As you sink into your post-Thanksgiving food coma, the name of an amino acid might pop into your mind: tryptophan, a molecule found in high levels in turkey that's known to induce drowsiness. While scientists say that the tryptophan in turkey is probably not the source of holiday fatigue, a possible new role for tryptophan has recently been uncovered. It appears to affect our sense of trust.

Tryptophan is a chemical precursor to serotonin, one of the brain's most important signaling molecules and the target of the most commonly prescribed classes of antidepressants: selective serotonin reuptake inhibitors. Scientists who study neuroeconomics--the way the brain makes decisions--are beginning to study the role that serotonin plays in normal behavior. Robert Rogers and his colleagues at Oxford University are using game theory to study serotonin's role in social interactions.

In the Oxford study, the researchers asked volunteers to play a two-person game known as the prisoner's dilemma. Players can choose to make a move that wins them money and garners money from the other player, or make a move that wins both players money. The latter move maximizes earnings for each player. Over time, the optimal strategy for both players is to cooperate. Under normal circumstances, players cooperate about 75 percent of the time.

In the new study, presented earlier this month at the Society for Neurosciences meeting in San Diego, half of the volunteers were given a drink that depleted their tryptophan levels prior to the start of the game, thereby decreasing serotonin levels in their brain. Rogers and his team found that dampening serotonin activity significantly decreased the level of cooperation among the players, and that this group also rated fellow players as less trustworthy. "The findings suggest that a serotonin deficit might impair sustained cooperation," says Rogers.

While it's not clear why serotonin has this effect, previous research has shown that mutual cooperation might be rewarding in its own right: it enhances activity in the brain circuits that play a role in positive reinforcement. Rogers hypothesizes that reducing the chemical also reduces the reward value of cooperating.

Given that the use of serotonin reuptake inhibitors has exploded in the past decade, should we be concerned that the nation as a whole has become overly trusting? Probably not, says Rogers. It's not clear precisely how these drugs affect serotonin activity in the brain, he says, especially in clinically depressed patients, who are likely to have abnormal serotonin levels to begin with.

Your holiday turkey probably isn't going to significantly boost your trust levels either. The researchers haven't yet determined how boosting tryptophan affects trust, although they are starting on those experiments. And while turkey is high in tryptophan, it's probably not high enough in the typical holiday dinner to have an impact on the brain.