N.B. This science review was originally published in Optimyz Magazine in February 2012 by Mandy Wintink, PhD.
My academic grandfather (i.e., my PhD supervisor’s supervisor), Dr. John Pinel from UBC, was the first person I heard argue that males and females are better considered along a continuum of maleness-femaleness rather than as a dichotomy. Considering sex along a continuum may, indeed, may be difficult, but there is substantial biological reason to suggest we are better off doing so if we are interested in more accurately understanding male-femalesness.
First, consider the genetics of sex. We categorize sex according to the chromosomes X and Y. Males have an X and a Y and females have two Xs as their 46th chromosome. Mothers pass on their X chromosomes to offspring, because they only have Xs, whereas fathers pass on both their X and Y. When the combine during fertilization, they give rise to either a male (XY) or female (XX). During normal development subsequent sex-specific hormones - testosterone and estrogen - are produced and circulate through the body to give rise to what we typically think of as males and females, respectively.
At face value, this is simple to understand, until, of course, we consider that there are conditions under which there is a discrepancy among the genetic sex, the individuals response to sex hormones, and physical sex-specific characteristics like genitals and body type - medical conditions called “intersex”.
Here’s one example of intersex. A person has XX chromosomes and ovaries, but developed male external genitals. This results when a female fetus receives too much exposure to the masculinizing sex-hormone testosterone during critical periods in the womb. The labia (lips of the female external genitalia) fuse forming a scrotum and the clitoris enlarges to form a penis. Both of these tissue can develop in either direction, normally, and do so in response to circulating sex hormones (the default is female, btw). The most common cause of this is Congenital Adrenal Hyperplasia, a condition in which the individual lacks the a specific enzyme and effectively increases androgen (a hormone group that includes testosterone).
A similar condition, Aromatase Deficiency, happens at puberty. Aromatase is an enzyme that converts androgens to estrogens. This conditions goes unnoticed until XX girls hit puberty and respond to the excess of testosterone by beginning to develop into teenage boys!
Similarly, there are individuals who look like a woman but are genetically XY male. We tend to hear about these women around the time of olympics when questions concerning their exceptional performance evolve. In this condition, the individual is biologically unable to respond to androgens and therefore develop as females (the default remember). This condition, Androgen Insensitivity Syndrome, can go unnoticed by women until they attempt to have children and realize they have male, not female, internal organs.
These examples beg the question, what constitutes men and women? Is it genetics? Physical characteristics? Gender identification? And I didn’t even get to touch on individuals who are XYY or XO genetically!
I find this topic really interesting to bring up for several reasons, particularly as a reminder that categories, although helpful in many circumstances, limit our understanding. I also hope, in highlighting this science, to promote tolerance for those who don’t fit into such categories, which can include varying sexual orientations. On that note, I will leave you with this final science tid-bit: There are several brain areas that specifically correspond to being male or female and develop later in prenatal development compared to genitals, suggesting a potential a mechanism by which gender and physical characteristics can be misaligned, like in transsexual individuals who undergo sex-reassignment surgery. There is significant evidence that these areas correspond to the sex felt, not born as (see research by scientist Dr. DF Swaab for many studies in this area).
In 2009 I reviewed how hormones work and discussed the effects of pseudo-estrogens (like Bisphenol A) on reproduction. I have reposted it on my A Science Perspective blog for those interested.