Intrapair differences in personality and cognitive ability among young monozygotic twins distinguished by chroion type

Abstract

We evaluated placentation effects on behavioral resemblance of 44 pairs of monozygotic (MZ) twin children. Tested at ages 4-6, the twins' zygosity and placental type had been determined at their delivery. The sample included 23 monochorionic (MC) and 21 dichorionic (DC) MZ twin pairs: DC-MZ twins result from separation of blastomeres within 72 h of ovulation; MC-MZ twins arise from later duplication of the inner cell mass. Twins were individually administered the McCarthy Scales of Cognitive Ability, while their mothers separately rated each cotwin on an individualized 280-item form of the Personality Inventory for Children (PIC). Absolute differences between MC-MZ cotwins were smaller than those between DC-MZ cotwins for all 20 PIC scales, significantly so for 3 of 4 factor scales, 8 of 12 clinical scales, and 2 of 4 validity/screening scales from the PIC; in contrast, no consistent differences in intrapair resemblance of mono- and dichorionic MZ twins were found for the McCarthy Scales. The chorion differences found in the PIC data cannot be due to genetic differences, because all pairs are monozygotes; nor are they associated with differences in parity, gestational age, birth weight, maternal education, palmar dermatoglyphic asymmetry, or maternal knowledge of chorion type. We interpret our findings as suggestive evidence that variation in timing of embryological division, with effects on MZ twins' placental vasculature, has significant consequences for some dimensions of their behavioral development, as well.

Genetic and environmental influences on human psychological differences

“…A small number of studies comparing very small numbers of MC and DC twins suggested that MC twins are more similar than DC twins on some, but not all, measures of mental abilities (Melnick et al, 1978;Rose et al, 1981). A greater number of small-sample studies (Brown, 1977;Welch et al, 1978;Sokol et al, 1995;Gutknecht et al, 1999;Riese, 1999), however, failed to replicate these chorion effects. Moreover, a recent, large epidemiological study ) using a near-representative sample from the East Flanders Prospective Twin Survey could not replicate the specific effects previously reported and found no chorion effect on total IQ (r MC ϭ .83, n ϭ 175 pairs; r DC ϭ .82, n ϭ 95 pairs; r DZ ϭ .44, n ϭ 181 pairs).…”

Effects of Chorion Type on Genetic and Environmental Influences on Height, Weight, and Body Mass Index in South Korean Young Twins

“…Unfortunately, however, twin studies of cognitive abilities and personality that have investigated chorion effects have produced mixed results. For example, whereas Melnick et al (1978) and Jacobs et al (2001) found greater similarities for MCMZ than DCMZ twins on measures of cognitive abilities, Sokol et al (1995) found no significant differences between the two types of MZ twins. Similarly, in a study of neonatal temperaments using MZ twins of known chorion type, Riese (1999) found that chorion type was not related to co-twin similarity on any of the temperament ratings.…”

The Influence of Twin Pair Permutation on Likelihood-Based-Estimates of Genetic Variance That Require Ordering of Twin-Pairs

“…Results from the MZ twins in this cohort and details of twin ascertainment were reported by Sokol et al (1995). From this study three variables were chosen using the ACE model so one variable had a highly significant genetic effect (weight at 4 years), one had a nonsignificant genetic effect (length at birth), and one had a not quite significant genetic effect (occipitofrontal circumference at birth).…”