The Psychology of Sex Differences – 5 Revealing Insights From Our Primate Cousins

By Christian Jarrett

There are behavioural differences, on average, between the sexes – few would dispute that. Where the debate rages is over how much these differences are the result of social pressures versus being rooted in our biology (the answer often is that there is a complex interaction between the two).

For example, when differences are observed between girls and boys, such as in preferences for play, one possibility is that this is partly or wholly because of the contrasting ways that girls and boys are influenced by their peers, parents and other adults (because of the ideas they have about how the sexes ought to behave). Studying non-human primates allows us to identity sex differences in behavior that can’t be due to human culture and gender beliefs.

Learning more about the biological roots of behavioural sex differences should not be used as an excuse for harmful stereotyping or discrimination, but it can help us better understand our human nature and the part that evolved sex differences play in some of the most important issues that affect our lives, including around diversity, relationships, mental health, crime and education.

Earlier this year, as part of a special issue of the Journal of Neuroscience Research – titled “An Issue Whose Time Has Come: Sex/Gender Influences on Nervous System Function” – Elizabeth Lonsdorf at Franklin and Marshall College published a useful mini-review detailing some of the sex differences observed among monkey and ape infants and juveniles.

“Many sex differences in behavioral development exist in nonhuman primates,” she writes, “despite a comparative lack of sex-biased treatment by mothers and other social partners”. Here is a digested account of five of these behavioural sex differences:   

 Boy chimps spend more time away from their mothers

Screenshot 2017-10-02 12.04.53.png
Graph shows time spent travelling independently by age for males and females

In human cultures across the world, boys tend to spend more time away from their mothers as compared with girls, which may reflect an early tendency toward greater risk taking. The same is true in chimps. For a 2014 paper, Lonsdorf and her colleagues, including famed zoologist Jane Goodall, conducted the most detailed ever analysis of the development of wild chimpanzees, observing 40 chimps from birth to age 5. They found that, by age 3, male infants showed greater physical distance from their mothers than females. The males began moving about on their own earlier than females and covered more distances independently. These and other findings from the study “suggest that some biologically based sex differences in behavior may have been present in the common ancestor of chimpanzees and humans, and operated independently from the influences of modern sex-biased parental behavior and gender socialization,” the researchers said.

Baby girl monkeys are more social 

In humans, girls tend to show more precocious social interest and development than boys. For example, at less than two-days old, girls spend more time than boys looking at a human face than at a mechanical toy. Though a replication attempt of this specific finding for humans is needed, a very similar pattern has been found in rhesus macaque monkeys. For a paper published last year a research team led by Elizabeth Simpson observed 48 two- to three-week old monkeys as they were presented with videos of a macaque monkey face. The females spent more time looking at the videos than males and they later displayed more social behaviour toward a human. “In sum, converging evidence from humans and monkeys suggests that female infants are more social than males in the first weeks of life, and that such differences may arise independent of postnatal experience,” the researchers said.

Boy monkeys enjoy more rough and tumble 

In human children, average sex differences in play preferences are among the most striking, consistent (and controversial) behavioural contrasts found between boys and girls. It never seems to take very long for little boys to pick up sticks and pretend that they are armed with a sword. In contrast, girls typically spend more time pretending to nurture toy dolls or similar. Of course, many say this simply reflects the way that boys are raised to be more aggressive and girls more nurturing. And yet very similar sex-linked play preferences are seen in our primate cousins.

Screenshot 2017-10-02 14.10.37For instance,  Gillian Brown and Alan Dixson studied 34 macaque monkeys for the first six months of their lives and found that the males (filled circles in the graph on the left) engaged in more rough and tumble play than females, including wrestling, pinning down, as well as chasing and gentle hitting. Males also initiated play more often than females. In her review, Lonsdorf notes that similar findings have been observed in blue monkeys, patas monkeys, Japanese macaques and olive baboons.

Girl monkeys spend more time playing parent

Human boys, on average, are less interested than girls in babies. There’s a similar pattern among non-human primates. Observational research at the Yerkes Regional Primate Research Center with one-year-old Rhesus Macaque monkeys found that the females spent significantly more time than the males with infants. “The focus of young females’ attention on infants has been suggested to be important for the development of later maternal skills,” the researchers said. They noted that a similar observation has been found for another species: “Juvenile female vervet monkeys that live in the wild carry and cuddle small infants, an experience that may provide them with the motor skills necessary for later matemal behavior, as weIl as give them exposure to the maternal role. It is likely that this experience is similarly useful for a yearling rhesus monkey”. Lonsdorf observes in her review that a female bias for nurturing style play has also been seen in lowland gorillas and blue monkeys.

