Mother’s report of twins’ birth weight was also obtained. Because twins are usually born 3-4 weeks premature and are, on average, 30% smaller than singleton births (Plomin, DeFries, McClearn, and Rutter, 1997), the established definitions of low birth weight (< 2500 g) and very low birth weight (< 1500 g) were not applied to this sample. Rather, low birth weight was defined by birth weights in the lowest 10th percentile of the distribution in this sample. This was equivalent to a birth weight of less than 1700 g (3.75 lb).
