A large sample of adult twins (1117 pairs), who were concordant for having had children were asked to report on their child-rearing practices. A 14-item version of the Parental Bonding Instrument (PBI) was used to assess rearing practices of parent twins. The two factors of Care and Overprotection, commonly found in other studies, were recovered from this analysis of the PBI's parent form. Model-fitting analyses indicate that human parental behavior is under significant genetic influence. Findings further suggest that this influence is sex limited, with a higher heritability in mothers than in fathers, and that it may result partly from the expression of dominant genes. For both PBI factors and both parents, the best-fitting models invariably assumed sex-limited genetic effects and unique environmental influences only. Broad heritability ranged from 19% (father overprotection) to 39% (mother care). These results are interpreted in the broader perspective of gene-culture theory.