Nonshared Environmental Influences on Individual Differences in Early Behavioral Development: A Monozygotic Twin Differences Study

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Abstract
The monozygotic (MZ) twin differences method was used to investigate nonshared environmental (NSE) influences independent of genetics. Four‐year‐old MZ twin pairs (N= 2,353) were assessed by their parents on 2 parenting measures (harsh parental discipline and negative parental feelings) and 4 behavioral measures (anxiety, prosocial behavior, hyperactivity, and conduct problems). Within‐pair differences in parenting correlated significantly with MZ differences in behavior, with an average effect size of 3%. For the extreme 10% of the parenting‐discordant and behavior‐discordant distributions, the average NSE effect size was substantially greater (11%), suggesting a stronger NSE relationship for more discordant twins. NSE relationships were also stronger in higher risk environments, that is, families with lower socioeconomic status, greater family chaos, or greater maternal depression.