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Does Nature Need to be Nurtured?

The views expressed are those of the author and are not necessarily those of Scientific American.


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Some say that the differences between boys and girls are just aping nature, but studies of primates tell a more complex story

"Boys will by boys" by Nathaniel Gold

"Boys will by boys" by Nathaniel Gold

“Boys will be boys” is a popular refrain in schools. A bit of rough and tumble at break time? That’s natural. Likewise, “girls will be girls” is accepted without question. Some feel justified in assuming that female students will choose to compete only in small peer groups and will do so by criticizing or excluding one another. Again, this is deemed “natural.”

These preconceptions about the behavioral tendencies of the sexes tend – consciously or unconsciously – to influence how teachers react to students’ actions and thus how behavior management in schools is conducted. But would we respond differently if it could be proved that the behaviors we labelled as natural were nothing of the sort? Could it be that we are encouraging behaviors that are nurtured rather than innate?

Classroom dynamics are complicated and teachers are rightly conflicted about how to proceed when certain gender norms seem pervasive. How do we reconcile nature and nurture when conducting our lessons?

I am the father of a five-year-old boy and have had experience in elementary school classrooms, teaching students aged 4 to 12, from kindergarten through to the 6th grade. You would think that this would provide a lot of answers. But most of what I have learned about childhood behavior comes from the study of primates.

During my PhD in evolutionary anthropology, my research looked at the common features we share with monkeys and apes. Because our species relies so heavily on socialization, it can be difficult to untangle the nature-nurture issue. However, it turns out that we can answer questions about human nature by looking to our closest evolutionary relatives.

The first fact to make clear is that all social mammals play. Chasing, climbing and play fighting are ways for young animals to gain the behavioral and social skills they will need as adults. But to understand why we evolved to “monkey around,” we are best off looking at primates.

Like us, monkeys and apes have a long period of childhood dependency and rely on nuanced social cues to navigate their world as adults. By investigating how primates use social play we can better understand the same behavior in ourselves.

In February this year, the internationally renowned primatologist Jane Goodall joined a team of scientists for a study of play behavior among chimpanzees at the Gombe National Park in Tanzania. Led by Elizabeth Lonsdorf of Franklin and Marshall College, the researchers analyzed the behavior of 20 male and female chimps aged 30-36 months, a period when they begin playing out of arm’s reach of their mothers. By measuring how many individuals the young chimpanzees came into contact with, how they played and who they played with, the researchers were able to find patterns among the sexes.

It turns out that male chimpanzees have a much larger social network than females – nearly twice as large – and are much more likely to engage in rough-and-tumble play, a pattern the researchers note has also been documented in multiple monkey species. The preconceptions of teachers, it would seem, are right.

In their report, the researchers conclude: “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.”

In other words, boys and girls play differently today because gendered behavior allowed more of our ancestors to survive and reproduce.

However, this is only half the story. Humans and chimpanzees are sister species, but we have a rarely acknowledged third sibling known as the bonobo. All three shared a common ancestor approximately 6 million years ago. Most studies looking to understand the evolution of human behavior use chimpanzees as a model to explain who we were and how we came to be the way we are today. But if we want to understand whether certain behaviors are natural for humans, we need to compare ourselves to both our close relatives.

It turns out that when we include bonobos along with chimpanzees, we get a very different picture about the potential range of natural behavior. In 2006 at the University of Pisa in Italy, Elisabetta Palagi conducted a study comparing the social play between bonobos and chimps that turned the gender question on its head. What she found was that bonobos engaged in significantly more rough-and-tumble play than chimpanzees and that young females were the roughhousing champions, displaying the behavior more than twice as often as males. So are females just as naturally wired for this form of play?

The answer may have less to do with gender itself and more to do with the mechanisms of gendered behavior. The hormone testosterone has a catalyzing effect on behavior, analogous to the way that yeast is required for bread to rise.

