Seems like a strange connection, but "New research from the Child Development Laboratory at the University of Maryland shows that shyness in kids could relate to the manner in which a stress-related gene in children interacts with being raised by stressed-out parents."
"Like all genes, the particular serotonin-related gene examined in this study has 2 alleles, which can be long or short. The protein produced by the short form of the gene is known to predispose towards some forms of stress sensitivity.
Fox's research found that among children exposed to a mother's stress, it was only those who also inherited the short forms of the gene who showed consistently shy behavior.
"If you have two short alleles of this serotonin gene, but your mom is not stressed, you will be no more shy than your peers as a school age child," says Fox. "But we found that when stress enters the picture, the gene starts to show a strong relationship to the child's behavior," says Fox. "If you are raised in a stressful environment, and you inherit the short form of the gene, there is a higher likelihood that you will be fearful, anxious or depressed.""
This is a nice description of the interplay between genetics and environment. Both parts need to be present for a particular effect to manifest.
Clinical implications: Yet another argument for medicating psychiatric illnesses when people are undecided. This is especially true for post partum depression, when new moms have an episode of major depression shortly after childbrith (within the first month). Some worry about medicating women who are breastfeeding because of risks of antidepressant exposure via breastmilk to the infants, but research (including this study) supports that babies do better psychologically and developmentally when moms are not depressed, and are better able to bond, and attend to their infants.