Outliers: The Story of Success · chapter 1 · id outliers-c1-08-birth-seasonality-is-not-perfe

“Birth seasonality is not perfectly uniform across quarters -- actual birth distributions vary by country and era, with summer months typically having slightly higher birth rates, which weakens the 25%-per-quarter baseline Gladwell uses.”

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The Relative Age Effect in Sports: It's Complicated, World of Psychologysource alive

Birth rates are not evenly distributed across months. In many countries, summer and early fall months see slightly higher birth rates, meaning the expected distribution is not exactly 25% per quarter.

Buckles & Hungerman (2013), Demography 50(5)FLAGGED: DOI not in registry

Birth seasonality varies significantly -- winter births are more common among younger and less-educated mothers. Using uniform 25% per quarter as a baseline introduces systematic error.

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