The more the question of whether or not or to what extent behavioral differences between the sexes are biologically determined has been investigated, the greater a consensus has grown that there is indeed a relatively intractable biological basis for this sexual dymorphism. According to Larry Cahill, the University of California's professor of Neurobiology and Behavior, "These variations [differences in brain structure, function and chemistry] occur throughout the brain, in regions involved in language, memory, emotion, vision, hearing and navigation"(Cahill, quoted in Sex Differences Extend Into The Brain).
The differences are so profound that they extend far beyond hackneyed (though certainly important) questions which tend to pervade popular discourse, such as whether or not women are biologically hardwired to be more maternal and empathetic, whereas men tend to be more aggressive and prone to violent crime, into questions of sex-specific treatments for conditions such as "Alzheimer's disease, schizophrenia, depression, addiction and post-traumatic stress disorder," according to Cahill, who says particularly of Alzheimer's disease,
"There are growing indications that the disease pathology, and the relationship between pathology and behavioral disturbance differ significantly between the sexes...Let us consider Alzheimer's disease-related pathology. Alzheimer's disease-related neurofibrillary pathology associated with abnormally phosphorylated tau protein differs in the hypothalamus of men and women: up to 90 percent of older men show this pathology, whereas it is found in only 8-10 percent of age-matched women"(Cahill, quoted in Sex Differences Extend Into The Brain)
The same is true of schizophrenia. Cahill says, for example, that "men with schizophrenia show significantly larger ventricles than do healthy men, whereas no such enlargement is seen in women with schizophrenia." As Margaret M. McCarthy, Arthur P. Arnold, and Geert J. De Vries point out, gender is a huge predictor of the frequency and severity of neurological disorders such as ADHD, autism, schizophrenia, depression, eating disorders, dyslexia, depression, anxiety disorders and stuttering (2012).
One of the most important determinants of behavioral differences between the sexes seems to be the role of early exposure of the brain to testosterone. One study involved examining the effects of controlled testosterone exposure on the brains of mice. "...as mammals develop in the womb, testosterone and related hormones trigger cell death in some regions of the male brain and foster cell development in other regions. In this way, the hormone sculpts the male brain and how it will differ from the female version...Remove or add testosterone to mice shortly after birth, and their brains develop according to the presence of the hormone, regardless of their sex"(Onion, 2005). Moir and Jessel emphasize the importance of the disparity:
The hormones, as we will see, determine the distinct male or female organisation of the brain as it develops in the womb. We share the same sexual identity for only the first few weeks after conception. Thereafter, in the womb, the very structure and pattern of the brain begins to take specifically male or female form. Throughout infant, teenage, and adult life, the way the brain was forged will have, in subtle interplay with the hormones, a fundamental effect on the attitudes, behaviour, and intellectual and emotional functioning of the individual. Most neuroscientists and researchers into the mysteries of the brain are now prepared, like the American neurologist Dr Richard Restak, to make the confident assertion "it seems unrealistic to deny any longer the existence of male and female brain differences. Just as there are physical dissimilarities between male and females . . . there are equally dramatic differences in brain functioning". The way our brains are made effects how how we think, learn, see, feel, smell, communicate, love, make love, fight, succeed, or fail. Undesrtanding how our brains, and those of others, are made is a matter of no little importance.
About the same time, if the baby is female, genetically XX, the reproductive machinery develops along female lines, produces no significant amount of male hormone, and results in a girl baby. Just as the six-week-old foetus wasn't recognisably male or female in appearance, so the embryonic brain takes some time before it begins to acquire a specific sexual identity. If the embryo is genetically female, nothing very drastic happens to the basic pattern of the brain. In broad terms, the natural template of the brain seems to be female. In normal girls it will develop natually along female lines.
In boys it is different. Just as male gender depended on the presence of male hormone, so a radical intervention is needed to change that naturally female brain structure into a male pattern. This literally mind-altering process is the result of the same process that determined those other physical changes - the intervention of the hormones.
Embryonic boy babies are exposed to a collosal dose of male hormone at the critical time when their brains are beginning to take shape. The male hormone levels then are four times the level experienced throughout infancy and boyhood. A vast surge of male hormone occurs at each end of male development: at adolescence, when his sexuality comes on stream, and six weeks after conception, at the moment his brain is beginning to take shape. But, as with the development of the rest of the body, things can go wrong. A male foetus may have enough male hormones to trigger the development of male sex organs, but these may not be able to produce the additional male hormones to push the brain into the male pattern. His brain will "stay" female, so he will be born with a female brain in a male body. In the same way, a female baby may be exposed in the womb to an accidental dose of the male hormone - we'll see later how this can happen - and end up with a male brain in a female body (Moir & Jessel, 1992).
