Factors influencing the sex ratio at birth in the United States from a historical perspective

Abstract

Many of the studies on the sex ratio at birth (SRB) are based on a small number of cases over a short period of time. Taking a multivariate approach to a dataset consisting of nearly 199 million birth records in the United States from 1968 to 2019, we present a detailed analysis of several possible factors that might affect the sex ratio at birth (SRB) and its patterns of variation. We found that race/ethnicity is the variable with the strongest influence on this index. Parental age, birth order and solar radiation also have a bearing on the SRB, albeit only to a moderate degree. The historical evolution of the SRB among Black and American Indian and Alaska Native populations remains unexplained.

Keywords: Sex ratio at birth; United States; differences in skin tone; ethnicity; father’s age; mother’s age; race; solar radiation; standardization; vitamin D.