Twins and their singleton siblings differ in language, cognition, and social-emotional development

This study compared the language, cognition, and social-emotional development between twins and their younger singleton siblings at the ages 2, 3, 4, and 7 years. Data came from 851 sibling trios from the Twins Early Development Study (TEDS), a family cohort born in 1994-1996 in England and Wales (50.2% female; 93.2% White British). Twins performed worse than singletons in language, cognition, and social-emotional development across ages, with small to moderate effect sizes (d=0.17-0.62). An exception was language at age 7 years, when twins exceeded their singleton siblings (d=-0.35). These findings suggest that (a) twins and their singleton siblings differ in development across domains and (b) their differences in language reverse during childhood.