Oldest siblings tend to make more money. Researchers have a new theory that explains why.

• An NBER working paper found that older siblings tend to make more money than their younger siblings. 
• Higher hospitalization rates among younger siblings could impact their future earnings. 
• Prior research has documented a similar trend in siblings' earnings and outcomes.

For years, researchers have found that the eldest child tends to earn more money and perform better on cognitive tests than their younger siblings. However, experts have disagreed on what factors are the biggest drivers of this trend.