The brain stem, in every species in which it appears, is focused on the management of energy to the purposes of personal survival. It sees other creatures as resources to be exploited for the continuance of the individual and the species.
The brain stem rides atop the spinal cord. It has three distinct components, rising sequentially into the limbic system: the midbrain, the pons, and the medulla oblongata.
The brain stem also regulates most of our autonomic functions: heartbeat, breathing, digestion, etc. Biologically, replacing the brain stem is a prohibitively large evolutionary leap.
In the human species, I believe that masculine personalities have the stronger cerebral integration with the brain stem. They think and act independently, and are significantly more aggressive than their feminine counterparts. They are more likely to use force than persuasion or suggestion. They are also less likely to feel the weight of social opprobrium, and so are more likely to pursue extraordinary aspiration and achievement — whether constructive or destructive in its consequence.
Obviously, men are not uniquely entitled to its capacities and defects. If we consider that the patterning of the brain stem makes it more susceptible to entanglement with the reptilian mind, we gain insight into the ancient epithet: "dragon lady".