Does contact lead to similarity or similarity to contact?

Evidence from the Finnish Twin Registry (e.g., Rose et al., 1988) shows that adult monozygotic (MZ) twins are more similar, within pairs, in personality if the cotwins are presently cohabiting or in frequent contact than if they are seldom in contact. Results of a follow-up study led Kaprio et al. (1990) to conclude that "changes in social contact between monozygotic cotwins precede (and causally contribute to) changes in their intrapair similarity" (p. 9). If true, this conclusion has important theoretical implications, e.g., many heritability estimates would have to be revised downward. We adduce evidence suggesting that similarity leads to contact, rather than the other way around. Low correlations between twins' frequency of contact and their absolute within-pair difference on all traits thus far studied indicates that, whichever the direction of causality, the relationship between MZ within-pair similarity and their frequency of contact is very weak.