Problem: In Chapter 4 of "Unthinkable" by Ripply, researchers examined sets of twins to investigate post-traumatic stress syndrome (PTSD) and its correlation with brain structures. They found that the men most likely to develop PTSD had smaller amygdalas, the part of the brain involved in processing emotions and fear responses. This suggests that individuals with smaller amygdalas may be more susceptible to developing PTSD in response to traumatic events.