Perception of intimacy could depend on the activity of serotonin, a brain chemical.

Understanding the powerful influence of such chemicals is important as supportive close relationships are known to protect against the development of mental illnesses and to promote recovery in those affected by psychiatric conditions. The opposite is also true: dysfunctional relationships can be triggers for those at risk of these conditions.

In the Oxford University research, two groups of volunteers were given amino acid drinks in order to manipulate blood concentrations of the amino acid tryptophan, a vital ingredient in the synthesis of serotonin. One group received drinks that contained tryptophan. The other group received drinks that did not contain tryptophan. They were then asked to make judgments about sets of photographs of couples. Differences in the judgments made by the two groups reflected changes in their serotonin activity.

"Serotonin is important in social behavior, and also plays a significant role in psychological disorders such as depression," explained Professor Robert Rogers who led the research. "We wanted to see whether serotonin activity influences the judgments we make about peoples' close personal relationships."

The volunteers who received the drink without tryptophan consistently rated the couples in the photos as being less 'intimate' and 'romantic' than the participants who received the control drink.

It is already known that serotonin function drops in episodes of depression, where the capacity for intimacy is often compromised.