The brain chemical serotonin plays an important role in the judgments we make about peoples' close personal relationships, says a new study.
Healthy adult volunteers recruited by experts at the University of Oxford were made to watch photos of couples, and rate how 'intimate' or 'romantic' they perceived them to be.
The volunteers, whose levels of serotonin activity had been lowered, rated couples in photos as being less intimate and less romantic than volunteers with normal serotonin activity.
The approach involved giving amino acid drinks to two groups of volunteers in order to manipulate blood concentrations of the amino acid tryptophan, which is 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," said Prof Robert Rogers of Oxford University, 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.
This finding is an important reminder that our relationships with other people are influenced by processes beyond our awareness and control.
Although much more research is necessary before a drug might come to market that can help promote intimacy, it is clear for now that our chemistry has an impact on nearly aspect of our lives, from our most public actions to our most private, as we see here with human intimacy and romantic feelings.
The study is published in Biological Psychiatry.