Experiments with monkeys have shown that injecting testosterone into the uterine environment during the late stages of pregnancy causes females to engage in rough play at nearly the same rate as males. During this period of fetal development, testosterone triggers a cascade of brain organization that permanently alters the range of behaviors that an individual will engage in. Too much testosterone will result in the masculinization of female sexual anatomy, but the right amount at this key stage is required for certain gendered behaviors to emerge in the first place. Eliminate testosterone entirely and rough play won’t arise in either sex.

It is worth noting that it is known that stress causes an increase in circulating testosterone levels for pregnant mothers in all primate species.

Last year the Harvard evolutionary biologist Victoria Wobber revealed that testosterone production in infants was the key to explaining the difference between certain chimpanzee and bonobo behaviors. She found that bonobos – both male and female – had significantly higher testosterone levels than chimpanzees throughout their childhoods. Chimp males will experience a spike in testosterone at puberty (just like human males) that more than doubles the amount of this circulating hormone. But bonobos have significantly higher testosterone levels until then, which seemingly explains why they are more inclined to roughhousing.

Wobber et al. (2013)

Testosterone levels in chimpanzees and bonobos at different developmental stages. Figure from Wobber et al. (2013)

This all suggests that so-called natural behavior for boys and girls is highly flexible, depending on their levels of testosterone.

Interestingly, however, like Palagi, Wobber also notes that female bonobos will consistently outrun, outclimb and outcompete their male counterparts. Yet both male and female bonobos have about the same testosterone levels throughout infancy and adulthood, revealing that this hormone promotes but is not the exclusive cause of female dominance in this species.

So the study of primates does not give us an unequivocal answer about the root cause of certain behaviors, but it can at least teach us that how much, or how little, a child chooses to engage in certain behaviors may be determined less by sex than the biological environment they are exposed to.

It should also remind us that myriad influences shape a child’s personality, so we shouldn’t assume that gender is a fixed variable when considering our strategies for managing behavior. Teachers need to be careful not to reinforce ideas about actions deemed natural to a certain sex. Nature is, of course, an important component of who we are as a species, but the right environment is just as crucial.

Cross-posted at Times Education Supplement

References

Lonsdorf, E V, Anderson, K E, Stanton, M A et al (2014) “Boys will be boys: sex differences in wild infant chimpanzee social interactions,” Animal Behaviour, 88: 79-83.

Palagi, E (2006) “Social play in bonobos (Pan paniscus) and chimpanzees (Pan troglodytes): implications for natural social systems and interindividual relationships,” American Journal of Physical Anthropology, 129: 418-426.

Wallen, K (2005) “Hormonal influences on sexually differentiated behavior in nonhuman primates,” Frontiers in Neuroendocrinology, 26/1: 7-26.

Wobber, V, Hare, B, Lipson, S, Wrangham, R, Ellison, P (2013) “Different ontogenetic patterns of testosterone production reflect male reproductive strategies in chimpanzees and bonobos,” Physiology and Behavior, 116-117: 44-53.

Eric Michael Johnson About the Author: Eric Michael Johnson has a Master's degree in Evolutionary Anthropology focusing on great ape behavioral ecology. He is currently a doctoral student in the history of science at University of British Columbia looking at the interplay between evolutionary biology and politics.

Follow his work on Facebook and Google+. Follow on Twitter @primatediaries.

The views expressed are those of the author and are not necessarily those of Scientific American.





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  1. 1. rationalrevolution 1:00 pm 06/12/2014

    This article doesn’t seem to make any sense at all. The headline and initial thesis seems to be saying that sexual differentiation is a product “nurture”, but then the article goes on to show that this isn’t the case at all, merely that nature” is different in different species. Well duh.

    This always annoys me about people who talk about bonobos as if it means anything.

    The fact that different species have different biology, which can result in different sex roles, says nothing at all about “nurture”, or the role of the environment in determining behavior. All it shows is that the biological mechanisms that determine behavior are hard wired differently in different species.

    It does not all all show that behavior isn’t hard wired, just that different species have different wiring.

