The idea that eldest siblings are more self-confident and natural leaders, while younger ones are always trying to make up for never having had their parents' sole attention is deeply ingrained in popular culture. However, it turns out that where we fit in the birth order of siblings is no more important to our personalities than the positions of the planets at the time of our birth.

The story goes firstborn children are confident and favor order, while younger ones' insecurities make them seek attention. Even people's politics, or their enthusiasm for new scientific theories, have been explained this way, with first children thought to uphold the established order, while younger siblings seek to overthrow it. The popular belief was formalized by psychologist Frank Sulloway.

Dr Tomás Lejarraga of the Universitat de les Illes Balears, Spain, looked at one particularly popular aspect of this idea – that younger siblings are more inclined to take risks to establish their place in the world. In Proceedings of the National Academy of Sciences, he tests the theory by asking a group of people to choose between risky and safe options. Lejarraga also examined birth order in a sample of a test that measures underlying propensity for risk-taking.

On most measures, birth order made no difference, and the few measures were it did were as likely to show younger siblings being more risk-averse as risk-prone. This is one myth that is definitely busted.

Lejarraga begins his paper with the story of the von Humbolt brothers who embody the myth. While the elder settled in his birthplace in a safe but boring job, Alexander “challenged established ideas, befriended world leaders, traveled to terra incognita, climbed the highest known peak of his time, and navigated the unchartered waters of the Amazonian forest.”

However, Lejarraga reveals, the reverse could equally be true. Among a sample of explorers of distant lands and prominent revolutionaries, younger siblings were not over-represented.

Early research on this topic supported Sulloway's theories of birth order's importance. However, such studies either used small sample sizes or had such glaring defects in their design (for example, comparing teenagers' personalities with younger children), it is amazing they were published. More recent work has found differences in intelligence and personality based on birth order to be so small as to be effectively irrelevant.

Lejarraga acknowledges there is evidence that younger siblings are more likely to be involved in accidents, particularly near drownings. However, this is when they are children, and seems to have less to do with risk-taking personalities than the danger of being around stronger, older children.