Data Notes
Why do pre-1935 records look two-thirds female?
While analyzing 100+ years of American baby names, we noticed that... 67% of babies born in 1910 were girls?! That can't be right. The biological sex ratio at birth is approximately 105 males per 100 females, or about 51% male, and has been consistent across human populations throughout recorded history.[1][2][3][4]
Here's the key insight: The Social Security Administration wasn't established until August 14, 1935. The pre-1935 data is retroactively compiled from people who later applied for Social Security cards and reported their birth information.[5] This creates a significant sampling bias:
- People born in 1910 who lived long enough to apply for Social Security (starting in 1936-37)[6]
- People who actually applied (not everyone did immediately)
- People whose birth records could be verified
OK, but why were girls over-represented in early data rather than boys?
Our guesses:
- Women lived longer: In 1910, life expectancy at birth was approximately 50 years for white males and 54 years for white females.[7] By the time Social Security launched in 1935-37, more women from the 1910 birth cohort were still alive to register. This survival advantage compounds over the 25+ year gap between birth and registration.
- Marriage and documentation: Women who married often needed birth certificates to document name changes for legal purposes.[8] This may have prompted more complete registration of female births when the SSA system launched.
- Workforce participation patterns: Social Security was initially tied to employment in covered occupations. In the 1930s-40s, men were more likely to work in agriculture and domestic service—occupations that were initially excluded from Social Security coverage.[9] This could have affected registration timing and completeness.
- State birth registration practices: Birth registration wasn't standardized nationally until all states joined the Birth Registration Area by 1933.[10] Before then, registration practices varied widely by state, region, and possibly by sex of the child.
How this affects our work at nobadnames.com
We'll be cautious with pre-WWII data. However, the early name counts can still still useful: Even if the sample is biased, trends in which names were popular can still be informative. Additionally, we'll use metrics scaled by sex in some of our analyses.
Thanks for reading!
References
[1] Grech, Victor. "The Sex Ratio at Birth." Early Human Development, vol. 91, no. 12, 2015, pp. 817-822. https://doi.org/10.1016/j.earlhumdev.2015.10.009
[2] Mathews, T.J., and Brady E. Hamilton. "Trend Analysis of the Sex Ratio at Birth in the United States." National Vital Statistics Reports, vol. 53, no. 20, CDC, 2005. https://www.cdc.gov/nchs/data/nvsr/nvsr53/nvsr53_20.pdf
[3] Orzack, Steven Hecht, et al. "The Human Sex Ratio from Conception to Birth." PNAS, vol. 112, no. 16, 2015, pp. E2102-E2111. https://doi.org/10.1073/pnas.1416546112
[4] Catalano, Ralph, et al. "Exogenous Shocks to the Human Sex Ratio: The Case of September 11, 2001 in New York City." Human Reproduction, vol. 21, no. 12, 2006, pp. 3127-3131.
[5] Social Security Administration. "Background Information for the Social Security Area Population." https://www.ssa.gov/oact/babynames/background.html
[6] The first monthly Social Security benefits were paid beginning in January 1940, though lump-sum payments began in 1937. Social Security Administration. "Social Security History: FAQ." https://www.ssa.gov/history/hfaq.html
[7] Arias, Elizabeth. "United States Life Tables, 2003." National Vital Statistics Reports, vol. 54, no. 14, CDC, 2006. Table 12. https://www.cdc.gov/nchs/data/nvsr/nvsr54/nvsr54_14.pdf
[8] Hacker, J. David. "New Estimates of Census Coverage in the United States, 1850-1930." Social Science History, vol. 37, no. 1, 2013, pp. 71-101.
[9] DeWitt, Larry. "The Decision to Exclude Agricultural and Domestic Workers from the 1935 Social Security Act." Social Security Bulletin, vol. 70, no. 4, 2010. https://www.ssa.gov/policy/docs/ssb/v70n4/v70n4p49.html
[10] National Center for Health Statistics. "History and Organization of the Vital Statistics System." CDC. https://www.cdc.gov/nchs/nvss/about_nvss.htm