Humans appear to have a reasonable amount of diversity in their sexual orientations, in what is often referred to as “gender” and in adult behavior generally. When convenient, people will point to “genes” as the “cause” of any particular subset of th is diversity (or all of it). When convenient, people will point to “culture” as the “cause” of … whatever. The “real” story is more complicated, less clear, and very interesting. And, starting now, I promise to stop using so many “scare” quotes.
Prior to birth there are a number of factors than can influence things like gender or sexuality in a human. You have probably heard of the finger-index (not the index-finger … actually called the 2D:4D ratio). The ratio of length of two of your fingers seems to be associated with certain trends; Men with a certain ratio tend to be more athletic or more gay, for instance. The mechanism for the finger ratio variation is probably straight forward but has not been nailed down yet. Likely, a surge of steroid hormones enhances growth rate of whatever bones are forming at that time (I simplify somewhat) and if such a surge occurs at a certain time, a slight shift in bone length ratio affecting fingers occurs and because of the timing, a slight change in something else also occurs, something having to do with what will eventually be adult behavior.1 I am not arguing here for the strength of this association or its meaning, but available evidence shows that there is something going on. To the extent that this particular relationship is true, we see an adult outcome (related to gender, sexuality, or other behavior) being the result of something that is biological and prenatal, but not likely genetic. While the overall pattern of the hormonal environment of a fetus may be broadly determined by genes, variations in the details are just as likely determined by other things. In many contexts, one steroid hormone looks a lot like another, or can convert into another as they float around in the blood supply, so any large surge of steroids could act like sex hormones or growth hormones even if they are merely stress hormones, and there is an exchange of hormones between the mother’s blood supply and that of the fetus. Since the mother’s hormonal environment is heavily influenced by her environment (especially stress hormones), the ultimate cause of steroid hormone-mediated developmental variations in a human is very likely to be strongly environmental, if not entirely environmental, even though it all happens before birth.
Then there is the stuff that happens after birth. One study carried out in Australia seems to show that adults in a specific culture (Australian middle class) treated infants very differently depending on their knowledge of the infant’s sex.2 For instance, a boy would be moved around more, tossed about a bit, handed boy-specific toys, and so on, while a girl would be held more calmly, not tossed about, hugged more, and handed girl-specific toys. In that study, the “sex” of the infant (boy vs. girl) was “known” to the adult on the basis of obvious clothing choices and pronoun use, and in fact, the infant was always a boy. After months of treatment as one sex or the other, depending on what that treatment consisted of, one could potentially get a gendered difference. Movement, touch, voice, etc. all form part of the environment in which the infant’s neural system, including the infant’s brain, develops. This would make a difference.
And so it goes throughout development; At numerous stages along the way, a human is affected by hormones, bathed in gendered behavior, and eventually, starts to observe her or his own environment and act accordingly. One study seemed to show that at about Kindergarten age, boys were more conscious of how they would fit into a group than girls, paying special attention to what other boys were doing before making certain choices. If this was a general pattern in a particular group of people, one might see girls engage in a wider range of available stereotypes while boys restricted themselves to a narrower range.
While it is possible that there is some hidden Jungian subconscious difference between nominal boys and girls resulting in different thematic tropes in their behavior (i.e., girls like circles and boys like lines or some such thing), the degree to which kids past a certain age … say six or so … gravitate towards gender specific toys or other objects, or engage in gender specific behaviors, is way too finely tuned to be the product of anything other than high cognitive function. While we know that across cultures, different colors are associated with different genders, within a culture most boys and girls know what the boy vs. girl colors are and to varying degrees express this knowledge as strong preferences, perhaps with boys expressing a narrower range of preferences than girls. Most likely, culturally specific gender preferences for things like toys and clothing are learned early, become deeply ingrained, are unlikely to be genetically determined at any level of detail (if at all) but may be attended to by boys more than girls.
There are many factors that would determine a person’s gender over a lifetime. The above mentioned inter-uterine hormonal conditioning is probably fairly complex, with multiple moments in time when one or another thing might happen, and where one version of the developmental scenario would lead towards one gender orientation than another. After birth, more of the same but less hormonal and more cultural, and later on, with puberty, the hormones kicking in again, but with a twist: Early conditioning may determine the nature of later hormonal activity by setting up differences in receptor sites or sensitivity, or other aspects of hormone feedback systems.
