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  • Researchers have tried for decades to generate evidence to support birth-order hypotheses—without much success. Tweet This
  • Birth order is like astrology or the Myers-Briggs Type Index: fun to read about, but not something to take seriously. Tweet This

I recently received an email from a mom who was agonizing over whether she and her husband should have a third child. She was concerned that her present “baby” would become the “middle child” and suffer from middle child syndrome once a new sibling arrived. As she wrote, “When I told someone what we’re thinking, they immediately said with horror that ‘poor Sophie will be a middle child!’ What does that mean? I never thought of it being a problem, so now I’m wondering what all the fuss is about.”

Birth order proponents have argued for decades that firstborn kids are leaders, who are responsible and intelligent. They say middle children feel left out and isolated, are laidback, and learn to negotiate well. Some argue that evidence shows they’ll likely move far from home to get out of their older sibling’s shadow. And last-borns? According to birth order researchers, they can be seen as a little spoiled and manipulative. And they’re seen as not being too disciplined.

Researchers have been trying to generate evidence to support birth-order hypotheses for decades. But the evidence they’ve gathered is poor, lacks reliability, isn’t easily replicated, possesses significant methodological challenges (to put it politely), and generally doesn’t stack up to anything meaningful at all. Here’s a snapshot of their findings in a number of areas.

Early communicationResearch indicates that there are birth order differences, but they’re so small as to be essentially meaningless, accounting for less than one percent of the overall variance among the study’s children.

Intelligence: The data show a miniscule difference in intelligence (measured by IQ). A Norwegian study of more than 240,000 children focused exclusively on males and found an IQ difference of just three points from firstborn to last-born—in other words, nothing. A study of nearly 300,000 Taiwanese students showed a relationship of both gender and birth order on intelligence, but it seems that parental income is the critical issue on test scores, rather than birth order. Other researchers have discredited studies promoting the idea that birth order influences intelligence.

Mental Health: A recent study linked birth order with conduct and emotional disorders. However, according to the same study, family structure, gender and the number of children also contribute to risk—and the sample did not come from the general population. Several other studies (like this one) do not implicate birth order in psychosis and mental illness—including ADHD—at all.

Personality: In this study, children’s personalities were assessed with no evidence found that birth order was related to personality. This is consistent with several other studies that provide compelling evidence comparing personality and birth order both within and between families.

Goals: One area that shows some support for birth order effects is in the kinds of goals we set for ourselves. Research indicates firstborn children are strongly mastery-oriented and are willing to learn, while later-born children prefer to prove what they can do by setting performance goals.

Risky behavior: In this area, findings are mixed, but once again, when birth order is implicated in risky behavior, it tends to mean very little. Some children may be more or less risk-averse based on their birth order, but other factors such as gender, siblings, etc., can impact whether the birth order effect exists. (For examples, see here and here.) One exception may be that middle- and last-borns may take more risks with alcohol and sex. However, this may be less due to birth order than parenting style: parents may become more relaxed with later-born than firstborn children.

Dozens of other categories have also been investigated in relation to birth order. Sometimes evidence supports a birth order effect. In fact, for many of the categories described above, there will be at least one study to support the birth order proponents. Many of those studies that support birth order effects are based on old evidence and less rigorous, less valid scientific methods. But the weight of evidence does not.

And at the risk of getting all scientific, studies that compare “across” or “between” families really aren’t valid because of huge differences within families. We need research within families to really get the data we need—and that data tends to dissuade us from believing the birth order hype.

In short, there may be small effects of birth order on some aspects of who we become. But research suggests our choice of friends, the influence of our parents and (in later life, partners), and a wide variety of other factors appear to have a much more significant impact on who we become.

Birth order is like astrology or the Myers-Briggs Type Index. By all means, read about it, do the surveys, and laugh about how well the description fits you. If you look for similarities, you’ll find them.

But don’t take it too seriously. And don’t let it define you—or your kids.

Dr. Justin Coulson is the author of What Your Child Needs From You: Creating a Connected Family. He and his wife have six children. Find him on Facebook.