For most of my life I’ve heard and read that personality is greatly affected by birth order. As a first-born, I could expect to be ambitious, self-confident, outgoing and a natural leader. My middle brother is supposed to be social, a peace-keeper and fairness-obsessed. And then there’s the baby of the family, my youngest brother, who by birth order should be a risk-taker, charming and a free spirit. You’ve probably heard similar categorizations in terms of birth order.

These birth order personality traits actually describe the three of us pretty well. Thinking about your own family and reading various descriptions of birth order characteristics you may agree that birth order shapes who we are. However, the order of one’s birth has very little – meaning almost zero – effect on personality.

Birth Order Has Almost Zero Effect on Personality

In the past five years, two definitive studies have shown there is almost zero birth order effect.

In the first study nearly 400,000 high school students were tested regarding intelligence and various personality factors and analyzed as compared to their family size and birth order. The study concluded that “overall, the average absolute association between birth order and personality traits was .02, whereas the one between birth order and intelligence was .04.” Note that a .02 and .04 association is nearly zero.

The second study was summarized in an article in the Proceedings of the National Academy of Sciences as follows: “The results show that birth order has null effects on personality across the board, with the exception of intelligence and self-reported intellect, where firstborns have slightly higher scores.”

Further, when the two studies are taken together, the article concluded that “when combined, the two studies provide definitive evidence that birth order has little or no substantive relation to personality trait development and a minuscule relation to the development of intelligence.”

Prof. Stefan Schnukle, of Leipzig University, an author of the second study stated:

“The real news of our study is that we found no substantial effects of birth order on any of the personality dimensions we examined. This does not only contradict prominent psychological theories, but also goes against the intuition of many people.”

Why Does It Seem That Birth Order Affects Personality?

Popular books and articles discussing birth order effects were based on older studies that were flawed or inconclusive. For example, as discussed by Dr. Joshua Hartshorne in Scientific American, an example of data in a study might be that 21 of the first 23 astronauts in space were first born children. Based on that astronaut statistic it seems reasonable to conclude that first-borns are more driven and likely to be successful! However, instead of birth-order being the cause of first-borns more likely to be astronauts, it could possibly be explained by household size as explained below:

Put simply, birth order is intricately linked to family size. A child from a two-kid family has a 50 percent chance of being a firstborn, whereas a child from a five-kid family has only a 20 percent chance of being a firstborn. So the fact that astronauts are disproportionately firstborns, for example, could merely show that they come from smaller families—not that firstborns have any particularly astronautic qualities . . . More children mean that parental resources (money, time and attention) have to be spread more thinly. Perhaps more telling, family size is associated with many important social factors, such as ethnicity, education and wealth. For example, wealthier, better-educated parents typically have fewer children. If astronauts are more likely to have well-educated, comfortable parents, then they are also more likely to come from a smaller family and thus are more likely to be a firstborn.

Many of the studies that led to the books and articles that created the birth-order myth suffered from the type of problem illustrated by the astronaut example above or suffered from small sample sizes. The article referenced above from the Proceedings of the National Academy of Sciences digs into the flaws in the prior studies.

Why does birth order effect seem so real? One explanation is that we tend to believe in things like the birth-order effect due to the Forer Effect which also explains astrology and some personality tests. IFOD on Forer Effect here

Birth Order and Intelligence

There is a slight birth order effect on intelligence with later-born children having slightly lower levels of measured intelligence on average. Why? The leading theory summarized in Medical News Today is “that family environment influences the variations in intelligence. While there are more adults and fewer children in the home, the overall intellectual environment is richer, but the arrival of younger children dilutes it. The ratio of adults to children changes, and those born later have less undivided attention from the parents.”

Note that while overall birth order and intelligence has a slight relationship, this pattern is true only in very large samples – it is not a hard and fast rule on the individual level as 40% of later-born children are more intelligent than their older siblings.