Over the years, a large body of work has been devoted to studying PTSD symptoms in second-generation survivors, and it has found signs of the condition in their behavior and even their blood — with higher levels of the stress hormone cortisol, for example. The assumption — a perfectly reasonable one — was always that these symptoms were essentially learned. Grow up with parents afflicted with the mood swings, irritability, jumpiness and hypervigilance typical of PTSD and you're likely to wind up stressed and high-strung yourself.