Why do little boys tend to behave differently from little girls? Why do boys and girls play differently, for instance, choosing different toys as their favorites?
Ask these questions and you invite a firestorm — of more questions.
Is the premise behind these queries even accurate? Aren't our sons and daughters really more similar than different, after all? And when behavioral sex differences do occur, aren't parents who inflict sex-stereotypical expectations on their children largely responsible?
Seven experts on chimpanzee behavior, led by Elizabeth V. Lonsdorf of Franklin and Marshall College and including the world-famous primatologist Jane Goodall, have published a paper in Animal Behaviour that speaks, they say, to these issues. Their data on wild chimpanzees from Gombe, Tanzania, indicate that human sex differences in childhood are primarily the result of biological, evolutionary mechanisms.
The scientists analyzed data on the behavior of 12 male and eight female chimpanzee youngsters, ages 30-36 months. At that age, chimpanzees, who develop quite slowly compared with many other mammals, are still considered infants. As a rule, chimpanzees spend most of their day in close proximity to their mothers clear through their ninth year of life.
In the Gombe study, male infants were found to be more gregarious than female chimpanzees; they interacted with significantly more individuals outside the immediate family, including more adult males, than did females. This result held even when the number of the mothers' social partners was controlled.
In adulthood, Gombe females are less gregarious than males, and male-male bonds are particularly strong. So the study's take-home message is that the behavioral patterns of the adult apes at Gombe have their roots in the biology of early ontogeny.
These results, and the relevance Lonsdorf et al. see in them for our species, interest me because I tend toward social constructionism in my perspective on human development. That is, I expect (or predict) that the behavioral tendencies of our children are largely molded by cultural forces, including the actions of parents and other caretakers.
Of course, the old nature vs. nurture debate is by now dead, and deservedly so.
Across the disciplines of anthropology, biology and psychology, scientists accept that development proceeds according to multiple, intertwined biological and environmental influences.
But we don't always agree on the nature of that interlacing.
So, the publication of Lonsdorf et al.'s paper seemed to me an opportunity to reflect critically — not only on their perspective but also on my own. This process was enhanced by corresponding, earlier this week, with Elizabeth Lonsdorf, whom I wish to thank here.
BJK: How would you answer the suggestion that 30 months is too late a developmental stage to point to a primary influence of biology in the existing sex differences? Couldn't the moms have differentially influenced their sons' and daughters' social tendencies even during the first 2 1/2 years?
EVL: Yes, as you well know it is difficult to disentangle the chicken or the egg when it comes to mother-infant interactions. That is why we specifically focused on the 30-36 month age group, when chimpanzee infants begin to spend the majority of their time out of arm's reach, so are theoretically more free to choose whom to interact with.
Of course, it would be ideal to do neonatal testing as is done in human infants to get more direct measurements of biological bases, but that is impossible given the wild setting. We did not find differences in mothers' group sizes during this time, but as you say, there could be differences beforehand, and we are actively investigating this at the moment.
BJK: Is your argument, then, that these results among wild chimpanzees point to a certain fixed pattern of sex differences in humans, or instead that sex differences in human populations will be facultative or malleable according to population-specific patterns that result in heightened reproductive success?
EVL: I would say that, yes, I would expect social constructs and adult gender roles of a given society to interact and perhaps modulate such differences. However, given the basic constraints of primate male reproductive success (access to females), I would still expect differences in some areas irrespective of the social environment.
It's important to note, then, that Lonsdorf and her team are not arguing that human kids' sex differences should exactly mirror the patterns they found among chimpanzees. The researchers do, though, use their data, along with sex-specific data from some other primate species, to assert a role for noncultural mechanisms in human development:
While gender socialization in humans may play a role in magnifying the differences between young males and females, these behavioral sex differences are fundamentally rooted in our biological and evolutionary heritage.
After thinking about all this some more, I emailed Lonsdorf again and asked if she would, based on the nonhuman primate data, predict that boys (human boys) would be more gregarious cross-culturally.
Yes, I would, and in fact, it has been found in some studies that boys tend to have larger play groups, while girls tend to play more in pairs.
For me the developmental question remains one of emphasis. Lonsdorf et al. recognize the influence of caretaker behavior on children's sex differences, but give it a secondary sort of role. By contrast, Lise Eliot, a neuroscientist whose book Pink Brain, Blue Brain I assign when I teach my course on evolution and gender, flips that emphasis: She weights caretaker behavior far more heavily. This paragraph from Eliot's website conveys her book's thesis in a nutshell:
Eliot argues that infant brains are so malleable that small differences at birth become amplified over time, as parents, teachers, peers — and the culture at large — unwittingly reinforce gender stereotypes. Children themselves exacerbate the differences by playing to their modest strengths. They constantly exercise those "ball-throwing" or "doll-cuddling" circuits, rarely straying from their comfort zones.
Even as the nature-nurture debate itself fades, important questions remain about the comparative contribution of parental choices and evolutionary conserved characters in our kids' sex-difference behaviors.
Barbara's most recent book on animals will be released in paperback in April. You can keep up with what she is thinking on Twitter: @bjkingape