Just after the sun rose on July 7, 2008, Hans Reiser led police and prosecutors to Nina's shallow grave. Reiser was about to be convicted of strangling his estranged wife to death when he agreed to plead guilty to second-degree murder and reveal where he dumped Nina's body. In exchange, he would dodge the death penalty. Reiser was a moderately wealthy Internet entrepreneur who started college at age 15. Why wasn't he smart enough to just his wife?

I became familiar with Reiser's case because he hand-wrote a four-page appeal from his cell at San Quentin requesting a new trial. He cited my research as one rationale for why his conviction should be tossed out. In experiments run over the last decade, I have shown that an ancient molecule in the human brain, , makes us feel for others and causes us to behave morally. I call oxytocin the "moral molecule."

But here's the rub: Reiser didn't request an appeal because he believed he was oxytocin-deficit and wasn't responsible for his actions. He claimed that his lawyer lacked oxytocin and was empathy-deficient and consequently did not appropriately represent him in court. Reiser's complete lack of insight is astounding. And diagnostic of his pathology.

So how do human beings go from good to bad to evil? My experiments have shown that 95 percent of the thousands of people I have studied release oxytocin when they receive a positive social signal. Oxytocin-releasers include having someone trust you with their money, being touched, and even watching an emotional movie. Five percent of those I have tested do not release oxytocin after such stimuli. These individuals have many of the traits of psychopaths: They are charming, deceptive, and even self-deceptive. And, when there is money that can be shared with others, they unabashedly keep it all for themselves. Greed, you will remember, is one of the seven deadly sins.

Knowing the chemistry of gives us keen insights into why most of us are good most of the time, and why some people like Hans Reiser are evil. Let's start with evil. Rodents that genetically lack receptors for oxytocin behave like psychopaths—they do whatever they want without regard for others' safety or welfare. They are loners in permanent survival mode. These behaviors also occur for many victims of abuse; the oxytocin circuit in the brain needs nurturing to develop properly. The abuse victims I have studied are also in survival mode and have impaired social behaviors.

And then there is petty evil. High stress inhibits oxytocin release and makes us temporary psychopaths. We know that we are not our best selves when we are out. Stress narrows one's focus on oneself and we cease being socially competent. Actions we call "virtuous" or "moral" are those that put another's needs on par, or above, one's own: honesty, trustworthiness, compassion, fairness. Oxytocin does this by subtly changing the self-other balance towards caring about another's well-being. My experiments have shown this both by measuring oxytocin release in blood after an act of kindness and by manipulating oxytocin levels in human brains to show that oxytocin directly causes virtuous behaviors. Yes, there is a moral molecule.

But, I have worried lately that the carefully controlled laboratory experiments I have done may not apply to people's daily experiences, so I've taken my lab on the road. We have studied the many ways humans connect and willingly cooperate with each other. Experiments with soldiers marching, a rugby team warming up before a match, and people praying in church showed that these activities cause the brain to release oxytocin. A spike in oxytocin produces a feeling of closeness and a willingness to help others.

My field experiments have even taken me to some of the farthest reaches of the earth. I recently traveled to the highlands of Papua New Guinea to study isolated subsistence farmers in the rain forest. Highland people live much like our ancestors did millennia ago. I took blood before and after a ritual dance and found that it caused the release of oxytocin in the majority of the men I tested. The moral molecule appears to be a human universal.

The human desire to connect not only with friends and family but complete strangers is, I have found, what makes us moral. It is our social nature, our need to be around others, that makes us good most of the time. Oxytocin makes us feel what others feel and this not only motivates us to avoid doing things that hurt others but actually makes us feel pleasure when we bring others joy. Sneaky evolution! Gregariously social creatures like us need to have an internal moral governor that sustains our place in the social group. Being ostracized from one's group is as maladaptive for humans as it is for wolves. Behaving morally—roughly, being nice to others who are nice to us—keeps us ensconced the warmth and protection of our pack.