Longitudinal Study of Language of Twins at Ages 9 and 14 Years: Twinning, Zygosity, and Heritability.
Mabel L Rice, Stephen R Zubrick, Kathleen Kelsey Earnest and 1 others
Twins still score below age-matched singletons on language and nonverbal IQ at 9 and 14 years.
The gap narrows for vocabulary but widens for broader language. Genes account for a large share of language differences. Boys outscore girls in vocabulary at both ages.
Key findings
1Twins scored below age-matched singleton expectations on omnibus language and nonverbal IQ at both 9 and 14 years (seven of eight sample means fell below the expected population score). Receptive vocabulary showed a mixed pattern, with one mean below and one above expectation. Scores improved with age on vocabulary but declined relative to age peers on broader language and nonverbal IQ.
2Heritability was highest for nonverbal IQ at age 14 years (.85) and for omnibus language at both ages (.71 to .72), and lowest for receptive vocabulary at age 9 years (.33). Heritability did not differ significantly between ages 9 and 14 for any phenotype.
3Boys scored significantly higher than girls on receptive vocabulary at both 9 and 14 years, with a predicted difference of about 2 standard score points at each age (Cohen's d of about 0.20).
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What it does not show
The sample is 90.24% White, so findings may not generalise to more diverse populations. Children with hearing impairment, neurological disorders, or developmental disorders were excluded, so the results say nothing about twins with co-occurring conditions. The Raven test has a limited score range (seven possible percentile outcomes), which constrains the precision of nonverbal IQ estimates. The study did not examine heritability separately for children at low levels of performance, so it cannot speak to the genetic architecture of language impairment specifically. The CELF-4 and Raven were new measures at these ages, replacing the TOLD-P:3 and Columbia Mental Maturity Scale used at 4 and 6 years, so direct comparison across the full trajectory is limited. Testing occurred over a wide window (up to 13 months from the target age), and while the authors controlled for this statistically, it adds noise to the age comparisons.
Declared interests
Funded by multiple NIH grants (R01DC005226, R01DC001803, P30DC005803, R42DC013749, P30HD002528) and the Australian Research Council Centre of Excellence for Children and Families Over the Life Course (CE140100027). No industry funding. No conflicts of interest declared.
The easy way to misread this
Do not read a twin's below-average language score as a language disorder, or read the heritability figures as meaning a child's language difficulty is fixed by genes. The twinning effect is a group-level finding across 594 pairs, and heritability is a population-level statistic about variance, not an individual prediction. A child's score still needs the same clinical assessment and intervention planning as any other child's.
Summarised by AI from the full paper, without a clinician reviewing it. Check it against the source before it changes what you do. Read it on PubMed →