Did Premodern Wars Impact Sex Ratios at Birth? The Case of 19th-Century Basque Country

This study examines the impact of pre-moder wars on sex ratios at birth. Specifically, it investigates whether wars involving premodern weaponry and more limited destruction increased stress for pregnant women such that the resulting hormonal changes in their bodies may have led to a decrease in the ratio of live-born boys to girls. To address this question, we analyzed all baptismal records from the Basque Country during the 19th century, encompassing nearly 1.2 million individuals. Our findings confirm that the First Carlist War (1833-1840) exercised the most significant demographic impact on the region. However, all wars resulted in variations in demographic behavior, reducing in sex ratios at birth by around 1%. This effect is even more pronounced when focusing on the main conflict zones, where sieges and major battles took place, leading to an impact of 3.2%. The effect was not only immediately visible in sex ratios at birth but became particularly evident during the first three months after the events. Furthermore, prolonged battles were found to have the most substantial influence, with an average effect during the three months after the battle of almost 7% on sex ratios at birth. In conclusion, our study demonstrates that premodern wars reduced sex ratios at birth.