Comrades and Cause: Peer Influence on West Point Cadets' Civil War Allegiances

Do social networks and peer influence shape major life decisions in highly polarized settings? We explore this question by examining how peers influenced the allegiances of West Point cadets during the American Civil War. Leveraging quasi-random variations in the proportion of cadets from Free States, we analyze how these differences affected cadets' decisions about which army to join. We have four main findings. First, there was a strong and significant peer effect: a higher proportion of classmates from Free States significantly increased the likelihood that cadets from Slave States joined the Union Army. Second, the peer effect interacted with geography and economic circumstances: almost all cadets from Free States joined the Union Army (if they decided to join the war), and most cadets from Slave States that had more than a third of their population in slavery joined the Confederacy. The cadets who were most highly influenced were from Slave States that had less than a third of their population in slavery. Third, we analyze how cadets' decisions affected their military rank and career outcomes. Fourth, we show that having served together in the Mexican-American war increased the peer influence, providing additional evidence that peer interaction was influential.