The Biology Behind Why Good People Do Bad Things

What makes people cross the ultimate moral line -- pulling the trigger and killing someone?

Part of the answer to that question lies in the part of the brain called the amygdala. That is the part of your brain that registers fear. We kill people ultimately because we fear them. That fear could be because they are breaking into our house, or because they are wearing the other army's uniform or because they are simply on another side of the political spectrum than we are.

Picture a scene in World War I. German and English soldiers sit in trenches opposite one another and fire guns, throw hand grenades and launch charges at each other. Thousands die. Then a truce is called: The Christmas Truce of 1914 .

The truce is called so that soldiers from each side of the line can retrieve dead bodies laying out in "no man's land" between the trenches. Soon both German and English soldiers are walking between the trenches carrying dead bodies. And then they are helping each other carry the other side's dead bodies and bury them in the frozen earth. Then they are having Christmas dinner together, singing carols and praying together. Some soldiers are even exchanging gifts and astonishingly, exchanging addresses to be able to visit one another after the war is over.

Eventually a soccer game breaks out, and the soldiers are playing almost as hard as they were trying to kill each other just a day ago. Once the truce is called off, the camaraderie gets so addictive that officers must come in and threaten to shoot the men if they don't go back to killing each other. So they do, and the war resumes.

Clearly these men did not fear each other during the truce. They did not fear each other so they did not shoot each other. When it came right down to it, they almost became friends and ended the war with a game of soccer. The fear in their minds was washed away and they were suddenly able to socialize and communicate like friends. Their amygdalas were not triggered, so they did not pull the trigger.

According to Robert Sapolsky, a neuroendocrinologist and professor at Stanford University, some clues can be found in why we pull the trigger based on our genes and what has been recently happening to our frontal cortex in the moments right before the trigger is pulled.

"Despite your sex," says Sapolsky, "if you have experienced higher levels of testosterone in your body in the preceding days you are more likely to pull the trigger. Your amygdala will be in a heightened state and your frontal cortex will be slower to respond. Furthermore, if you are in pain or exhausted or hungry, your frontal cortex is not going to work as well."

Sapolsky also states that the development of the brain is something that happens early in life. "The frontal cortex does not fully mature until you are 25," he says. "And thus adolescence and early adulthood are the years where environment and experience sculpt your frontal cortex into the version you are going to have as an adult at that critical moment."

Genetics is a factor in predicting whether someone will be violent. However, genes are unique because they react differently in different environments.

Sapolsky adds "Genes can also be a factor. There is a variant of a gene called MAO Alpha and if you have that variant, you are far more likely to commit anti-social violence, if and only if, you were abused as a child."

This is not excusing bad behavior. It certainly does not excuse homicide, but it does give us some insight into what goes on in the brain before someone commits a heinous act of violence.