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The authors tested the proposition that among humans (1) differences in lactational duration result in differences in costs of reproduction even under rich nutritional conditions; and (2) elimination of factors postulated to favor male-biased parental care will be reflected in elimination or reversal of sex-biased care. To do so, the authors examined the relationship between lactactational duration and fertility among Hutterites, a communal-living human population in which the levels of nutritional resources and fertility are high, breast feeding is the norm, contraceptive use is limited, and the collective social and economic system results in low resource variance among individuals. The authors demonstrate that even under good nutritional conditions, duration of nursing was a significant predictor of the length of time to next pregnancy and that nursing continued to suppress fertility after the resumption of menses. Moreover, the authors find that daughters were nursed longer than sons, leading to a longer interval to next pregnancy. The authors examine this uncommon, but not unique, finding of female-biased human parental care in the light of Hutterite social structure, and they explore the consistency of this finding with the most applicable models of parental investment.