Phillip Skaliy
Genetics
Ely
November 4, 2011
Human Mate Preference: Genetic and Social/Environmental Influences
Creatures and animals across the world do not prefer all members of the
opposite sex equally. Some research has been done in this area, but it is extremely
hard to figure out the entire story about mate selection due to the fact that many
different systems and thought processes control it. Mate choice is very important to
an organism’s success and the success of its family and species due to the fact that
the genes of the parent are eventually passed down to the children. Thus, someone
will want to mate with a person who has “good genes” because those genes are
eventually inherited by the offspring. One would assume that organisms, including
humans, take this decision of who to mate with seriously because not only does it
have major impacts on their descendents but also because humans want to mate
with individuals of certain characteristics that meet their specific needs. Therefore,
not only are the interests of the children involved, but also there are some personal
interests involved with mate choice. When it comes to these personal interests,
most people would like to mate with successful, intelligent, and attractive people.
But certain questions arise, like which of these traits is most influential and what
happens when one compares long-term relationships to short-term relationships?
Many other questions surround this topic, like which genes code for what
characteristics dealing with mate choice, and what other non-genetic factors
influence mate choice. Another question could be if someone would be inclined to
mate with a person of a more diverse genotype due to the fact that diverse
genotypes tend to lead to healthier children, and how would someone recognize a
different genotype than their own. These types of questions are important, and
most have been answered at least partially by many different scientists around the
world. Ultimately, recent research has found that there is more of a genetic
component to mate choice than was thought in the past, but there are still some
environmental, social, and familial influences to mate choice that cannot be ignored.
One non-genetic influence that many people can relate to with regards to
mate choice is that of the parents (Zietsch et al. 2011). Family influence on mate
preference in past societies has been proven through anthropological evidence, and
it has shown that parents throughout history and throughout many different
cultures have significantly influenced mate choice with regards to income and age
(Buunk et al. 2008). To test whether this hypothesis was actually true, Zietsch et al.
(2011) conducted a twin study and compared the spouses of female twins to see if
those spouses correlated significantly on income and age, along with a variety of
other factors. The results showing the correlations between twin pair partners for
age and income can be seen in table one. As one can see, the correlations between
spouses of female twins for income and age are fairly strong and very significant,
especially when compared to other traits (Table 1, the other traits are not shown
due to the size restriction of this page). On top of that, the correlation is strong
between spouses of monozygotic (or identical) female twins and spouses of
dizygotic (or fraternal) female twins, showing that this outside pressure is a family
influence and not a genetic influence. If this was a genetic influence, then the
influence would be much greater when comparing spouses of identical twins
because monozygotic twins share the same genome. Zietsch et al. (2011)
hypothesized that parents have an evolutionary interest in ensuring that their
daughters mate with older and higher income men. Parents often want to ensure
that their daughters live a comfortable lifestyle, and marrying a successful man is a
good way in making sure that happens. Thus, when looking at mate preference for
females, it seems that females tend to stress the income and age of their mates,
which is why income and age are two of the most important and most studied traits
of a man by researchers today.
Table 1. Correlations between twin pair partners (i.e., twin pair correlations
for mate choice), uncorrected and corrected for assortative mating
Mate choice
Raw data:
MZF
MZM
DZF
DZM
DZOS
Controlling for
assortative
mating:
MZF
MZM
DZF
DZM
DZOS
Income
Age
.30 (.04)**
.12 (.06)
.37 (.05)**
.24 (.08)*
.06 (.06)
.93 (.01)**
.92 (.02)**
.90 (.02)**
.92 (.03)**
.90 (.02)**
.24 (.05)**
.07 (.06)
.30 (.06)**
.21 (.09)
−.09 (.07)
.19 (.03)**
.09 (.06)
.18 (.05)**
−.02 (.04)
.00 (.07)
Note. Values in parentheses are standard deviations, except where indicated. “Overall” correlations (and
95% CIs) are estimated by constraining twin correlations to be equal across all traits (except for age in the
raw data). MZF = monozygotic female pairs, MZM = monozygotic male pairs, DZF = dizygotic female
pairs, DZM = monozygotic male pairs, DZOS = dizygotic opposite-sex pairs.
* Significant at P < .01, ** Significant at P < .001
Another hypothesis involving parents and their influence on mate choice is the
sexual imprinting hypothesis. Sexual imprinting is the process by which individuals
acquire mate-choice criteria during development by using their opposite-sex parent
as the template of a desirable mate. So, for example, a man would search for a
woman like his mother. In past studies, sexual imprinting has been seen in less
advanced animals, but its role in humans is not very clear (Oetting et al. 1995). This
study compared the similarity between a person’s spouse and his or her opposite
sex parent, relative to other family members (Table 2) (Zietsch et al. 2011)).
