Over the past 125 years, many studies have documented significant birth-order differences, indicating the formative influence of birth order on a person’s personality. For example, one study suggests that while parents may encourage first-born or only children to follow a more prestigious path, such as becoming a doctor or lawyer, they might take a more relaxed approach with later-born children, which can influence personality.

A 2007 survey of corporate leaders conducted by an international organisation of CEOs stated that 43% of CEOs were first-borns, 33% were middle-borns, and 23% were last-borns.

A US study of 67 firstborn and 99 later-born male college students found that, in general, later-borns are more likely to engage in dangerous sports than firstborns. The study compared the subjects’ preference for participation in sports such as judo, football, lacrosse, sky diving, ski jumping and motorcycle racing among other sports.

This reflects the theory that, in general, parents may be more anxious about the activities of an only child, while parents of several children may have a more relaxed approach.

A survey carried out in the US asked 76 pairs of 13 to 18 year old siblings to rate themselves on various personality aspects. It found that more younger siblings rated themselves as more sentimental and forgiving than their older siblings.

The same survey of 76 pairs of 13 to 18 year old siblings found that more older siblings tended to state that they were perfectionists.

In a US study of sibling relationships, researchers theorised that younger siblings are mentored by older siblings, feel the need to be more competitive and have more confidence in overcoming obstacles because they’ve watched their siblings face those same challenges.

Francis Galton did not count female children when reporting his results in  “English Men of Science: Their Nature and Nurture,” which was published in 1874. Galton considered a male subject a firstborn even if he had five older sisters.

Parental money and time are finite resources, and in larger families, these are spread thinner than in smaller families. Also, family size can be associated with several other variables, including ethnicity, education and financial concerns.