Birth Month
How common your birth month is, relative to the yearly average. August has about 9% more births than an average month and February about 8% fewer (US data, 2000 to 2020).
Post-holiday conceptions dip slightly
January-borns are
-4%
less common than the monthly average
Birth frequency by month (relative to average)
The relative age effect
January-borns are among the most over-represented in elite sports due to the relative age effect.
Notable January-borns
In context
The relative age effect is one of the better replicated findings in sports science. In any age-grouped programme with a fixed cutoff, the children born just after it are nearly a year older than the youngest in the group: bigger, stronger, better coordinated. They get picked, get coached, and the gap compounds. Barnsley, Thompson and Barnsley (1985) found about 40% of NHL players born in the first quarter of the year and 15% in the last. The physical gap closes in adulthood; the career head start does not.
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About this test
Pick your birth month. The module shows how common it is relative to the average month in the US, from CDC vital statistics averaged over 2000 to 2020, and notes the relative age effect: where your month lands in school and sports cut-offs.
The spread is small but consistent. August is the busiest month at 9% above average, September 8% above, July 6% above. February is the quietest at 8% below, partly because it is short. The percentile shown is your month's frequency as a share of August's.
What is the most common birth month?
August in the US, at about 9% more births than an average month, followed by September (8% above) and July (6% above). Those are November, December and October conceptions. Source: CDC/NCHS National Vital Statistics, 2000 to 2020.
What is the rarest birth month?
February, about 8% below average, and part of that is simply having 28 days. Per day, the gap is smaller. December (5% below) and January (4% below) also run under the average.
Does birth month matter?
A little, through the relative age effect. Children born just after a school or sports cut-off are the oldest in their cohort and are over-represented in elite sport (Barnsley et al., 1985; Cobley et al., 2009). The effect fades in adulthood.
Percentiles above come from the same parameters the Birth Month module uses. See the methodology for how they are calculated.