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Details of the population pyramid of living Herero and Mbanderu of Botswana suggest that infant and childhood mortality of males has been substantially greater than that of females. Direct tests from reproductive histories show that the hazard ratio is approximately 3 to 1 in favor of female survival in infancy and 2 to 1 in childhood. This biased mortality began in about 1960 in concert with recovery from infertility. A model of asymmetric fitness benefits between siblings is weakly supported. Logistic regression shows that heterogeneity among mothers explains much of the mortality of children under 2 years of age. A direct test for heterogeneity provides strong support for the hypothesis. Field methods appropriate for anthropologists are contrasted with those that are standard in demography. Finally, a contrast between these data and those from south Asia suggests that strong sex preferences may exist without being culturally articulated. Cultural norms do not necessarily coincide with behavior.