It's probably even the case that if you stoked up some Buddhist monks with tons of testosterone, they'd become wildly competitive as to who can do the most acts of random kindness.

Until you appreciate something crucial - It is incredibly easy to manipulate us as to who counts as an Us, who as a Them.

The gigantic challenge is the magnitude of the individual differences in the optimal set point for "good stress." For one person, it's doing something risky with your bishop in a chess game; for someone else, it's becoming a mercenary in Yemen.

The problem isn't testosterone and aggression; it's how often we reward aggression. And we do: We give medals to masters of the "right" kinds of aggression. We preferentially mate with them. We select them as our leaders.

Stress is not a state of mind... it's measurable and dangerous, and humans can't seem to find their off-switch.

We all seek out stress. We hate the wrong kinds of stress but when it's the right kind, we love it - we pay good money to be stressed by a scary movie, a roller coaster ride, a challenging puzzle.

Perhaps most excitingly, we are uncovering the brain basis of our behaviors - normal, abnormal and in-between. We are mapping a neurobiology of what makes us us.