To study the concept, economists are unlocking the role of trust in financial transactions, while neurobiologists have located centers of the brain responsible for trusting cooperation. Psychologists, meanwhile, now believe that happiness, altruism and mood-elevating neurochemicals all enhance trust behaviors.

A trusting smile

Happy people are more trusting than sad people, say J. Keith Murnighan and Robert Lount, researchers at Northwestern University in Evanston, Ill.

Intuitive, yes — but little scientific research has explored this concept, according to the researchers.

Using an experiment that evaluates trust in an investment setting — the so-called "Trust Game" — Murnighan and Lount discovered that happy participants committed acts of trust more often than sad participants.

In the Trust Game, players exchange certain amounts of money they are initially given with the possibility of a greater return from their partners. Researchers study how much a person is willing to risk in assuming the other will play and pay by the rules — in effect how much trust can be created between the players.

"Research on happiness suggests that it reduces mental processing — we work less hard cognitively," Murnighan said. "We might miss subtle cues when we are happy that we would otherwise detect and be concerned about."

Lending a hand

Altruism — that philanthropic urge to lend a hand — increases trust the same way, said George Dent, a law professor at Case Western Reserve University in Cleveland.

"I gather that evolution has linked altruism to happiness — groups that practice altruism are more likely to survive," said Dent, who studies trust and altruism in interracial settings. "Altruism may also be linked to opportunities to reproduce."

Altruism and trust tend to be reciprocated among members of similar groups, Dent said. Evolution may promote xenophobia and has programmed us — to be wary of what is different and foreign— and consequently, who is worthy of help, he said.

Neurobiology of trust: just follow your nose

Following Dent's reasoning, something that facilitates reproduction and lowers social inhibitions might also increase trust behaviors.

Indeed, researchers have recently shown that oxytocin, a naturally occurring reproductive hormone, may represent an act of evolutionary good will toward humans who need to trust to survive and pass on their genes.

Using the same Trust Game, Paul Zak, Robert Kurzban and William Matzner, neuroeconomics researchers at Claremont Graduate University in Claremont, Calif., published a landmark study in 2004 showing that trust behaviors were correlated to naturally increased levels of the reproductive hormone oxytocin in the blood. 

In a June issue of Nature, Zak and  neuroeconomic researchers at the University of Zurich in Switzerland, reported that oxytocin dramatically increased a trust-based money transfer. The researchers gave some subjects three squirts of oxytocin in each nostril. Those subjects handed over significantly more money during the Trust Game than subjects who didn't get the hormone.

Never trust a stranger?

But what about trust between strangers who have no contact except through cyberspace? EBay claims more than 100 million registered users who buy and sell millions of items each day on the Internet.

Researchers at California Institute of Technology in Pasadena, Calif., and Baylor College of Medicine in Houston used hyperscan functional magnetic resonance imaging to study the brains of subjects 1,500 miles apart engaged in a trust-based financial transaction game. 

"Hyperscanning is the simultaneous scanning of the socially interacting brains of two or more people," said P. Read Montague, a Baylor neuroscience professor. "We were very interested in how trust evolves when subjects are not near one another and anonymous to each other," he said.

Montague and colleagues Brooks King-Casas and Steven Quartz measured how and when their subjects made trust decisions by measuring changes in blood flow where the — intention-to-trust mechanism— occurs in the brain.

Over time, subjects became less reactive and more proactive in their willingness to trust one another.

"The most intriguing results were that the intention to trust shifted in time and developed in a part of the brain that also does very basic reward learning," Quartz said. "We have shown that trust, one of the highest-level concepts we possess, is a reflection of basic brain mechanisms.