SAME-AGE UNRELATED SIBLINGS - A UNIQUE TEST OF WITHIN-FAMILY ENVIRONMENTAL-INFLUENCES ON IQ SIMILARITY

Pairs of unrelated siblings of the same age, reared together from infa ncy (UST-SA), uniquely replicate the rearing situations of dizygotic ( DZ) twins. These dyads offer a new behavioral-genetic design for exami ning genetic and environmental influences on behavior. An IQ intraclas s correlation of .17, based on 21 UST-SAs, is substantially lower than the correlations of .86,.60, and .50 reported for monozygotic (MZ) tw ins, DZ twins, and siblings, respectively. This finding supports an ex planatory model of intelligence that includes genetic factors. The ver y modest IQ similarity between UST-SAs, despite their common rearing, suggests that the shared environment has a very small effect on intell ectual development and supports the position that individuals respond to environments in ways consistent with their genetic predispositions. The results also challenge some critics' views that the behavioral re semblance of MZ twins is primarily a function of shared experience.