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. Undertanding 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).
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/