It is clear that twins’ partners were not more similar in any trait to the twins’
opposite-sex parent than to the twins’ same-sex parent. In fact, twins’ spouses were
much more similar to the twin and co-twin than the twin's opposite-sex parent.
Thus, parents have an influence on who their children marry when it comes to
income and age, but there was no evidence that they serve as a template for the
future mate of their child .. Since other studies have found evidence of sexual
imprinting Bereczkei et al. (2004), more work needs to be done in this area to resolve
the conflicting results. Zietsch et al. (2011) suggest that maybe different traits
should be considered than the ones that were tested in this experiment and see
whether the results change. Another suggestion was to compare newlyweds to
older couples and see if there are any environmental and genetic differences
between the couples. Pawlowski et al. (1999) examined how market value affected
a person’s selection of mate. The term market value simply means how appealing
someone appears to the opposite sex in a competitive market. Market value is
dependent on many different characteristics, but one characteristic that is especially
important is age. After examining personal advertisements in newspapers,
Pawlowski et al. decided to observe if there was a correlation between age and
market value and whether people with a greater market value were more
demanding when searching for partners.
The data indicated that the optimum age for females was between twenty
and thirty years old which correlates with their maximum fecundity and
reproductive value. Thus, one could infer that males put high importance on child
rearing. For men, the age at which they had the best market value was between
thirty and forty years old. Pawlowski and Dunbar proposed that male market value
is attributed mostly to income and commitment to the relationship, meaning that
the male will not get a divorce or die over the next twenty years. Twenty years is
the optimum amount of commitment time for a woman when looking for a mate
because that is about how long it takes to raise a child,. Ultimately the idea is that
women often view older men as safer, better behaved, and wiser than younger guys,
so they are more prone to have a relationship with them because they are a safer
choice. Pawlowski and Dunbar also compared market value and the number of
traits that person sought in a mate, and the researchers found a correlation between
a high market and the number of traits they sought in their partner. This
observation suggests that people of high market value impose strong demands in
their mate search criteria, and the opposite is true for people of low market value. It
seems that mate choice is very complex and is almost like a negotiation in the way
that people stress some traits over others. Overall, it may seem surprising that
people are waiting somewhat later in life to enter into relationships and have
children, but Pawlowski and Dunbar (1999) explain that when someone considers
the amount of money that it costs to raise a child, it is not surprising that people
would have smaller family sizes and wait until later in life to have their children.
Another important factor in mate choice is intelligence. It seems that as
intelligence increases, that would make a mate more highly valued. Most likely, that
intelligent person would be more successful and have more access to essential
resources, especially for child rearing. Studies have already shown that intelligence
seems to be a necessity and not a luxury in mates (Li et al. 2002). There is also
evidence now to suggest that intelligence could be a fitness factor, meaning that it
could be beneficial to conceive children with more intelligent mates (Miller 2000).
Prokosch et al. (2009) hypothesized that women would prefer men with higher
intelligence and also higher creativity, a trait closely related to intelligence, in both
long and short term relationships. They also studied whether preferences for
intelligence and creativity shifted across the menstrual cycle. If intelligence or
creativity cues men with good quality genes, then these characteristics should
become especially important during the menstrual cycle of a female in short-term
relationships. In order to assess intelligence, the men took a test and participated in
videotaped interviews. These videos would be shown to the women, and they
would decide for themselves whether the person was intelligent or not. This
process is called perceived intelligence. The study found that there was some
variability, but overall, in both long and short-term relationships, women preferred
men of higher intelligence. But, picking mates on perceived intelligence seemed to
be more inaccurate than when compared to intelligence measured form the test.
Women also preferred more creative men. The researchers believe that women
stress intelligence because of financial security and dependability. The researchers
also hypothesized that intelligence was important because intelligence shows that
someone has “good genes,” but there was not a shift in importance for intelligence
while the women were menstruating. Therefore, it is not certain whether
intelligence is an indicator of good genes according to this study. This study also
suggested that further investigation needed to occur in order to test whether
creativity is more related to physical attractiveness or intelligence. Overall, it seems
as though women appear to prefer more intelligent men, but more research needs
to be done in order to more clearly understand what effect intelligence has on mate
choice (Prokosch et al 2009).
All of the factors that have been described up to this point are mainly nongenetic factors. That means that these factors are external influences with very little
genetic basis, or these are characteristics that are specifically and consciously
looked for in a mate. But there has been some evidence that mate choice is not only
just a product of environmental factors but also genetic ones as well (Lie et al.