Let us look at some other studies on the topic:
Typically, XX individuals will possess female genitalia and XY individuals will possess male genitalia. This occurs around 3 months after the fetus begins to develop. Before this, embryos possess female genitalia. Whether or not, to what extent, or in what way or ways, hormones affect behavioral differences among the sexes, however, is hotly disputed.
The relation of hormones to genes clearly plays a significant role in this behavioral distinction. For example, those with androgen insensitivity syndrome appear to be female, but are genetically male. Likewise, individuals with congenital adrenal hyperplasia are genetically female, but may possess varying degrees of male genitalia. Both are oftentimes raised as females.
Females with CAH tend to be interested in stereotypically "male" activities, as well as tending to choose males as often as females for playmates, as opposed to biologically ordinary females, who choose females 80 to 90 percent of the time. This remains the case even when these girls are encouraged to exhibit more stereotypically feminine behavior. Genetically ordinary females also tend to exhibit more stereotypically masculine preferences if they are exposed to only slightly higher levels of testosterone. The same thing has been experimentally confirmed in female monkeys exposed to higher degrees of testosterone.
Eg Reiner and Gearhart (2004) studied 16 genetic males with almost no penis. Two were raised as males and the other 14 were castrated shortly after birth and raised as females. 8 of the 14 who had been raised as females re-assigned as males by age 16, whereas both of those raised as males remained males. Of the 8 who decided to change their gender identity to masculine, 6 of these went gender re-assignment surgery to acquire masculine traits. This provides strong evidence that there is a strong biological component to which gender norms we adapt to.
In one unfortunate instance, twin boys Bruce and Brian were born to a Janet Reimer in 1965. Both boys were circumcised, but Bruce's penis was destroyed. The Reimers were influenced by an important Johns Hopkins University psychologist who argued that gender was entirely socially constructed. They made an appointment with the psychologist and decided to give him gender re-assignment surgery, castrating him at 21 months. They renamed him Brenda and decided to raise him as a girl.
While they tried to raise Brenda as a girl, she repeatedly rejected this, mimicking her father's behaviors, such as shaving, and rejecting her mother's behaviors, such as applying makeup. Brenda complained that she felt like a boy as she grew up. She fantasized about being a big, tough guy with muscles and a nice car in middle school, and insisted on urinating while standing up. The psychologist the parents had previously consulted with advised the construction of a vagina but Brenda threatened suicide if this was performed. It was at this point, at 14 years of age, that Brenda's parents revealed the truth of her childhood. Brenda therefore cut her hair and renamed himself "David." David removed the breasts that had developed as a result of estrogen treatment and received testosterone injections to re-masculinize her features. David also had numerous surgeries in an attempt to reconstruct a penis. David lived out the rest of his life as a heterosexual male, but tragically committed suicide in 2004.
Unfortunately, most of the data we have to go on is in animal studies. Male animals typically behave in a masculine manner. Female animals typically exhibit stereotypically feminine personality traits. Castrated females exhibit stereotypically feminine traits. Castrated males typically exhibit stereotypically feminine traits. Females treated with testosterone typically exhibit stereotypically masculine traits. Castrated males treated with testosterone likewise exhibit masculine personality traits. These treatments are imposed 7 days within the birth of the animal in question.
A similar difference was found in comparing the personality differences between females with congenital adrenal hyperplasia with control females (Berenbaum & Resnick, 1997):
Males are more likely than females to show aggressive behavior across species, ages, and situations, and these differences may be partly influenced by early hormones. We studied aggression in three samples of subjects with congenital adrenal hyperplasia (CAH), who were exposed to high levels of androgen in the prenatal and early postnatal periods. Controls were siblings and first cousins similar in age. In Sample 1, adolescents and adults completed the Multidimensional Personality Questionnaire (MPQ), which includes an Aggression scale. In Sample 2, adolescents and adults completed the MPQ and a paper-and-pencil version of Reinisch's Aggression Inventory. In Sample 3, parents rated the aggression of children aged 3–12, using a modification of Reinisch's Inventory. In all three samples, control males had higher aggression scores than control females. Further, as predicted, females with CAH had higher aggression than control females, but the difference was significant only in adolescents and adults. These results suggest that early androgens contribute to variability in human aggression.
Onion, Amanda (2005). Scientists Find Sex Differences in Brain. Retrieved from: http://abcnews.go.com/Technology/Health/story?id=424260&page=1
no author. (2008). Sex Differences Extend Into The Brain. Retrieved from: http://www.sciencedaily.com/releases/2008/02/080229171609.htm
Moire, Anne & Jessel, David (1992). BRAIN SEX: The real difference between men and women. Dell Publishing (paperback), New York, 1992. Retrieved from: http://theabsolute.net/misogyny/brainsx.html
Margaret M. McCarthy, Arthur P. Arnold, [...], and Geert J. De Vries. Sex Differences in the Brain: The Not So Inconvenient Truth. J Neurosci. Feb. 15, 2012; 32(7): 2241-2247. Retrieved from: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3295598/