The study, published in the Archives of General Psychiatry, found 40 percent of differences in cortisol production were genetically determined, but growing up in difficult family circumstances overrode this genetic effect.

"A transient rise in cortisol level is a normal response to stress. But continuously high levels of this hormone could be harmful to the child's development in the long run," study leader Michel Boivin of Universite Laval said in a statement.

Cortisol levels were measured in 130 identical and 216 fraternal twins -- 19 months old -- before and after they were brought into a room with a clown and noisy robot.

"These are not traumatic events, but they are sufficient to cause behavioral changes in most children of that age," Boivon said.

A quarter of the study participants had three or more specific risk factors known to affect cortisol levels in children classifying them as having a "difficult family context" -- including tobacco use during pregnancy, low family income, low education level, single parenthood, very early parenthood, low birth weight, maternal hostility toward the child -- and classified as having a "difficult family context."