Two psychologists, one from Brock University and the other from the University of Calgary in Canada, have found evidence showing that middle children with several siblings are more honest and cooperative than other siblings and children who grow up without siblings.

In their paper published in the journal Proceedings of the National Academy of Sciences, Michael Ashton and Kibeom Lee wrote how they analyzed data from thousands of individuals who had completed profiles on a personality website and shared the findings they discovered through this research.

Studies on whether birth order affects personality have had mixed results. Some researchers have found patterns, while others have not, and some have disagreed with each other’s findings. Because of this, it's unclear if such patterns exist. In this new study, the researchers took a different approach by looking at personality differences between children with different numbers of siblings and those who grew up as only children.

Before this new study, the researchers developed a system that categorizes six key personality traits: Honesty-Humility, Emotionality, Extraversion, Agreeableness (versus anger), Conscientiousness, and Openness to Experience (HEXACO). They also created a website where visitors could take a HEXACO personality test. The new study analyzed data from 710,797 people who visited the site and completed the inventory.

The researchers found that middle children (those with both older and younger siblings) scored the highest in honesty-humility and agreeableness. Next were the youngest siblings, followed by the oldest, and then only children. They also discovered that the more siblings a person had, the higher they scored on these two traits.

The researchers suggest that their results make sense: having more siblings means more compromises, which leads to greater agreeableness. It also encourages honesty, as it becomes harder to get away with lies in larger families.