Abstract

This paper analyzes a key theoretical assumption of the "classical twin method": the so-called "equal environment assumption" . The purpose of the discussion is to determine whether this assumption, which states that identical and fraternal twins experience similar environments, is valid. Following a brief discussion of the origins of the twin method and the views of its main critics, the arguments of its principal contemporary defenders are examined in detail. This discussion is followed by a critique of several studies which have been cited as evidence in support of the equal environment assumption. It is concluded that the equal environment assumption does not stand up to critical examination, thereby calling into question the claim that the twin method measures genetic effects on human behavior and personality differences