Appraisal of Parental Bias in Twin Studies.Ascribed Zygosity and IQ Differences in Twins

Abstract

A criticism of twin studies has been that the difference between the behavioral similarities of identical and fraternal twins is largely created by parental influences based on their perception of the twins' zygosity. This issue is examined for differences in the IQ scores found within pairs classified by parents and bloodtyping. The systematic differences in IQ scores could be attributed to zygosity classified by bloodtyping rather than by parental belief. The available evidence indicates that the twin method is still appropriate for human behavior genetics.

A test of the equal-environment assumption in twin studies of psychiatric illness

“…Consistent with some (Matheny, 1979;Lytton, 1977), but not other (Scarr, 1968;Munsinger and Douglass, 1976), reports the disagreements between self-reported and assigned zygosity were not symmetrical in this sample; many more MZ twins considered themselves to be DZ than vice versa. In an attempt to understand this finding, we compared the reasons given for belief about zygosity in MZ twins where both considered themselves DZ vs. twins who were, in our judgment, both correct in considering themselves either MZ or DZ (Table III).…”

“…In a population-based sample of female-female adult twin pairs, self-reported zygosity in both members of the twin pair agreed with assigned zygosity in about 85% of pairs. This is a somewhat higher rate than in most previous reports (Loehlin and Nichols, 1976;Scarf, 1968;Scarr and CarterSaltzman, 1979;Matheny, 1979;Munsinger and Douglass, 1976). However, these studies examined younger twins and often asked the parents of the twins rather than the twins themselves.…”

“…), this method provides a global test for all potential environmental influences that are dependent on zygosity associated attitudes and expectations. Studies using this method, which requires that a certain proportion of twins be misinformed about their zygosity, have examined traits such as intelligence, attitudes, hyperactivity, and personality and have found little evidence that perceived zygosity influences twin resemblance (Scarr, 1968;Munsinger and Douglass, 1976;Matheny, 1979;Scarr and Carter-Saltzman, 1979;Goodman and Stevenson, 1989). In this report, we apply, for the first time to our knowledge, an assessment of the impact of perceived zygosity on twin resemblance for psychiatric disorders as diagnosed by personal structured psychiatric interview in a large population-based sample of female-female twin pairs.…”

Twins: a test of the equal environments assumption

“…Such an effect could arise when parents and others know that MZ twins are genetically identical and on the basis of that knowledge treat them identically, even when the twins do not want or deserve such similar treatment. Studies of twins whose zygosity has been mistaken by their parents indicate, however, that parental beliefs regarding their twins' zygosity have no effect on the similarity of their twins' personalities or abilities (19)(20)(21)(22). For example, Scarr & Carter-Saltzman (22) found that MZs misdiagnosed as DZs were as similar as correctly diagnosed MZs.…”

Gene × environment interaction of vigorous exercise and body mass index among male Vietnam-era twins

“…assumption by zygosity suggest that it holds for many traits (42)(43)(44). In addition, a lack of assortative mating by phenotype is typically assumed.…”