BEIJING (Xinhuanet) -- Social factors strongly influence differences in children's alpha amylase levels, according to a study in the April issue of Journal of Social and Personal Relationships.

Scientists have already relied on biomarkers in saliva to study stress vulnerability in adults, and this study shows for the first time such methods can also work in children.

Dr. Douglas A. Granger at Penn State University of U.S. found that 4-year-old children with higher alpha amylase levels were more susceptible to illness and had less close relationships with their preschool teachers.

Alpha amylase, an enzyme secreted by the salivary glands, is a marker for the response to the sympathetic nervous system, and therefore salivar may answer why kids are happy or sad, the study said.

But, the study made it clear, amylase does not specifically cause stress -- it is only a marker present when stress levels are elevated.