Neuroscientists contend that oxytocin is not so much a trust serum as a kind of brain messenger that primes animals to overcome their natural aversion to others. This may be an especially important ability in people with autism.

Scientists have long known that the hormone used in the study -- oxytocin, which circulates widely in the body during childbirth and lactation -- prompts warm relations and mating in other mammals.

But they say the Swiss study, which appears in the Thursday issue of the journal Nature, is the first to show that a simple administration of a hormone in humans can consistently alter something as socially sensitive as trust.

The new finding could help researchers not only understand the biological system underlying social judgments but perhaps correct it when it goes awry, as in conditions like autism, scientists say.

Workings of the Human Brain

The finding follows others about the workings of the human brain recently. Scientists recently identified romantic love as a biological urge not unlike hunger or thirst. Some weeks ago, two scientists disclosed that they had found a way to peer directly into your brain and tell what you are looking at, even when you yourself are not yet aware of what you have seen.

In the new study, the participants played an investment game with anonymous partners. Those who were given oxytocin invested more money with the partners than did those who did not receive the hormone, the researchers found.

Ernst Fehr, a professor of economics at the University of Zurich and the senior author of the paper, said, "Most experts were very pessimistic" that the study would find anything of significance. But he said that the clear results should "induce a new wave of oxytocin research in humans."

"I have the hope that this research will lead to clinical applications in psychiatric disorders that are associated with a lack of trust," Fehr said.

Neuroscientists who were not involved in the study said that any hopes for new treatments or fears of abuse of the hormone were premature.

'Elegant, Simple and Plausible Finding'

"One always wants to see something like this replicated, and we understand very little about how this oxytocin system works," said Dr. Antonio Damasio.

"But this is a very elegant, simple and plausible finding, and is significant because it joins cognitive processes to an underlying biological regulation," said Damasio, a professor of neurology at the University of Iowa, who wrote an accompanying editorial in the journal.

The oxytocin nasal spray used in the study is prescribed in Europe and elsewhere for inducing contractions or stimulating lactation, according to Novartis, which makes the product.

In the study, the Swiss researchers had 178 male college students play a simple investment game. Investors began the game with an allowance of 12 monetary units, of which they could send 12, 8, 4 or none to an unseen, anonymous "trustee." The amount was tripled before being transferred to the trustee, who then chose how much of this income to share with the investor.

In previous experiments using this game, economists have shown that investors are guarded with their money at first, increasing their investments only after seeing evidence that their partner is playing fair. The oxytocin study did not allow for this adjustment: Investors knew that they would be dealing only once with four different partners.

Yet those who inhaled oxytocin before playing the game invested an average of 10 monetary units, 17 percent more than did players who got a placebo spray. In the oxytocin group, 45 percent invested all their money, compared with 21 percent in the placebo group.

Brain Messenger

Neuroscientists, including the Swiss researchers, contend that oxytocin is not so much a trust serum as a kind of brain messenger that primes animals to overcome their natural aversion to others. This may be an especially important ability in people with autism. Whether oxytocin or other hormones could affect such behavior is unclear, but the oxytocin study suggests it is worth investigating.

"To put it succinctly: I believe that oxytocin may help those people who have a pathologically low trust level," Fehr said in an e-mail message. "But you cannot induce a pathologically high trust level in normal people by giving them oxytocin.

"If I abuse your trust once or twice, you are not going to trust me a third time even if I give you a high dosage of oxytocin," he said.