 It is also the case that non-combat twin brothers of Vietnam veterans with PTSD also had smaller hippocampi than other controls, raising questions about the nature of the correlation.A 2016 study strengthened theory that a smaller hippocampus increases the risk for post-traumatic stress disorder, and a larger hippocampus increases the likelihood of efficacious treatment.The hippocampus has a generally similar appearance across the range of mammals, from monotremes such as the echidna to primates such as humans.