(CNN) -- The longest debate since humans have been having debates is whether we are good or evil. It underlies the stories of Adam and Eve, Cain and Abel, Jesus and Judas.

What is our human nature? Of course, the answer is we can be both good and evil. But what determines which part of our character emerges?

About a decade ago, my lab made an unexpected breakthrough in the understanding of good and evil. We discovered that the neurochemical oxytocin makes people trustworthy. We then found oxytocin was responsible for many other moral behaviors, from being generous to sacrificing to help a stranger.

An interview with Paul Zak Wait -- morality is chemical? In my TED talk, I describe how I made the unlikely discovery of the moral molecule, how I was roundly discouraged from even looking for such a chemical, and what drove me to persist in my search.

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Morality has traditionally been the domain of theologians and philosophers, often providing prescriptions of what we must do. But in the past decade, neuroscientists have started analyzing brain activity while people think about, and engage in, moral or immoral acts. These findings have changed the inquiry into morals from prescriptive to descriptive. As I discuss in my talk, I have even done studies that have manipulated brain chemistry in human beings to show that oxytocin directly causes people to be moral.

I also talk about what having a chemical that affects morality means for individuals, organizations and entire societies. For example, does "my chemicals made me do it" absolve people from legal or moral responsibility? If we have a moral molecule, where does evil come from?

By the way, oxytocin doesn't only cause morality in a laboratory setting -- I've done studies in churches, on sports fields and among indigenous people to show that the biology of morality is a human universal.

While neuroscience has provided new insights into our human nature, the philosophy of morality has not gone away. My talk identifies the philosophers whose insights and arguments are consistent with the way oxytocin works in the human brain. Two hit the mark: Aristotle and Adam Smith. Aristotle claimed that the reason to be a virtuous person is because it makes us happy. I found the same thing: Those who release the most oxytocin in the lab are more satisfied with their lives (watch the talk to find out why).

And then there is Adam Smith. Yes, the same Adam Smith who is considered the "father" of economics was a moral philosopher. In 1759, Smith published a book called "The Theory of Moral Sentiments" that nearly perfectly anticipated my findings. Smith's book caused a sensation when it came out because of his radical claim that morality comes from humans' social nature, not from God.

Sociality, said Smith, means we inevitably share the emotions of others. This is just what I found: When the brain is flooded with oxytocin, people feel empathy for others. It is this emotional connection that causes most of us, most of the time, to behave well toward each other.

I've also found that societies that are more moral (for example, more trustworthy and more tolerant) also have higher standards of living. Smith understood why: Morality undergirds economic exchange, opening up more opportunities for the creation of wealth that individuals in a transaction can share. And, prosperity (perhaps surprisingly) can make societies more moral. All this occurs as part of our human nature, our brains adapting to evolving social environments.

So, this ancient and tiny molecule, oxytocin, has taken us from being social creatures to, increasingly, being tolerant, empathic and prosperous ones. Quite a nice trick for a tiny molecule that traces its lineage back at least 400 million years.