A Small Grant for Exploratory Research (SGER). Unrelated siblings of the same age, reared together from early infancy, uniquely replicate the rearing situations of dizygotic twins. These dyads offer a new behavioral-genetic research design for examining genetic and environmental influences on behavior. Early findings show a substantially smaller IQ correlation for unrelated siblings reared together than for monozygotic twins, dizygotic twins and siblings, respectively. This finding supports an explanatory model of intelligence that includes genetic factors. The very modest IQ similarity between unrelated siblings reared together, despite their common rearing since infancy, suggests that the shared environment has a very small effect on intellectual development, and supports the position that individuals respond to environments in ways consistent with their genetic predispositions. The results also challenge the views of some critics that the behavioral resemblance of monozygotic twins is primarily a function of shared experience. An additional ongoing analysis is examining the similarity of unrelated siblings reared together on scales from the Child Behavior Checklist and comparing the magnitude of similarity with published data for twins, siblings and adoptees. Outcomes from this study can assist parents and educators concerned with the progress of near-in-age siblings, both unrelated and related. Differences in intellectual outcome, despite similarity of educational and social opportunities, should increase sensitivity to the individual talents of each child, and encourage informed appraisal of the educational aims and goals for each sibling.