Boy monkeys like learning from their dads, girl monkeys from their mums

There’s little doubt that biological differences between the sexes are amplified by parenting practices and parent-child relationships, including boys’ tendency to want to emulate their dads and girls to emulate their mothers – as seen both in humans and our primate cousins. For instance, in a study of capuchin monkeys, Susan Perry looked to see whether infants adopted one of two different techniques (pounding or scrubbing) for extracting seeds from fruit – girls, but not boys, typically adopted whichever technique was used by their mothers. In wild tufted capuchins, meanwhile, male juveniles typically stick with their dads and copy their dietary preferences, such as favouring eating animals over fruit (the latter preferred by females). It’s a similar story for chimps and termite fishing with research showing how females are better at this task than males and that female infants spend more time than their boy peers watching and learning from their mothers.

“Taken as a whole,” Lonsdorf concludes, “these consistent and accumulating reports of sex differences in primate behavioral development suggest that, although gender socialization in humans plays a role in magnifying the differences between young males and females, these behavioral sex differences are rooted in our biological and evolutionary heritage.”

Sex Differences In Nonhuman Primate Behavioral Development

Christian Jarrett (@Psych_Writer) is Editor of BPS Research Digest

21 thoughts on “The Psychology of Sex Differences – 5 Revealing Insights From Our Primate Cousins”

  1. We need to keep in mind that our primate cousins are also social creatures and thus subject to their own social programming. Isn’t it true that the Bonobos have a matriarchal social government system which differs from the usual sex roles?

    1. are you high? “social programming” so you claim that the PRIMATE societies, which have no actual concept of social hierarchy and work on natural instincts alone, somehow created a social patriarchal structure that program gender stereotypes and in truth, chimp gender is a spectrum.

      😀 this is the funniest thing i ever heard of, its like you are living in a cartoon, *THOSE PIGEONS LIVE IN A PATRIARCHY!*

    2. Absolutely agree Marvin. I was thinking along those lines myself ( I wasn`t aware of the Bonobos information though ). It would be absurd to believe that primates have no social learning influencing their behaviour.

      1. The key point is that the animals’ behaviour is not influenced by human ideas about how the genders ought to behave. Note also that there is little evidence among non-human primates of sex-biased treatment of the young, meaning the sex-based behavioural differences that emerge are likely to be driven predominately by sex-linked genetic influences and related downstream biological mechanisms.

    3. “Many sex differences in behavioral development exist in nonhuman primates,” she writes, “despite a comparative lack of sex-biased treatment by mothers and other social partners”.

  2. Not stupid at all! They do live in matriarchal groups where the leader is always female, they do have social hierarchy with lower and higher ranked individuals, as many other animals who live in groups do (you never watch national geographic?) So primates clearly have roles in their groups that are reserved for one gender and not the other. What makes us so sure then that they don’t exhibit gender biased behavior?

    1. What makes you so sure that it could? What is your evidence for the assertion that the researchers did not factor that possibility in their findings? And in the studies of primate infants, how could socialization possibly affect the results?

      1. What about investigator effects? All these studies are based on human observation, not blind to gender of infant being observed presumably. This was the big criticism of the human infants eye gaze study – babies are labelled at birth (or before) and pink / blue baby grows, balloons and cards on their cribs prime observers to notice particular behaviours in the infants.

  3. The point here, I think, is not that there is no scope for ‘gender biased behaviour’. Rather, the point seems to be that some (surely not all) sex differences observed in humans have close analogues in other closely related species. It’s a counter to the ‘it’s purely because of culture’ argument that has become so popular in some quarters. Such blank slatists will have to explain why evolution saw fit to remove any genetically influenced sex differences in human beings, only for social learning processes to reinstate them again. To do that you’d have to argue that such differences were both maladaptive (hence, selected out of our genes) and adaptive (hence, selected in to our culture). This state of affairs isn’t impossible but it is, prima facie, hugely improbable. Personally, I’ll be happy to accept this as an explanation only when there is a lot of very sound evidence for it.

  4. “Inhuman children, average sex differences in play preferences are among the most striking, consistent (and controversial) behavioural contrasts found between boys and girls. ”

    The fact that we have to tip-toe around “controversial” issues like this says far more about our inability to reason than our ability to be reasonable.

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