Many people assume that brain differences between men and women result from sex hormones like estrogen and testosterone. This is not true. In fact, increasing numbers of studies find large reliable distinctions between the sexes. These effects are often seen early in development and before sex hormones are expressed. The research examining sex-specific brain mechanisms finds variability in structure, neurotransmitters, and the expression of specific genes in males and females.
Sex differences in size, shape, and function occur in every region of the brain. For example, sex divergence is seen in the hippocampus, an area of the brain that is important for learning and memory. The hippocampus makes up a larger portion of total brain size in women than in men. But that doesn’t necessarily mean that women learn better than men. Interestingly, some sub-regions of the hippocampus are larger and contain more cells in men. Further studies will be needed to fully understand the implications of such sex differences.
Additional variation occurs in the levels of neurotransmitters, the chemical messengers, in the brain. Males and females differentially produce serotonin, a neurotransmitter that affects mood. These differences may have significant effects on gender-specific treatment of diseases, such as depression.
Gene expression differences between the sexes are also seen across many species. Scientists have found groups of genes with sex-specific expression that are conserved, or maintained, across primate species, suggesting that such patterns are inherited across evolution.
What is the evolutionary advantage to having sex differences in the brain? While we can only guess, one hypothesis is that such variability is important in the selection animals make in finding mates and raising offspring. Whatever the cause, differences in the brain affect development and health during adulthood. Many of the genes with differential expression are also associated with neurological diseases such as Alzheimer’s disease, schizophrenia, and cerebral ischemia. As such, sex-specific treatments of these diseases should also be studied.
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