & Sons, Inc. 2007
Huffman: Psychology in Action (8e)
Birth vs. step children: education, health; nutrition
•
Anne Case – economist at Princeton
Plight of Cinderella (children and step
mothers)
• Sig decrease in food expenditure when
non-bio mother in home
• Children raised with non-bio mother sig
less educational attainment compared to
bio mothers
• In mixed families, exclusively non-bio
offspring that suffer reduced educational
levels (see graph left)
• Non-bio mother: fewer visits to doc,
dentist; reduced expenditures on fruits,
vegetables, milk, more on alcohol,
tobacco.
Sibling rivalry
• Mom is equally related to all
her offspring
• Sibs are 100% related to
themselves; only half
(genetically) related to each
other
• All things being equal (which
often they aren’t) Mom has
natural incentive to be “fair”
to offspring
• Each sib has incentive to
extract more resources from
Mom then sibs
• “That’s not fair”
Sexually Dimorphic Maturation Rates
•
•
•
•
Put simply – girls mature faster than boys. Why?
Females: Secondary sexual development precedes
gametogenesis (fertility); they look sexually mature
before they are fully mature (probably more
exaggerated in ancestral past then today, in most
traditional societies there’s 2-3 year period of
infertility after onset of secondary sexual traits)
Boys: vice-versa
14 year-old boy vs. 14 year old girl
Sexually Dimorphic Maturation Rates
• Males: Evolved divergent mating strategies: Early maturing – Alpha
strategy; more short-term mating opportunities. Late maturing -stealth strategy; sexually fertile but physically non-threatening.
• Females: “Practice” for mothering skills
• Evidence mixed: Faster maturing boys = higher status, more
popular, slower maturing boys = more creative
• Girls do more babysitting, child care, but does this mean maturation
rates selected for this purpose, jury still out.
Future stability and female maturation rates
• K vs. r reproduction strategies: unstable future – live fast, reproduce
early and often (r)
• Factors related to faster maturation rate in human females: (1)
domestic instability and stress; (2) father absence; (3) lack of
grandparental figures, esp grandfathers; (4) economic deprivation.
• Male investment unreliable; reproduce early while familial support
present
Sex ratio, poverty, and reproduction
• Operational sex ratio: ratio of sexually active males to sexually receptive
females.
• <1; more females than males; intense female mate competition. Few
investing males; marital instability; more single moms; promiscuity, etc
• >1; more males than females; intense male mate competition. Low status
males lose out; greater marital stability; higher crime.
For lower socio-eco females
(dashed and dotted lines) as osr
decreases (fewer males) birth rate
increases for younger women. But
higher s-e women have lower birth
rate at younger ages as osr
decreases and higher birth rate at
older ages. With few investing
males, poor women “speed up” life
history; rich women “slow down.”
Ref: Chipman & Morris, 2013 study
Crazy Bastard Hypo
•
•
•
•
•
•
•
Young male propensity for risk-taking: signal
of formidabilty.
Signal to whom? Male rivals and allies
Males who take risks are perceived as taller,
bigger, more muscular more prone to
violence.
Turn off to females as long-term mate, but
may be desirable as “protector” in
dangerous environment.
Absence of fathers usually produces more
violence among boys. Does propensity
toward risk taking increase if boys do not see
many older male role models around?
In other words, where there is long-term
mating (evidenced by fathers) a boy's mating
strategy shifts toward attracting long-term
mates and thus reduced risk-taking.
Self-Reinforcing Cycle: violent social context
-> risk taking males for allies -> less long
term mating -> female preference for
formidable male (genetics, protection) ->
more risk taking -> more violence
Parental Grief
•
•
•
Correlates with
reproductive value
curve for children
more so than just
age.
Tends to peak at
early adolescence.
Max investment at
threshold of
reproductive payoff.
Matches life history
pattern of
traditional huntergatherers, more so
than contemporary
life history curves.
Demographic Transition
• DT: Wealthy countries have
below replacement birthrates
• Evo paradox. Why?
• Greater investment per child
to maintain status
• Status seeking conflicts with
parental investment
• However, in long run higher
investment reduces fitness!
• Genetic/personality factors
important: Extro males and
agreeable females more
fertile
• High status males more
fertile; high status females
less fertile.
Religion & Fertility
source: last three slides see*
Worship Attendance Adults / No. of Children
82 Nations & Germany
World Value Surveys 1981 - 2004, IW 2007
2,6
2,4
2,2
2
1,8
1,6
1,4
1,2
1
2,5
2,23
2,01
1,67
1,39
1,78
1,66
1,98
1,8
1,44
never
on
once per once per > more
Holidays
Month Week
often
Children Germany
Children globally
Data Source: Dominik Enste, Institut der deutschen Wirtschaft Köln 2007
Swiss Census 2000
Denominational category
(CFR) Births per
woman
% academic
education
% higher occupational
class
Hinduism*
2,79 (1)
17,0% (12)
7,4% (14)
Islam*
2,44 (2)
11,4% (15)
6,1% (15)
Jewish
2,06 (3)
42,7% (1)
42,4% (1)
Other (smaller) Protestant
2,04 (4)
20,1% (5)
19,2% (6)
New Pietism / Evangelical
2,02 (5)
19,2% (6)
17,9% (8)
Pentecostal
1,96 (6)
17,1% (11)
15,7% (10)
Other (smaller) Christian
1,82 (7)
39,1% (2)
31,8% (2)
Didn’t answer
1,74 (8)
19,1% (7)
5,3% (16)
Christian-Orthodox*
1,62 (9)
18,0% (10)
9,8% (13)
Swiss Average
1,43
19,2%
19,6%
Buddhist*
1,42 (10)
20,3% (4)
13,4% (11)
Roman-Catholic
1,41 (11)
16,8% (13)
18,5% (7)
New Apostolic
1,39 (12)
13,9% (14)
17,6% (9)
Reformed Protestant
1,35 (13)
18,9% (8)
22,2% (4)
Yehova’s Witnesses
1,24 (14)
6,8% (16)
11,2% (12)
Christian-Catholic
1,21 (15)
18,4% (9)
22,2% (5)
Non-affiliated
1,11 (16)
30,6% (3)
26,7% (3)
0,054
-0,269
r / Spearman Rank Correl.
Religiosity is offering POTENTIALS to culturally diverse,
reproductive strategies
 We still found NO demographically successful nonreligious population!
Hutterites,
Haredim, Old
Order Amish etc.
NonAffiliated
USA, China,
France,
Sweden,
Austria etc.
Shakers
*Why are the Non-Religious lacking Children
Exploring the Evolutionary Adaptivity of Religion
Dr. Michael Blume
( www.blume-religionswissenschaft.de )
„Philosophical Anthropology of Religion“,
Wuppertal University, March 2014