Just how long we gaze at faces may be partly determined by our genes.
A new study focused on a single gene, the one coding for the cannabinoid receptor 1 (CNR1) in brain cells. Participants with certain mutations in this gene spent more time looking at happy faces than others did.
The results suggest these mutations may be involved in differences in how people process emotions on faces, the researchers said.
In practical terms, the findings may provide insight into the genetics of autism. Individuals with autism have trouble understanding facial emotions and look less at people's faces, the researchers said.
"There are genetic factors that influence the most basic aspects of our social behavior that we do not really think about" ? for instance, how long we look at a happy face, said study researcher Bhismadev Chakrabarti, an assistant professor at the University of Reading in England. Future research may uncover more genes involved in this process, Chakrabarti said.
The study will be published online June 29 in the journal Molecular Autism.
Infants tend to look longer at certain stimuli, including happy faces, presumably because they prefer these objects. One hypothesis is that infants are being "rewarded" by their brain, through the release of certain chemicals, when they look at pleasing objects.
Cannabinoid receptor 1 is involved in the brain's response to rewards. It controls the release of dopamine, the brain's "feel-good" chemical. (Compounds from the cannabis plant also stimulate these receptors.) From earlier work, the researchers suspected the CNR1 gene might help determine gaze fixation.
Chakrabarti and his colleague, Simon Baron-Cohen, director of the Autism Research Center at Cambridge, analyzed the gazes of 28 adults as they watched videos depicting happy faces, and for comparison, faces showing disgust. The researchers measured how long the subjects looked at the eyes and mouths of faces in the videos.
The found a link between certain mutations in the CNR1 gene and how long the subjects looked at happy faces, but not at faces of disgust. This fit their theory, because faces of disgust would not be rewarding.
The mutations, known as single nucleotide polymorphisms, occur naturally in DNA and do not necessarily cause disease.
It's not clear whether the subjects actually experienced a change in their mood upon looking at the happy faces, the researcher said.
Rewards from faces
It's possible people who have certain natural genetic mutations don't get the same rewards from looking at happy faces as other people do. So children with these mutations may be at risk for developing social problems, the researchers speculated.
"As a child, if you are not looking enough at the faces of your parents or caregivers, you may not be seeking appropriate social feedback" or trying to engage people socially, Chakrabarti said.
"It is possible that this can drive you down a route where your social behavior starts showing atypicalities" such as those seen in autism, Chakrabarti told MyHealthNewsDaily.
However, Chakrabarti emphasized, autism is a complex condition that likely involves many genes and their interaction with the environment.
It's also possible similar genetic differences exist in people who have problems socializing without having autism. Autistic traits occur in the general population along a continuum, Chakrabarti said, and only those who cross a certain threshold might merit a diagnosis.
An earlier study found human faces may hold more meaning for socially outgoing people than for introverts.
Pass it on: Variations in a specific gene may determine how long we look at happy faces.