Objective: To determine the role of parental and biological factors on the U.S. male birth fraction from 1964 through 1988.
Design: Logistic regression on annual U.S. male births by race group.
Setting: Population-based data.
Patient(s): Live births in the United States 1964 through 1988.
Main outcome measure(s): Annual U.S. male birth fraction by parental and biological factors.
Result(s): During the study period, the annual U. S. male birth fraction showed changes based on race group, parental age, and low birth weight. The overall influence of parental age on the U.S. male birth fraction is strong and is stronger in nonwhites than in whites. The U.S. male birth fraction is also strongly influenced by the percentage of low birth weight infants in nonwhites, but not in whites. The male birth fraction declines with increasing age of either parent and with an increase in the percentage of low birth weight infants.
Conclusion(s): The relative magnitude of influences on the U.S. male birth fraction depend on the race group, which may be a reflection of the range of observed data rather than biological differences. The developed models have reasonable predictive power and are an appropriate first step in understanding the factors influencing the male birth fraction. These types of parental and biological variables should be included in models before examining other exogenous and population level variables.