Finally, the masculinizing of the gonads (making testes, penis, and prostate) occurs before the masculinizing of the brain. Owing to variability in the pathways controlled by genes and the interactions among the items in the suite of neurochemicals, sometimes the masculinizing of the brain does not follow the typical path and may be incomplete in various ways. You could have male genitalia and a female brain.
Rats have been the model animals used in figuring out the fundamentals of the gonad-brain story. But you are not just a big rat. So, is the human hypothalamus like the rat hypothalamus? Pretty much. The basic anatomical differences in hypothalamic structures of male and female lab rats are also seen in humans. Nevertheless, human sexual and social behavior is vastly more complex. Your very large brain, especially your huge prefrontal cortex, means that your flexibility in navigating your social world and your capacity to control sexual behavior is far more rich and varied and involves much more learning than in the case of a rat. In large-brained mammals, the interaction between gene expression and learning-based changes in the brain becomes a dense thicket of complexity.
This chapter is focused on aggression, and given the statistics on male aggression, that means we need to consider what makes a male brain male. But that is not enough. To understand the masculinization of the fetal brain at a deeper level, it is useful to see that nature does not always follow the beaten path. Reflecting on these other paths allows us a broader understanding of how hormones can affect our very nature. Then we shall return to the matter of male aggressive behavior.