A brain hormone thought to play a role in social bonding makes us more trusting, but not blind fools, a new study suggests.

Oxytocin, also dubbed the "love hormone," has been shown to aid in the connection between mother and child and between mates. Some studies have found that even sniffing oxytocin makes humans more trusting of others.

But researchers in Belgium wondered whether all those warm and fuzzy feelings might cloud our judgment, making us indiscriminately trusting and gullible.

To find out, Moïra Mikolajczak, of the Université catholique de Louvain in Belgium, and colleagues had 60 men (average age 21) take part in a trust game. Before the game, the men were either given a nasal spray containing either oxytocin or placebo. (The study included only men because men and women may respond differently to the hormone.)

The subjects were given a sum of money and told they could share some or all of it with a partner. Any amount they gave to their partner would automatically triple. However, the partner could then decide to keep all the money, or give some or all of it back. Subjects who are more trusting would be expected to share more money.

To help the participants determine whether their partner is trustworthy, partners were described as either reliable (for instance, "they practice first aid") or unreliable ("they play violent sports"). Participants also played against a computer, which randomly determined how much money to give back.

Participants who received the oxytocin gave more money to the reliable partners and to the computer than did participants who only received the placebo. In other words, the oxytocin recipients were more trusting of these entities. However, the trust effect vanished when it came to giving money to unreliable partners. Subjects who received oxytocin gave about the same amount to unreliable partners as subjects who received the placebo.

The findings suggest oxytocin's trust-inducing effects depend on the situation.

Essentially, the results suggest oxytocin is most effective at boosting trust when that increase in trust is likely to bring benefits yet it has no effect when conditions are shady and so more trust could be detrimental, the researchers wrote in the August issue of the journal Psychological Science. "Thus, the higher the perceived risk, the lower the trust-enhancing effect of [oxytocin]."