Season of Birth and Later Outcomes: Old Questions, New Answers

TLDR

Season of birth is linked to later outcomes, yet the mechanisms are unclear; prior work has focused on conception conditions, whereas this study points to birth‑time maternal characteristics as a key driver. The study tests whether variation in maternal characteristics explains season‑of‑birth effects. Maternal characteristics differ substantially by birth season—winter births are more often from teenage and unmarried mothers—and these differences account for roughly half of the season‑of‑birth association with adult outcomes, driven by planned conception and absent in unwanted births.

Abstract

Season of birth is associated with later outcomes; what drives this association remains unclear. We consider a new explanation: variation in maternal characteristics. We document large changes in maternal characteristics for births throughout the year; winter births are disproportionally realized by teenagers and the unmarried. Family background controls explain nearly half of season-of-birth's relation to adult outcomes. Seasonality in maternal characteristics is driven by women trying to conceive; we find no seasonality among unwanted births. Prior seasonality-in-fertility research focuses on conditions at conception; here expected conditions at birth drive variation in maternal characteristics while conditions at conception are unimportant.