ABSTRACT
Behavior geneticists are fond of asserting that the family environment has little or no effect on similarity among siblings, noting that full siblings raised within the family are no more alike than unrelated children raised in different families (Plomin & Daniels, 1987; Rowe, 1990). This assertion is based on evidence obtained from extensive studies of twins, full siblings, half siblings, and adopted siblings. The preponderance of evidence in these studies appears relatively clear: The degree of similarity between siblings follows along genetic lines, with monozygotic twins sharing identical genetic material and having the most similarities, and adopted siblings sharing no genetic material and having the fewest similarities.
