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Sex ratios at birth vary significantly among human races, from about 0.502 (Negroid) to 0.514 (Caucasoid) to 0.518 or higher (Mongoloid). Because a race-specific factor affecting sex ratio could be expressed by either parent (or both), an evaluation of interracial births was undertaken to assess the relative contribution of maternal and paternal effects. Live birth records for the United States during 1975 were sorted by offspring sex, live-birth order and race. Logistic regression and chi-square comparisons show a clear trend toward a paternal effect (by paternal race and age) on sex ratio in 31,837 interracial births. There was no suggestion of a maternal-race effect, or that simply averaging the two relevant parental race sex ratios would produce a particular interracial sex ratio.