New Delhi, June 2: A gene that sits on chromosome 17 in every human cell appears to play a key role in how quickly a person will break into a smile, burst into laughter or display distress when emotionally upset.

Scientists have documented the strongest evidence so far that a gene called 5-HTTLPR, already known to be associated with the display of negative emotions, is also linked to positive emotional expressions such as laughing and smiling.

A study by the researchers in the US and Switzerland contributes to earlier suggestions that a version of this gene amplifies both positive and negative emotional reactions. The findings of the new study were published on Monday in the research journal Emotion.

"We already know 5-HTTLPR is involved in the regulation of serotonin, a neurotransmitter implicated in depression and anxiety," Claudia Hasse, assistant professor of psychology at Northwestern University in the US and team member, said. "There is now emerging evidence that serotonin also plays a role in positive emotions - a short version of the 5-HTTLPR amplifies emotional reactions, both negative and positive," she told The Telegraph.

Earlier studies by several research groups, including by Hasse and her colleagues, had shown that the gene 5-HTTLPR could predict how upset a person can become when seeing others in distress or the intensity of emotional display during an embarrassing situation.

Humans inherit so-called two alleles of 5-HTTLPR, one from the mother and one from the father. The gene comes in two variants - long and short. The earlier studies had shown that people with shorter versions had greater, more intense, emotional reactions.

Those with shorter variants of the gene became more upset or physiologically aroused when watching others. In the new study, the scientists have found that people with the shorter variants also show greater positive emotional expressions -they laugh and smile faster.

Their study suggests genes work along with cultural and social factors that also influence the display of emotions. "Genes are only part of the (equation) when it comes to emotional differences among people," said Robert Levenson, a team member at the University of California, Berkeley. "Both nature and nurture are important - genes do not determine our emotional reactions, they're one piece in the puzzle of our complex emotional lives."

In their experiments conducted in Berkeley laboratories, the scientists exposed young adults to cartoons and young, middle-aged, and older adults to an amusing clip from a movie. The researchers coded smiling and laughter through a system that measures facial movements to grade them.

The study focused on genuine or real emotional expressions rather than expressions underlying attempts to be polite. "When you measure smiling and laughing, you want to be able to distinguish real laughs and smiles from the ones that aren't," team member Ursula Beermann at the University of Geneva, Switzerland, said.