mythbuster graphic The implications of birth order are a popular topic of debate because birth order is a concrete, measurable, stable, and uncontrollable aspect of family life that can be used as an explanation for behavior and performance. For years, researchers claimed that older children were more intelligent than their younger siblings. Such conclusions were based mainly on cross-sectional data, which compared children from different families across a population.

More recent studies have examined children within the same families and followed siblings over time. Using this more accurate method, researchers have found that birth order does not affect intelligence, and that differences in intelligence observed in previous trials are most likely due to external factors such as parents’ intelligence or economic disadvantages more often faced by larger families.