Tendency to laugh could run in the family: study

Working with a total of 336 participants, the research team demonstrated that a genetic variant that was once linked to negative emotions is actually what's behind the tendency to laugh and smile.

A gene called 5-HTTLPR is responsible for regulating serotonin, the neurotransmitter that contributes to feelings of well being.

Short alleles -- also known as genetic variants -- of the gene 5-HTTLPR make an individual more sensitive to the giggles.

"Our study provides a more complete picture of the emotional life of people with the short allele," says Claudia Haase of Northwestern University. "People with short alleles may flourish in a positive environment and suffer in a negative one, while people with long alleles are less sensitive to environmental conditions."

Each gene has two alleles, one of which is inherited from the mother, the other from the father.

"Having the short allele is not bad or risky," says Haase, an assistant professor in the Human Development and Social Policy program at Northwestern's School of Education and Social Policy. "Instead, the short allele amplifies emotional reactions to both good and bad environments."

In the first of three experiments, young men and women viewed cartoons from "The Far Side" by Gary Larson and "The New Yorker" magazine.

In the next, young, middle aged and senior adults watched what the researchers describe as a "subtly amusing" clip from the movie "Strangers in Paradise."

The last experiment was less funny: Middle aged and senior adults were asked to discuss conflicts in their marriages.

All experiments were videotaped and the researchers used the "Facial Action Coding System" to assess the participants' reactions down to small movements in the face, according to the study.

In this way, the researchers could discern whether participants were smiling or laughing just to be nice or whether they had genuine reactions to the material.

A genuine laugh shows in the crow's feet -- the wrinkles produced around the eyes when someone laughs -- which the researchers say can only be seen in an authentic response.

To assess the alleles of their participants 5-HTTLPR genes, the researchers collected saliva samples.

Results indicate that those with the short allele had a greater tendency to laugh than those with the longer version.

"This study provides a dollop of support for the idea that positive emotions are under the same tent as negative ones, when it comes to the short allele," says senior author Robert W. Levenson.