I was hooked by the first line of Beyond Revenge, a book by the research psychologist, Michael McCullough.

What he's learning, he said, is for people who want to bypass all of the pious sounding statements about the power of forgiveness,

and all of the fruitless sermonizing about the destructiveness of revenge.

Both revenge and forgiveness, he says, have their purpose in human biology and history.

But from neighborhood arguments to civil wars, Michael McCullough's science is showing how we can make forgiveness more possible and appealing.

Some of the baggage is that it's an ambi-pambi thing that doormats do,

but from everything I've managed to read and see and understand forgiveness is a brawny, muscular exercise that I kind of imagine someone with a great passion for life

and a great, hearty sort of disposition being able to take on.

Getting revenge and forgiveness, I'm Christopher Tippett, this is on being from APM American Public Media.

Michael McCullough directs the evolution and human behavior laboratory at the University of Miami.

He works with social scientific research, as well as emerging discoveries in biology and brain chemistry.

His focus on the biology of revenge and forgiveness has taken him into other related areas of human moral sentiments, gratitude and self-control.

I interviewed Michael McCullough in 2008, just after he'd published Beyond Revenge.

I think one of the important themes that comes through that I just, you know, they think it's important for us to talk about to lay the groundwork for what you have to say, what you're learning, is that we lay people, citizens, consumers of science and journalism, have to open our imaginations to think in new ways about subjects like revenge and forgiveness that there are certain boxes into which we've put these things.

That's right. One of the things that got me writing Beyond Revenge actually was the dissatisfaction with the kind of boxes that we all tend to put revenge and forgiveness in as human dispositions.

So if you turn on the news, you see certainly senseless acts of revenge.

But we don't really know what to do with those once we see those acts. What are the stories we tell ourselves about what causes those acts?

What kind of judgments do we pass about the people who commit them? Do we demonize them? Do we call them animals?

Those, I think, do tend to be the kind of conclusions we draw. And the more I read and the more I tried to dig deeply into not just the social sciences, but also the biological sciences, as you say, the worse that story really seemed to fit.

As it seems to me revenge is much more deeply etched into the human mind than those kind of stories would suggest.

I want to try to understand this because you really talk about two different kind of preconceptions or I can't tell if they're different or if they converge and I just don't get it.

On the one hand, there's this idea that human nature really is brutish and that positive characteristics like generosity and love and forgiveness are exceptions to human nature.

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