http://arstechnica.com/journals/science.ars/2008/03/18/study-identifies-gene-x-environment-link-to-ptsd is a link to a website that discusses the connection between a stress-related gene and the likelihood of developing PTSD.
The new issue of the Journal of the American Medical Association features an article by Dr. Kerry Ressler and several co-authors found that specific variations in the gene appeared to be influenced by child abuse. That interaction strongly increased the chances for adult survivors of abuse to develp signs of PTSD. The worse the abuse, the stronger the risk in people with the gene. (Though that might be true in any population.) From the Seattle Times: "The results of the new study suggest there are critical periods in childhood when the brain is vulnerable to ‘outside influences that can shape the developing stress-response system’. . .Dr. Thomas Insel, director of the NIMH, said the study is valuable for the light it sheds on military veterans. He said the results help explain differences in how two people see the same radside bomb blast. One simply experiences it as a bad day and goes back to functioning. The later develops paralyzing stress symptoms." See below for the Seattle Times link: http://seattletimes.nwsource.com/html/health/2004291429_ptsdgene19.html