September babies score higher: birth month tilts the iq scale, study finds
A one-month head start in kindergarten can still echo at 25. Children born in September—not earlier, not later—outperform every other birth cohort from first grade to graduation, according to a 1.2-million-student data set quietly released by the National Bureau of Economic Research.
The age gap that never closes
Florida public schools keep birthday cut-offs tight: turn five by 1 September or wait another year. That rule creates a 364-day spread within the same classroom. The September-born sit on the elder side, August-born on the youngest. The gap feels trivial to parents; to statisticians it is a chasm. Elizabeth Dhuey and David Figlio tracked math and reading scores from age six to fifteen. The elder group carried a 0.2 standard-deviance edge—roughly the difference between a B+ and an A–—every single year. Multiply by twelve grades and the cumulative advantage becomes the equivalent of an extra semester of learning.
Teachers rarely hold back the older kids; they simply look more “ready.” Ready children get picked for gifted tracks, pull-out labs, and magnet programs. Once inside the fast lane, acceleration compounds. By age fifteen the September cohort is 4.2 percentage points more likely to enroll in advanced math and 2.8 points less likely to see the inside of a juvenile detention center. The researchers ran sibling fixed-effects models to be sure: same parents, same zip code, different birth month—same pattern.
A gift that keeps on giving
The edge does not fade with puberty. Early access to rigorous courses predicts college application rates, and college completion later feeds into lifetime earnings. A back-of-the-envelope calculation by the same team values the September premium at an extra $1,350 in annual salary per year of career, discounted to present value. Over forty years that is a mid-five-figure birthday present signed by the calendar.
Parents of August babies scramble for redress: red-shirt the child, start kindergarten at six instead of five. Florida districts report a 300 % surge in delayed-entry requests since the working paper first circulated. Yet the paper warns the tactic only pays if the whole class does it; otherwise the late-starter simply becomes the new youngest. Collective action problems rarely end well for individual families.
Policy, not panic
Dhuey argues the fix is structural. Splitting each grade into quarterly cohorts, or rotating gifted screening twice a year, would dilute the age effect without parental arms races. Until then, the calendar remains the cheapest aptitude test ever invented—administered at birth, graded for life.
The takeaway is blunt: where you sit on the September-August seesaw predicts more than playground dominance. It nudges test scores, course selection, and eventually tax brackets. Genius is still rare; relative maturity is just common enough to masquerade as it.