Philadelphia ? Thomas Aquinas called greed a sin against God. Its profile rose briefly after the 1980s, when some people apparently took to heart the movie line “greed is good,” but this deadly sin has again fallen from grace thanks to the real estate and financial meltdowns.

While it’s widely seen as the poison that has sickened the world economy, researchers who’ve studied greed say it’s intrinsic to human nature, stemming from our evolution as social animals, our instincts to compete and, some say, our fears of being left behind the pack.

“Everyone feels greed. … It’s natural for people to want more than they need,” said Keith Murnighan, an economist at Northwestern University.

In which case, saying greed sank the economy is a little like saying water sank the Titanic. Only under certain conditions can greed seep in and take us over.

When he first set out to study greed, Murnighan said, he found it hard to detangle from the natural and healthy desire we have to pursue our own interests. “It’s really not an easy issue to define,” he said. “Greed in my mind is self-interest run amok.”

The trigger is almost always something akin to envy (also one of the seven deadly sins).

Imagine, he said, that everyone you know drives a Honda and you’re perfectly happy to drive a Honda, too. But now imagine a neighbor or co-worker acquires a Jaguar or a BMW, and suddenly all your peers assume this person is more successful than you. Then how do you feel?

Being social animals, we gauge our own wealth and success in comparison with other people.

Think back to fifth grade, he said, when most children become acutely aware which of their peers are prettier or smarter than they are, which ones wear more expensive clothes and even whose parents drive more expensive cars.

George Loewenstein, a professor of economics and psychology at Carnegie Mellon University, agrees that greed is rooted in competitive drives, but he says greed goes beyond a healthy desire to fit in.

He said that, for years, economic theory ran on the notion that people act out of a rational self-interest, and many economists equated greed with a reasonable pursuit of happiness.

But for Loewenstein, the kind of greed that gets us into trouble pushes us to act against our long-term interests. “Greed is a situation where your desire for acquisition causes you to behave in a fashion that’s either destructive to yourself or to others around you.”

In fact, the greedy desire to keep up with the pack can trump our more rational desires for financial gain. “In almost every scandal — every disaster that’s unfolded from the Latin American loan crisis to Enron and WorldCom — there’s always the same pattern of intensified competition leading to greed,” he said.

In the past few years, for example, financial institutions were making big profit by underwriting risky loans, he said, but rather than worry about whether the borrowers could pay them back, the leaders of those institutions were overcome by the fear that their competitors were making even bigger profit.

In booms and busts alike, he said, people are greedy because they’re worried that others will win while they get left behind. “They’re terrified about being the sucker,” he said.