Anyone with siblings knows they can differ from us in maddening ways. They share our parents and our family history, but their personalities can be so different. Birth order offers an intuitively appealing explanation for these perplexing differences.

The only problem is, it’s a myth.

Psychologists have speculated on the effects of birth order on personality for well over a century. Sir Francis Galton – pioneer of statistics, fingerprint analysis, weather maps and arithmetic by smell – supposed that firstborn children benefited from greater responsibility and undivided parental attention. As a result they were over-represented among high achievers.

Alfred Adler, protégé of Sigmund Freud, argued that the dethroning of firstborns by younger siblings left an enduring impression on their character.

Firstborns, he argued, feel weighed down by responsibility and have neurotic and authoritarian tendencies. Laterborn siblings are often overindulged and seek creative alternatives to conventional achievement.

Frank Sulloway’s Born to Rebel, published in 1996, made the strongest case for birth-order effects on personality. Referring to the popular big five personality traits, he proposed that firstborns tend to be more conscientiousness and neurotic than laterborns, are less agreeable and less open to new experiences. In essence, firstborns are anxious conservatives and laterborns are easygoing rebels.

Scouring the historical record, Sulloway found that laterborns were more likely than firstborns to support the French Revolution and the Protestant Reformation. They were also more likely to be at the vanguard of scientific revolutions, such as Darwin’s theory of evolution.

These links between personality and birth order ring true for many people. But decades of research have failed to show any consistent and substantial association between birth order and any personality trait.

Two studies published this month should drive the final nails into the coffin of birth-order effects.

In the first study, published today in the Proceedings of the National Academy of Science, researchers examined the big five traits (openness, conscientiousness, extroversion, agreeableness and neuroticism) in very large samples from the United States, Great Britain and Germany.

In every sample, there was no statistically reliable link between any trait and birth order, after controlling for factors such as gender, age and family size. Firstborns did not differ from laterborns, either when comparing siblings from different families or within the same family.

The second study examined the big five traits in 377,000 American high school students.

After statistically controlling for gender, age, family size, socioeconomic status and family structure, associations between personality and birth order were uniformly tiny.

The trivially small effects they found also contradicted common beliefs about birth-order effects. Firstborns were very slightly more conscientious than laterborns, but they were also very slightly more agreeable and less neurotic, contrary to expectation.

If the evidence for birth-order effects on personality is so flimsy, why do people continue to believe in them? This belief is a classic example of what psychologists call “illusory correlation”: the conviction that two things are associated when they are not.

One reason for this illusory belief is that birth order is confounded with age. Any differences in sibling personalities may simply reflect firstborns’ greater maturity.

Conscientiousness, for instance, increases over the course of childhood development. So, at any given time, firstborn children will tend to be more conscientious than their laterborn siblings.

A second reason for the illusory correlation involves birth-order stereotypes. People who are aware of common beliefs about birth order will bias their perceptions to confirm their expectations, even in the absence of supportive evidence.

This dynamic accounts for supposed correlations between astrological star signs and personality traits. Some weak associations exist, but only among people who are aware of the traits associated with their sign. These people perceive their personalities through the distorting lens of their astrological expectations.

The third reason for illusory correlations between personality and birth order is overgeneralisation. Birth order may indeed be associated with differences in behaviour in the context of early family life.

Older siblings may tend to be more dominant and responsible; young ones to be more indulged and free-spirited. However, differences in specific roles within the narrow confines of the childhood family environment do not generalise to broad, enduring personality traits in the big wide world of adult life.

But while birth-order effects on personality are illusory, it is now generally accepted that birth order influences IQ. Both studies mentioned earlier support this link.

On average, laterborn children are somewhat less intelligent than firstborns. Six times out of ten, the second of a pair of siblings will score lower on IQ than the first.

Birth-order effects may also extend to physical health. A recent study of more than 200,000 Swedish military conscripts found that firstborns have somewhat greater cardiovascular fitness than laterborns.

Another study of more than one million Swedes found firstborns were significantly less likely to die prematurely, especially of accidents and suicide.

Birth order clearly matters, just not for personality. Siblings loom large in our lives, and the extent of their individuality can be striking. Their differences cry out for an explanation, which unfounded ideas about birth order provide.

Nick will be on hand for an Author Q&A between 10 and 11am AEDT on Wednesday, October 21, 2015. Post your questions in the comments section below.