A relative age effect in men's but not women's professional baseball: 1943-1954.

Birthdates of professional female and male baseball players active from 1943 to 1954 (the beginning and final years for professional female leagues) were matched for year of birth and league years and then compared by birth quarters, with a cutoff date beginning August 1. A relative age effect was noted for males, as there was a significantly different distribution of players across birth quarter, but there was no such effect for female professionals. Since players were matched for birth year and league play, the difference was unlikely to be due to seasonality differences in birth. Instead, the absence of a "relative age" effect for female players can be attributed to the absence of organized adolescent baseball for girls prior to the establishment of professional baseball leagues for women.