Editors' Summary: Mary Jane Kellum uses the sociobiological theory developed by Trivers and Willard (1973) in her discussion of the variations in secondary sex ratios (sex ratios at birth) in human groups. This theory suggests that parents 'manipulated' the sex ratios of their offspring in ways which maximize parental genetic contribution to the succeeding generation. Kellum examines United States census data and finds that it appears to conform with predictions based on this model. Obviously parental 'manipulation' of secondary sex ratios cannot be conscious, but must be based on physiological responses to environmental variables. Kellum discusses the proximate mechanisms which have been identified as contributing to variance in human sex ratios and concludes that only some of these would produce the effects predicted by the Trivers-Willard model. Some of these proximate mechanisms are also considered in the final chapter by Michael Little.