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A nationwide Finnish sample of schizophrenics' offspring given up for adoption was compared blindly with matched controls, who were adopted offspring of non-schizophrenic biological parents. The adoptive families were investigated thoroughly using joint and individual interviews and psychological tests. The biological parents were also interviewed and tested. Among the 155 index offspring, the percentage of both psychoses and other severe diagnoses (borderline syndrome and severe personality disorders) was significantly higher than in the 186 matched control adoptees. This supports a genetic hypothesis. However, notable differences between these two groups only emerged in the families which were rated as disturbed. Thus the genetic effect (i.e. the differences between high and low genetic propensity) was only manifested as a psychiatric disorder in the presence of a disturbed family environment. The impact of disturbed family relations was strongest in the presence of the appropriate genotype.
Tienari et al. (Fri,) studied this question.