Abstract
4 Twin studies are important for research on genetic and environmental influences on child 5 development, but it is imperative to test whether findings can be generalized from twins to 6 singletons. Since the first months of life are defined by the emergence of important sleep 7 behaviors, we compared 451 (54.8% females) twins and 77 singletons (48.1% females) on a 8 range of sleep, settle, and crying behaviors at five months of age. No significant differences 9 were found regarding duration of crying or time until settled. However, singletons were 10 reported to wake up more frequently during nighttime than twins (F(1, 509) = 35.10, p <.001, 11 ηp 2 = 0.065), suggesting that, when reported in twin studies, this measure might be slightly 12 underestimated at a mean level in relation to singletons. In conclusion, despite the unique 13 challenges and additional caregiver load of infant twins, there seem to be few differences 14 between twins and singletons regarding parent-reported settle and crying behaviors in early 15 infancy. 16 17