A prospective twin study of birth weight discordance and child problem behavior

Background: We investigated whether low birth weight constitutes a causal r isk factor for child problem behavior, using a variation of the co-twin con trol method. Methods: In a representative sample of 745 twin pairs (monozygotic: 324 pai rs), birth weight was recorded at birth and child problem behavior at mean age 10 years was measured with the Child Behaviour Checklist (CBCL). Results: Lower birth weight was a continuous risk factor for later child pr oblem behavior (adjusted regression coefficient over units of 500 g: beta = -.15, p =.046), and greater levels of within-pair CBCL discordance did not result in a reduced effect size. Greater within-pair birth weight discorda nce was associated with greater within-pair CBCL score discordance (beta =. 35, p < .001). This latter effect was similar in monozygotic (beta = .34, p = .005) and dizygotic twins (beta =.37, p =.003). Conclusions: The fact that (1) the effect size of the association between l ow birth weight and child problem behavior was not reduced in pairs with gr eater levels of CBCL discordance, and (2) similar effect sizes were found i n monozygotic and dizygotic twins for the within-pair association between b irth weight discordance and CBCL score discordance, suggests that the obser ved relationship between low birth weight and child problem behavior is not due to a shared environmental or genetic variable that influences both cha racteristics. Lower birth weight is a causal risk factor for child problem behavior, the effects of which may well extend into adulthood. (C) 2001 Soc iety of Biological Psychiatry.