A key hormone helps determine whether we will trust lovers, friends or business contacts, scientists claim.
The study looked at monetary exchanges
Exposure to an oxytocin "potion" led people to be more trusting, tests by University of Zurich researchers found.
They report in the journal Nature that the finding could help people with conditions such as autism, where relating to others can be a problem.
But one expert warned it could be misused by politicians who want to persuade more people to back them.
Oxytocin is a molecule produced naturally in the hypothalamus area of the brain which regulates a variety of physiological processes, including emotion.
It also acts on other brain regions whose function is associated with emotional and social behaviours, such as the amygdala.
And animal studies have shown oxytocin is linked to bonding between males and females and mother-infant bonding.
The Swiss and American team of researchers suspected the same effect may occur in humans and invited 58 people to take part in a "trust test".
The participants in the study played a game, in which they were split into "investors" and "trustees". The investors were then given credits and told they could choose whether to hand over zero, four, eight or 12 credits to their assigned trustee.
If the investor showed trust, the total amount which could be distributed between the two increased, but the trustee initially reaped all the reward.
It was then up to them to decide if they would honour the investor's trust by sharing the profit equally - or if they would keep the lot.
At the end of the game, the credits were translated into real money, meaning both participants had a selfish financial incentive.
Investors and trustees were either given oxytocin via a nasal spray, or a dummy, or placebo, version.
Of 29 investors who were given oxytocin, 13 (45%) displayed "maximal trust" by choosing to invest highly, compared with six (21%) of the 29 investors who were given the dummy spray.
Oxytocin did not change the behaviour of trustees.
In addition, when trustees were replaced by a computer, the oxytocin effect was no longer seen on the investors.
The researchers, led by Dr Ernest Fehr, say this suggests the chemical promotes social interaction, rather than simply encouraging people to take risks.
And they say it appears to over-ride obstacles such as the fear of being betrayed.
Writing in Nature, the team says: "Oxytoxin does not increase the general inclination to behave prosaically. Rather, oxytocin specifically affects the trusting behaviour of investors."
They suggest this is because people in the position of "investors" have to take the first step.
The scientists say their findings could potentially be used to help people with conditions such as social phobia and autism which can be linked to persistent fear and avoiding social situations.
"Our results might lead to fertile research on the role of oxytocin in several mental health disorders with major public health significance."
In the same journal, Dr Antonio Damasio of the Department of Neurology at the University of Iowa College of Medicine, US, said some might fear the findings could be used by those trying to gain people's trust.
"Some may worry about the prospect that political operators will generously spray the crowd with oxytocin at rallies of their candidates.
"The scenario may be rather too close to reality for comfort, but those with such fears should note that current marketing techniques - for political and other products - may well exert their effects through the natural release of molecules such as oxytocin in response to well-crafted stimuli.
"Civic alarm at such abuses should have started long before this study."