We examined two alternative explanations, one demographic, the other sociological, for the uneven distribution of birth months of Major League baseball (MLB) players active between 1880 and 1999. Beginning in 1900, players born between August and October were significantly overrepresented, and this uneven distribution was almost identical for the next five 20-year periods. During the last 20-year period (1980-1999), the disparity in birth months became even more pronounced. Ethnicity, handedness, player position, accomplishment (winning an award), and career length were not significantly related to birth month. Prior to 1980, the distribution of births for MLB players did not differ significantly from the distribution for the general population, but after 1980, it did. We concluded that up until 1980, the uneven distribution of birth months in MLB originated in the demographic seasonality-related excess number of births in August and September in the United States. Beginning in the 1980s, this seasonality pattern was institutionally reinforced by the growing influence of Little League and related junior baseball leagues and their reliance on the August 1 birth date for age grouping.