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 were collected 1996-2004 from 851 sibling trios from the Twins Early Development Study (TEDS; 50.2% female; 93.2% White British). Distributions were comparable between twins and singletons across domains, indicative of mean rather than variance discrepancies. 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). Overall, twins and their singleton siblings differed in development across domains and their differences in language reversed during childhood.