INTRAPAIR DIFFERENCES IN PERSONALITY AND COGNITIVE-ABILITY AMONG YOUNG MONOZYGOTIC TWINS DISTINGUISHED BY CHORION TYPE

We evaluated placentation effects on behavioral resemblance of 44 pair s of monozygotic (MZ) twin children. Tested at ages 4-6, the twins' zy gosity and placental type had been determined at their delivery. The s ample included 23 monochorionic (MC) and 21 dichorionic (DC) MZ twin p airs: 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 Cog nitive Ability, while their mothers separately rated each cotwin on an individualized 280-item form of the personality Inventory for Childre n (PIC). Absolute differences between MC-MZ cotwins were smaller than those between DC-MZ cotwins for all 20 PIC scales, significantly so fo r 3 of 4 factor scales, 8 of 12 clinical scales, and 2 of 4 validity/s creening scales from the PIC; in contrast, no consistent differences i n intrapair resemblance of mono- and dichorionic MZ twins were found f or the McCarthy Scales. The chorion differences found in the PIC data cannot be due to genetic differences, because all pairs are monozygote s; nor are they associated with differences in parity, gestational age , birth weight, maternal education, palmar dermatoglyphic asymmetry, o r maternal knowledge of chorion type. We interpret our findings as sug gestive evidence that variation in timing of embryological division, w ith effects on MZ twins' placental vasculature, has significant conseq uences for some dimensions of their behavioral development, as well.