2010). The Major Histocompatibility Complex (or MHC) is found in all jawed
vertebrates and contains genes that are essential for many biological functions, like
the immune system (Doherty et al. 1975)and mate preference (Millinski 2006).
MHC alleles are expressed co-dominantly, meaning that increasing the diversity at
the MHC should be beneficial for biological systems like the immune system because
that organism will be better able to defend itself from more foreign pathogens
(Doherty et al. 1975). How the MHC influences mate choice is debatable, but most
researchers seem to think that heterozygosity at the MHC is most preferred when
looking for a mate (Carrington et al. 1999). One of the main hypotheses
surrounding the MHC is the disassortative mating hypothesis, which suggests
individuals prefer mates with different MHC alleles to avoid inbreeding and gain
indirect benefits like their offspring being more resistant to disease (Brown 1997).
Similarly, Brown (1997) proposed the good-genes-as-heterozygosity hypothesis,
which states that preferences for MHC diverse mates are desired because that will
most likely enhance the genetic diversity of the offspring, making them more fit and
better able to survive. Past experiments (Mitton et al. 1993, Roberts et al. 2005)
have proven that many different animals prefer mates of dissimilar and diverse
Major Histocompatibility Complexes, but the exact effect in humans is not very clear.
Lie et al. (2010)investigated whether MHC diversity and dissimilarity affected face
preference when searching for a mate in short and long-term relationships. When
someone looks at another person’s face, something in that face intuitively triggers
that person to know that he or she is looking at a person with a genetically different
MHC than his or her own. Therefore, Lie et al. (2010) expected that people would
prefer to mate with people who had diverse Major Histocompatibility Complexes in
long and short-term relationships.
The results of this experiment clearly showed that males prefer the faces of
MHC dissimilar females in both the short-term and long-term. This result was not
the same for females though. Some people might refute this claim of genetic facial
dissimilarity between partners because it has been shown in past experiments that
couples look more similar than just by chance alone (Hinsz 1989, Little et al. 2006).
Lie et al. (2010) suggests that these similar facial characteristics between mates
according to this other experiment might be controlled by environmental influences
and not have a genetic basis. Therefore, it is possible that facial characteristics can
be controlled by genetic influences and environmental influences at the same time.
An important question to ask is why males would seek genetically dissimilar
females to mate with. One answer to that question is that a male’s reproductive
success is heavily dependent on the health and fertility of the female (Buss and
Schmidt 1993). Thus, males should value fertility in their partners, and MHC
dissimilarity may serve to reduce fertility problems that are associated with MHC
allele sharing, which include spontaneous birth abortion and longer birth intervals
(Beydon et al. 2205, Ober 1999, Ober et al. 1992, Ober et al. 1998, see also Knapp et
al 1996). Another question might be why males were more sensitive to recognizing
female facial MHC dissimilarity than females were, and it seems that males put an
emphasis on visual aspects of their mates while females put an emphasis on odor
aspects of their mates. Therefore, it would be easier for a male to detect facial MHC
dissimilarity (Havlicek et al. 2008, Herz et al. 2002). For genetic diversity, females
preferred MHC diversity in both short-term and long-term mates (Lie et al. 20120).
This makes sense because heterozygosity is heritable and choosing a MHC diverse
mate is associated with fitness, meaning the mate can provide resources in the
short-run and parental care in the long run. Males showed no preference for MHC
diversity.
Overall, this study about the Major Histocompatibility Complex only tested
mate preference and not actual mate selection. Therefore, this experiment needs to
be supplemented with how mate preference ultimately leads to mate selection. As
discussed earlier, some people have a greater market value, and that allows them to
be choosier when selecting a mate. Maybe this theory is the same when one
considers genetic influences, and studying that relationship would be an interesting
experiment and lead to useful results. Also, this study is just one of many
experiments covering the Major Histocompatibility Complex. Other tests have been
done on facial attractiveness and odor (Roberts et al. 2008, Thornhill et al. 2003),
and all these results seem to conflict each other in some way. Therefore, more
research needs to be done in this area before researchers can consistently say how
the MHC controls mate choice. But, ultimately, these results do show that the MHC
does have some control and influence on mate selection, and that females and males
both experience it in some way when dealing with facial characteristics (Lie et al
2010).
In the end, mate choice is one of many examples of a human trait or action
that is controlled by both environmental and genetic factors. This is significant
because it is not only our genes that influence the way someone looks and acts, but
it is also controlled by environmental factors like how someone was raised by their
parents. Therefore, issues like mate choice are extremely hard to study and fully
understand because there are many different influences that can affect it. In order
to understand mate choice completely, future studies will have to be done to see
exactly what genetic and environmental influences affect mate choice and how those
influences interact with each other.
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