Four members of a sibship showing abnormal P50 auditory sensory gating in two members, with the other two normal. Sibling A, who has schizophrenia, has a smaller hippocampus than sibling B, who has never been mentally ill. A similar pattern was found in eight other such sibling pairs in other families. There was no significant difference in hippocampal volume between schizophrenics and their siblings with normal P50 gating. These data suggest that decreased hippocampal volume may be a second factor, in addition to the abnormality demonstrated using P50, that is present in schizophrenics. (From ref. 112)