    And for that matter, its a matter of individual wiring. Clearly testosterone isn’t the single and only hormone that plays a major role in determining behavior, but using it as a simple example, the fact that on average human boys tend to behave a certain way and girls another doesn’t mean that all boys and girls will behave that way, but it also doesn’t mean that the behavior of girls and boys isn’t biologically determined.

    It’s just that some boys have relatively low testosterone (as a simple explanation) and some girls have relatively high testosterone.

    So yes, we can’t stereotype and say that all girls fit mold X and all boys mold Y, but the reason we can’t make this stereotype has nothing to do with “nurture” and everything to do with biological diversity, and the fact that not all individuals’ biology fits the normal pattern.

    It still means that behavioral tendencies are biologically determined, some people’s biology just happens to be abnormal. There is nothing WRONG with that abnormality and it isn’t something we should worry about or persecute people over or try to “correct”, but by the same token we shouldn’t expect the norm to change just because of the existence of outliers.

    The outliers are biological outliers, not proof that biology doesn’t matter.

    If anything this study actually just reinforces the role of biology in determining behavior. It shows that the behavioral differences between bonobos and chimps regarding sexual differentiation is a product of biology, not “bonobo society”.

    Or, as I like to tell people (especially feminists), all society is a product of biology. All that “society” can do is simply amplify or restrain existing biological tendencies, but society doesn’t invent behaviors out of thin air, it merely reinforces existing biological tendencies or restrains them.

    Link to this
  2. 2. Eric Michael Johnson 1:37 pm 06/12/2014

    My point was to show that gendered behavior has multiple influences, in utero testosterone levels being just one example. This variable biological environment can adjust genetic predispositions in novel ways. The comparison between the closely related species of bonobos and chimpanzees demonstrates that this one factor can result in significant behavioral differences. There is a tendency for people, particularly those who see genetics as determining complex behaviors, to misapply the average for a population onto individuals within that population. This is incorrect. For example, most men who rail against feminists on blog posts usually have some deep-seated insecurities that they haven’t dealt with. But it would be wrong to assume that for any one individual.

    Link to this
  3. 3. jtdwyer 6:05 pm 06/12/2014

    Isn’t it correct to say that the principle factor statistically determining the physical and behavioral differences between the genders is testosterone levels?
    IMO it’s not fair to children to attempt to minimize natural behavioral distinctions – for typical or atypical individuals.
    I wonder, are (predominantly female) primary school teachers involved in identifying students with behavioral problems for treatment – predominantly in male children?

    Link to this
  4. 4. Pierre300 9:35 pm 06/12/2014

    “For example, most men who rail against feminists on blog posts usually have some deep-seated insecurities that they haven’t dealt with”

    Not to go off topic but I have to say scientific american is the last place I expected to find my burn of the week.

    Link to this
  5. 5. phalaris 4:58 am 06/13/2014

    The conclusion seems to be then, that behaviour is to some extent biologically determined, and hormones are a mechanism. Biologically determined would surely mean that there must be a genetic component in the pathway.

    As for the “men railing against feminists”, the study seems to suggest that feminists who argue dogmatically that behaviour is purely a cultural artefact, and society should be reconstructed around this fallacy deserve to be railed at.

    Link to this
  6. 6. OlgatheGreat1 10:25 am 06/13/2014

    So there’s an assumption that chimpanzees and bonobos have only nature, not “nurture”– that every behavior they display is caused by biology only.

    That seems… like it needs justifying. Because chimps definitely display differences from group to group.

    Link to this
  7. 7. Eric Michael Johnson 3:35 pm 06/13/2014

    This is an important point. It is true that there is a range of cultural practices that vary between groups in both chimpanzees and bonobos. I wrote about some of this research previously (see: http://j.mp/qm7Vpe). In the 2002 book “Behavioural Diversity in Chimpanzees and Bonobos,” edited by primatologists Christophe Boesch, Gottfried Hohmann, and Linda Marchant, they identify a range of behavioral traits in different populations that are each influenced by their unique environments. In the Preface, Boesch writes, “The picture emerging from all these studies was that each wild population presents many different behaviours, not only in the domain of tools and hunting, but in core, basic social interactions as well.” He ends his overview by encouraging more research into the behavioral diversity of different populations and to refrain from assuming universal “species-typical” behavior in either chimpanzees or bonobos. It goes without saying that the same applies for our own species as well.

