The discovery is the first direct evidence that a hormone called oxytocin, which evolved 100 million years ago to aid mating among fish and breast-feeding among mammals, also promotes trust between human beings, the scientists said.

The finding arises from a new wave of brain research designed to analyze how humans make decisions, by investigating the neural impulses that dart beneath the surface of self-awareness.

In an experiment published in the current edition of the journal Nature, researchers at the University of Zurich in Switzerland and Claremont Graduate University in California determined that people who inhaled two puffs of an nasal spray were measurably more likely to risk all their money with a stranger, without consciously knowing why.

''If I increase your level of oxytocin, I can induce you to overcome your anxiety in trusting a stranger,'' said Paul Zak, director of Claremont's center for neuroeconomic studies and a co-author of the research paper. ''It is a [biochemical] signal we induce unknowingly all the time by looking people in the eye or shaking someone by the hand.''

Although the researchers set up their experiment to test the effects of oxytocin on financial decisions, they believe the hormone is the keystone of a normal social life.

Trust is as crucial in love and diplomacy as in finance. Social phobias, which hinge on the inability to trust other people, are the third most common mental health problem.

''My conjecture is that it applies broadly to all kinds of social interactions where your trust could be abused,'' said economist Ernst Fehr in Zurich, the senior member of the research team. The finding is an important advance in understanding the biology of human behavior, two independent experts said.

''Oxytocin enhanced trust,'' said University of Iowa neuroscientist Antonio Damasio. ''The finding has powerful implications for understanding the brain. Remove trust and you compromise love, friendship, trade and leadership.''

Researchers have long known that the hormone was important in the social behavior of animals.

Oxytocin also is active during human childbirth, when it enhances uterine contractions during labor and often may be administered to ease a difficult birth. For nursing mothers, it eases the production of breast milk.

Until now, its role in human social behavior had been a matter of conjecture.