Abstract A cross-sectional twin design was used to study the developmental nature of genetic and environmental influences on morningness–eveningness (M–E). A total of 977 South Korean twin pairs aged 9–23 years completed 13 items of a Korean version of the Composite Scale through the telephone interview. The total sample was split into three age groups: preadolescents, adolescents, and young adults. Twin correlations did not vary significantly with age, suggesting that genetic influences on M–E are stable throughout the developmental span. Results of model-fitting analyses indicated that genetic and environmental factors explained, respectively, 45% and 55% of the variance in all three age groups. Environmental factors were primarily those factors that twins did not share as a consequence of their common rearing; family environmental factors in M–E were consistently near zero in all three age groups. The present study is the first to demonstrate genetic influences on M–E in preadolescent children as young as 9 years old. In spite of differences in culture and frequencies of genes between South Koreans and Caucasians, genetic and environmental influences on M–E found in the present sample were remarkably similar to those reported by previous studies on the basis of late adolescent and adult Caucasian twins.