The most recent chapter is a new study in the Proceedings of the National Academy of Sciences this week by researchers from the National Bureau of Economic Research. They found that the sons of Union Army soldiers who endured grueling conditions as prisoners of war were more likely to die young than the sons of soldiers who were not prisoners. This is despite the fact that the sons were born after the war, so they couldn’t have experienced its horrors personally. In other words, it seemed like the stresses of war were getting passed down between generations.

The effects on longevity showed up for the sons of men who were imprisoned in 1863 and 1864, when conditions in POW camps were especially bad. Crowding was extreme—each man was said to have had a grave’s worth of square footage to himself—and deaths from diarrhea and scurvy were common.

Because the study authors controlled for other factors that might have influenced the sons’ longevity, like socioeconomic status and the quality of the parents’ marriages, they believe this effect on mortality is working through epigenetics, or the process by which genes are switched on and off. These epigenetic changes are inherited by later generations, setting diseases in motion.