In the late 1990s, child psychiatrist Glenn Saxe launched a study of children with burn injuries, who are known to have a high risk of developing post-traumatic stress disorder. The researchers gave psychological interviews to two dozen children admitted to the Shriners Burns Hospital in Boston and then followed their progress for six months.

Saxe’s team tracked everything they could about the kids’ cases — severity of injury, length of hospital stay, pain ratings, heart rate, medications and surgeries, parental stress, even how much they smiled around other people — hoping to find patterns that could predict which children would develop the frightening memories, anxiety, guilt, and heightened state of arousal that are characteristic of childhood PTSD.

A predictive variable popped out of