Why Do Males and Females Exhibit These Differences?

Hormonal Differences
In prenatal development steroid hormones, particularly testosterone, are believed to act to organize the brain in either the male or female circuitry. The male develops in the presence of high testosterone, which not only develops the proper sexual organs but also organizes the brain. The female state is the default state, developing if there is no testosterone present. Interestingly, in the rat, where a lot of the brain studies were done, the masculinization of the brain is also dependent on oestrogen, a female hormone, which is converted from testosterone in the male brain. This conversion causes a high dose of oestrogen to be present in the male brain. Once the wiring is all set, the brain undergoes activation by an increased dose of testosterone during puberty. In humans, the hormonal exposure during puberty may be more important, which leaves room for outside interference in this critical period.

Testosterone and estrogen, being steroid hormones, cause cell differentiation via control of protein synthesis, cell division and migration, neuronal growth and axonal branching and synaptic remodeling. In the brain these hormones may control brain differentiation by selective cell death.

Hormones have been recently linked to brain disease. Many brain diseases are particularly associated with specific sexes and scientists discovered that hormone imbalances may lead to imbalances in the level of neurotransmitters. For example, oestrogen has been shown to protect cells against changes that have been known to cause Alzheimer's disease, while low oestrogen has been associated with an increased risk of schizophrenia. Differential brain disease in men and women may have an effect on their social interactions and behavior.

Environmental factors and social stimuli
Environmental factors and social stimuli have been known to effect hormonal levels in people and thus may effect the development of the brain. Hormonal drugs taken for other disorders may have this effect as well.

Evolutionary differences
Each sex probably had a very defined role in ancient societies: women remained with the offspring while men went out to hunt and provide procure food. This separation of labor may have caused the brain to develop differently in men and women. The parts that women used to perform their responsibilities increased in size while the parts that the men used for their activities (such as hunting) became larger compared to the female counterparts. Through evolution, this difference in development caused men and women to be better able to perform their respective tasks. According to Halloway, this model assumes "greater degrees of, and selection pressures for, sexual division of labor"(1990). Possibly, females were more skilled in social relationships and communication while men were more skilled in visuospatial tasks, as required by their respective activities. The difference in brain structure thus developed, driven by positive and negative feedback mechanisms.

Research Significance
Men and women have, surprisingly, enormous differences between their brain organization. Within each gender, there is even more variation from individual to individual. Most of the conclusions drawn about hominid brain evolution - especially on very ancestral species - were based on only a handful of skulls from each species. Hopefully, this section shows the errors that can result from that reasoning. Each individual will have variations from the norm, so even if Broca's area was found to be very underdeveloped in one specimen, that finding cannot rule out the possiblity that other individuals of that species possessed a more developed area. Until the paleoneurology community can acquire many more specimens, many conclusions about brain evolution may very well come under attack with new finds over the next couple of decades.