ABSTRACT
Save for those rare twins who are adopted by two separate families and, therefore, reared apart (Bouchard and McCue 1981; Bouchard 1984; Pedersen et al. 1988; Plomin et al. 1980; Bouchard Jr. et al. 1990; Bouchard Jr. 1994), each twin inevitably continues to be an essential part of the environment of the other after they are born. However, their link progressively takes on psychological/emotional dimensions. A ‘couple effect’ begins to be operative as well.
