A recent study by a law professor and a business professor has found that birth order has a small effect on whether someone eventually becomes a CEO: It found that, out of a sample of 650 CEOs, 47 percent are the oldest of their siblings, versus 35 percent of the general population who are the oldest siblings. CEOs were also found, on average, to have 2.2 siblings and come from an upper or middle upper class family background. 

Why are more CEOs eldest children? One possibility raised by the authors is that  previous studies have found that firstborn children tend to be more professionally successful than their younger siblings. Statistically they attain higher educational levels, higher earnings and even higher IQs. 

"Under this account, there are more firstborn CEOs because of greater familial resource allocations, rather than any inherent personality differences. In other words, firstborns are more likely to become CEOs because they are smarter, more capable, and better educated than laterborns on average, meaning there is a larger pool of talent from which to draw," said the report. 

The researchers found that birth order had a small effect on CEO behavior, though much more predictive was socioeconomic status growing up. Overall, the researchers found that firstborn CEOs from higher class strata tend to prefer safer investment and leverage policies, which lead to a lower firm value. 

"In summary, early childhood environment plays an important role in personality development, which has a strong life-long effect on preferences across multiple domains and plays a role in CEO management style formation and decision making," said the report.