State and regional differences in the male-to-female ratio at birth in the United States, 1995-2012

Abstract

Introduction: In humans, live male births slightly exceed females and this ratio is conventionally expressed as male live births divided by total live births (M/F). A wide variety of factors have been shown to influence M/F including latitude, stress, socio-economic status and race. This study was carried out in order to ascertain whether there are differences in M/F in different states and in different geographical regions in the United States of America (USA).

Methods: Annual live births by gender for the period 1995–2012 were obtained from the website of the US Centers for Disease Control and Prevention. These were grouped according to the four regions devised by the United States Census Bureau, Geography Division.

Results: This study encompassed 52 601 559 live births for the period 1995–2012 (M/F: 0.5117; 95% CL: 0.5116, 0.5118). Southern states tended to have a lower male-to-female ratio. Hawaii had a high M/F (p < 0.0001). The male-to-female ratio for the South region was significantly less (p = 0.004). This region had the highest proportion of Black mothers.

Conclusion: The high Hawaiian M/F is in keeping with Micronesian island findings. The M/F latitude gradient accords with that previously noted in the USA. Historically, Blacks have been shown to have a lower M/F than other races. Long-term stress related to maternal socio-economic status has also been shown to influence M/F, and Blacks are known to be disadvantaged to this day. It is possible that the low M/F historically noted in this race may be due to chronically poor socio-economic circumstances.