The purpose of this study was to clarify the genetic contribution to finger-sucking and nail- biting in childhood using the largest databases available on Japanese twins. The subjects were 1131 pairs of 12-year-old twin children, consisting of 1057 males and 1205 females. All data were gathered by maternal questionnaire, and responses to the questionnaire were checked in the medical interview. The prevalence of finger-sucking between 0 to 2 years was 40% in males and 43% in females (p = .0053). The prevalence of nail-biting up until the age of 12 years was 28% in males and 26% in females (nonsignificant). Concordance rates and polychoric correlations were all higher in monozygotic pairs than in dizygotic pairs, irrespective of the sex combination. Univariate and bivariate genetic analyses using structural equation modeling was performed. The results showed that the proportion of total phenotypic variance attributable to genetic influences was 66% in males and 50% in females for finger-sucking, and 50% in both males and females for nail-biting. A co-occurrence of finger-sucking and nail-biting was observed in 17.7% of males (tetrachoric correlation: r = .40) and 15.7% of females (r = .32), which was attributed partly to common genetic or environmental factors. The proportion of total shared variance explained by genetic factors was 67%.