In speaking of humans it is easy to assume that other animals, who lack the complex and often costly (and therefore presumably ‘important’ in some way) trappings of prolonged development and culture have simpler systems for determining gender. For the most part, I would argue that rodents do in fact have simpler systems of gender than do humans, with the caveat that I’ve just compared an entire order of mammals (and a rather speciose and diverse one at that) with a single species. But what would you make of a gender-shaping system in rodents that was actually very complex, in which ‘culture’ was the main determinant of adult male-ness?
In rats, males get to be males in large part because they have testes that secrete testosterone, which in turn causes other changes. But according to at least one study, the degree to which testes will secrete testosterone is determined by anogentital licking behavior of the mother. This behavior is, in turn, brought on by some sort of cue produced by the newborn male. Without this licking, the testes do not produce much testosterone and andorgenization of the rat does not take place.3
OK, so I was exaggerating slightly when I said that rat “culture” determines adult gender, but prior to hearing this you probably assumed that there was a gene or set of genes that simply coded for which sex the rat would be when it grew up. And yes, you can get some interesting results when the mother rat is replaced with a lab tech and various different variations of the licking thing are tried out. (Using tiny wet paintbrushes.)
And I could go on. But I want to make two points about development and behavior, especially gender. One is that whatever genetic component is working, most aspects of adult behavior and orientation are shaped by non-genetic factors and those genetic factors that may exist come in the form of basic species-specific (but almost certainly gender-differentiated) “drives.” I’ve discussed the importance of drives here, and if you want to read a whole book about the link between drives and everything you do in your life check out Mean Genes: From Sex to Money to Food: Taming Our Primal InstinctsSelf-Help Books)
The second point is that as something complex (and both personal and social) as gender orientation emerges in a person it must be true that it comes to whatever point it comes to after a series of many turning points. If every single factor is thought of as a simple binary choice (and I use the word “choice” with no reference to human decision making) between two cannalized options, then the number of possible outcomes could be thought of as 2n where ‘n’ is the number of times a binary choice is encountered. So, if there are, say, three hormonal moments in utero, and one more after birth (puberty) and, say, three life stages that have major influences on gender (and I oversimplify) then the number of possible routes a person may take from conception to adulthood would be 27. That is 128. If these different paths lead to mostly different outcomes, wouldn’t there be over 100 “genders” among humans?
The interesting thing about this is that a cursory examination of potential human gender diversity from a purely biological point of view suggests that there are at least dozens of “genders” but the vast majority of cultures define (or even allow) only a few. Perhaps culture, in this case, is more restrictive than biology. Which, to a behavioral biologist, is not much of a shock, though it might be if considered from a broader social science perspective.
So, the next time you are in charge of making a form to collect personal information from people, when you are designing the “gender” question, you might consider something other than a couple of checkboxes. Perhaps a drop-down list. Or, best of all, just have people write a short essay. Make ’em think, that will.
1Be careful with this idea: While I’m sure there are several aspects of 2D:4D research that are valid and interesting, it is often somewhat over-reported. Also, the numbers are tricky. The measurement is often done on fleshed and living fingers, but should really be done on the bones directly (using X-ray technology, not sacrificing the subject and defleshing them!). And the meaning of this trait is somewhat open to interpretation. I’d be comfortable sorting out males from females in a skeletal population with good preservation of hands but no pelvic remains, but more reluctant to use this for sorting out ethnic groups, gender orientations, or assertiveness levels. For a recent review see Bailey and Hurd, 2005. Finger length ratio (2D:4D) correlates with physical aggression in men but not in women. Biological Psychology. Volume 68, Issue 3, March 2005, Pages 215-222.)
2The specific research to which I refer was shown on a documentary about sex differences; For an exemplar published study on this work see Frisch 1977. Sex Stereotypes and Adult-Infant Play. Society for Research in Child Development. Vol. 48, No. 4 (Dec., 1977), pp. 1671-1675
3See this study and references therein: Moore and Morelli, 1979. Mother rats interact differently with male amd female offspring. Journal of Comparative and Physiological Psychology, Vol 93(4), Aug 1979, 677-684. doi: 10.1037/h0077599.
Moore, C., & Morelli, G. (1979). Mother rats interact differently with male amd female offspring. Journal of Comparative and Physiological Psychology, 93 (4), 677-684 DOI: 10.1037/h0077599