Sex Differences in Chimpanzee Behavior May Lead to New Model for Mental Illness Sex Differences in Chimpanzee Behavior May Lead to New Model for Mental Illness Thu, 03/10/2016 Elizabeth Doughman, Editor-in-Chief Chimpanzees exhibited significant genetic and neuroanatomical sex-related differences while scratching, a common indicator of anxiety, according to a new study from Georgia State University. This reveals their promise as a model of human mental illness.