    Link to this
  8. 8. jvkohl 11:39 am 06/14/2014

    Excerpt: “Last year the Harvard evolutionary biologist Victoria Wobber revealed that testosterone production in infants was the key to explaining the difference between certain chimpanzee and bonobo behaviors.”

    My comment: That suggests to me that the conserved molecular mechanisms of testosterone production and hormone-organized and hormone-activated behaviors are the key to explaining differences in primate behavior. Evolutionary theories that fail to incorporate what is known about how differences in testosterone arise seem more likely to attribute them to unknown natural mechanisms associated with mutations and natural selection instead of linking biological facts to differences in behavior.

    From Fertilization to Adult Sexual Behavior
    http://www.hawaii.edu/PCSS/biblio/articles/1961to1999/1996-from-fertilization.html
    Excerpt: “The general sense of the word “environment” as something exterior to the person is retained, even if that something influences intraperson processes. In addition, we focus directly on molecular events themselves. Here the “environment” involved can be that within a DNA segment. We also expand the notion of “biologically based sex differences.” Although many, and perhaps most, important sex differences arise from gonadal and hormonal development, also important are sex differences which are neither gonadal nor hormonal. All these factors affect the internal workings of the individual and intervene in structuring how the social environment might or might not modify sexual behavior. This discourse calls attention to features that are central to the so-called nature-nurture discussion.”

    Link to this
  9. 9. jvkohl 1:04 pm 06/14/2014

    The effects of perinatal testosterone exposure on the DNA methylome of the mouse brain are late-emerging http://www.bsd-journal.com/content/5/1/8

    This article attests to what we established in 1996 in our section on molecular epigenetics, which link sexual differentiation of cell types in yeasts to the differentiation of all cell types in individuals of all species via what we now know are conserved molecular mechanisms during life history transitions.

    The authors write: “…the emergence of sex differences in the brain may be a gradual process that is cemented over the organism’s life. Our data provide a new perspective by showing that most sex differences in CpG methylation are dynamic and not the result of acute modifications in response to hormones.”

    There’s a model for that: Nutrient-dependent/pheromone-controlled adaptive evolution: a model. http://www.ncbi.nlm.nih.gov/pubmed/24693353

    “Nutrients are metabolized to pheromones that condition behavior in the same way that food odors condition behavior associated with food preferences. The epigenetic effects of olfactory/pheromonal input calibrate and standardize molecular mechanisms for genetically predisposed receptor-mediated changes in intracellular signaling and stochastic gene expression in GnRH neurosecretory neurons of brain tissue. For example, glucose and pheromones alter the hypothalamic secretion of GnRH and LH. A form of GnRH associated with sexual orientation in yeasts links control of the feedback loops and developmental processes required for nutrient acquisition, movement, reproduction, and the diversification of species from microbes to man.”

    Link to this
  10. 10. hkraznodar 11:57 am 06/20/2014

    @rationalrevolution: Did we read the same article? I drew a completely different conclusion than you did and quite frankly, your rant seems to be more of a personal problem than a real discussion of science.

    @phalaris: I tend to rant at pretty much any extremist I encounter in person. That includes delusional males that try to use biology to justify bad behavior. I’m not including you in that category because you didn’t blanket slam feminists in general but only the extremist subset. Most feminists simply want to be able to feel good about themselves and pursue their own life goals. All real Americans must, by definition, support this. A surprisingly large number have life goals of being stay at home moms. Others don’t want children but choose career instead. The majority want a balance of career and family. Since the balance allows them a safety net if something bad happens to their spouse/partner, that is a good thing too.

    Link to this

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