The researchers demonstrated that people with a certain genetic variant smiled or laughed more while watching cartoons or subtly amusing film clips than others.
Those with short alleles of the gene 5-HTTLPR smiled more than people with long alleles, the findings showed.
An allele is a variant of a gene. Each gene has two alleles; humans inherit one allele from mother and one from father.
“The short allele amplifies emotional reactions to both good and bad environments,” said study author, Claudia Haase from Northwestern University in the US.
The study combined three experiments from different Berkeley labs. In the first experiment, young adults were shown cartoons.
In the second experiment, young, middle-aged and older adults watched a subtly amusing clip from the film “Strangers in Paradise.”
The final experiment asked middle-aged and older spouses to discuss an area of disagreement in their marriage.
The scientists videotaped the volunteers during the experiments.
Overall, 336 participants were included in the final analysis. The researchers collected saliva samples from the volunteers to analyse the 5-HTTLPR gene.
The data from the three experiments combined indicated that people with the short allele of 5-HTTLPR showed greater positive emotional expressions.
Specifically, people with the short allele displayed greater genuine smiling and laughing than people with the long allele.
The study appeared in the journal Emotion.