Longitudinal Study of Language of Twins at Ages 9 and 14 Years: Twinning, Zygosity, and Heritability

This unprecedented longitudinal twin study focused on the arc of language acquisition from first words to adolescence, with data collection at 2, 4, and 6 years of age, reported in four previous studies and now new data at ages 9 and 14 years.

The method is a classic twin study design for estimating inherited versus environmental effects. The sample consisted of 1,188 children from 594 twin pairs, ascertained from birth records and followed for studying language and cognitive development. Here, we report language and nonverbal cognitive phenotypes at ages 9 and 14 years. Previous research reports from the sample at younger ages, as well as data from cross-sectional samples in other studies, supported the following generalizations: (a) Twinning effects persisted into adolescence for young children, such that twin children's language milestones were delayed relative to norms for singleton children; (b) twin type, that is, zygosity effects persisted although perhaps reduced with age; (c) gender also played a role for young children with advantages for girls relative to boys; and (d) heritability increased with age.

As in earlier studies, twins' standard scores for language were below the expected mean scores of 100 provided by test developers, 97 (SD = 14) at age 9 years and 95 (SD = 15) at age 14 years, although the gap relative to age peers is closing. Twin type effects, in which there was a slight disadvantage for monozygotic twins at age 4 years, disappeared at ages 9 and 14 years. Gender was significant at ages 9 and 14 years for vocabulary only but in the opposite direction than expected-with boys outperforming girls; however, this is consistent with singleton longitudinal findings from our lab. Models of heritability included adjustments for gender effects and other covariates. Heritability estimates at ages 9 and 14 years did not significantly differ. The final heritability estimates at age 14 years were .49, .71, and .85 for vocabulary, omnibus language, and nonverbal intelligence quotient (IQ), respectively, suggesting greater environmental effects for vocabulary relative to language and nonverbal IQ.

The findings are consistent with previous reports, offering further empirical precision in estimation of the role of genetics in the transition from childhood to adolescence and further support for the need to consider genetics as well as the home environment in the lives of twin children.