If you believe that the birth order of siblings is a major factor in personality development, you’re not alone. Shelves of popular books have been written on the topic, with titles like The Birth Order Advantage and Born to Rebel and The Birth Order Book: Why You Are the Way You Are.
This is a pop psychology topic that’s sold a lot of books and filled a lot of talk show time, and many have assumed that it’s solid science. But according to the results of the largest study ever conducted on birth order and personality, it’s all much ado about nothing.
Researchers studied 377,000 high school students to find out how much birth order affected their personality development and intelligence. They found that firstborns do have slightly higher IQs than their later-born siblings, but only one point higher – a statistically significant but practically meaningless difference.
Firstborns also tend to score higher on certain personality traits like extroversion, agreeableness and conscientiousness, but the differences between their scores and those of later-borns are, according to the researchers, “infinitesimally small.”
Overall, the association between birth order and personality was statistically .02, which is well below the level of perception.
According to study co-author Brent Roberts, professor of psychology at the University of Illinois, “In some cases, if a drug saves 10 out of 10,000 lives, for example, small effects can be profound. But in terms of personality traits and how you rate them, a .02 correlation doesn’t get you anything of note. You are not going to be able to see it with the naked eye. You’re not going to be able to sit two people down next to each other and see the differences between them. It’s not noticeable by anybody.”
The study controlled for factors such as a family’s economic status and number and ages of the children. Wealthier families with fewer children are able to devote more resources to them, which in turn influences personality development and intelligence.
Previous birth order studies have relied on much smaller sample sizes and have tended to compare siblings within specific families. This study also examined a subset of children living within families and the results confirmed those of the larger study – differences between siblings were slight if any.
This may be a good example of a pop psychology topic becoming so influential that, over the years, it has affected how parents interact with their children. If parents begin with an assumption that their first born will be smarter and have a stronger personality than his or her siblings, that assumption can color how all of the children are treated. The assumption influences the outcome.
According to the authors of the latest study, it’s time to rethink those assumptions.
Quoting study co-author Rodica Damian, “The message of this study is that birth order probably should not influence your parenting, because it’s not meaningfully related to your kid’s personality or IQ.”
The study will be published in the Journal of Research in Personality.