Triparental Families: A New Genetic-Epidemiological Design Applied to Drug Abuse, Alcohol Use Disorders, and Criminal Behavior in a Swedish National Sample

Objective: The authors sought to clarify the sources of parent-offspring resemblance for drug abuse, alcohol use disorders, and criminal behavior, using a novel genetic-epidemiological design. Method: Using national registries, the authors identified rates of drug abuse, alcohol use disorders, and criminal behavior in 41,360 Swedish individuals born between 1960 and 1990 and raised in triparental families comprising a biological mother who reared them, a "not-lived-with" biological father, and a stepfather. Results: When each syndrome was examined individually, hazard rates for drug abuse in offspring of parents with drug abuse were highest for mothers (2.80, 95% CI=2.23-3.38), intermediate for not-lived-with fathers (2.45,95% CI=2.14-2.79), and lowest for stepfathers (1.99, 95% CI=1.55-2.56). The same pattern was seen for alcohol use disorders (2.23, 95% CI=1.93-2.58; 1,84, 95% CI=1.69-2.00; and 1.27, 95% CI=1.12-1.43) and criminal behavior (1.55, 95% CI=1.44-1.66; 1.46,95% CI=1.40-1.52; and 1.30,95% CI=1.23-1.-37). When all three syndromes were examined together, specificity of cross-generational transmission was highest for mothers, intermediate for not-lived-with fathers, and lowest for stepfathers. Analyses of intact families and other not-lived-with parents and stepparents showed similar cross-generation transmission for these syndromes in mothers and fathers, supporting the representativeness of results from triparental families. Conclusions: A major strength of the triparentat design is its inclusion, within a single family, of parents who provide, to a first approximation, their offspring with genes plus rearing, genes only; and rearing only. For drug abuse, alcohol use disorders, and criminal behavior, the results of this study suggest that parent-offspring transmission involves both genetic and environmental processes, with genetic factors being somewhat more important. These results should be interpreted in the context of the strengths and limitations of national registry data.