Complex illnesses may result from the interaction or addition of multiple f
actors. We examined the familial co-distribution of two abnormalities commo
n in schizophrenia: impaired auditory sensory gating and impaired flush res
ponse to niacin. In ten families, the obligate carrier parent had sensory-g
ating deficits, while eight of the ten parents without a family history of
schizophrenia had impaired flush response. No parents had both deficits. Th
e data are consistent with a theory suggesting the interaction of these two
factors in some cases of schizophrenia. (C) 1999 Elsevier Science B.V. All
rights reserved.