The Super-Normal Control Group in Psychiatric Genetics

Recent family, twin and adoption studies of psychiatric illness have often used “super-normal” control groups, from which individuals are excluded if they demonstrate any significant psychopathology. Relatives of such “super-normal” control groups will have lower rates of psychiatric illness than the general population. Since such screening is not usually applied to index probands, rates of “screened-out” disorders may be increased in relatives of index versus “super-normal” control probands, producing spurious evidence for coaggregation. An algebraic model demonstrates that this artifactual coaggregation will be strongest for “screened-out” disorders that are both highly familial and common in the population.