How Much Do Our Siblings Shape Who We Are?

As any first-year psychology student can tell you, our personalities are determined by a complex combination of our genetics and our environment. Among the wide array of environmental influences on personality, one stands out as having long intrigued both researchers and the general public: our siblings.

Informal theories about how siblings (or lack of siblings) influence personality traits abound. Are only children more selfish? Do middle children feel overshadowed by their siblings? New research published in the journal Psychological Science examines a specific component of sibling influence: the gender of one’s siblings. Would you be a different person if you had a sister rather than a brother (or vice versa)? Results of this large-scale study suggest the answer is “not so much.”

It’s easy to understand why people might believe our siblings could have a big impact on who we are. We tend to spend a lot of time with them, and we do so primarily during childhood, when our personalities are most in flux. The authors of this new research considered two possible ways that the gender match between a person and their sibling(s) could shape personality.

The first is social learning theory. In basic terms, this theory focuses on how we model or mimic the behaviors of close others. If you had a sister who played with dolls, you might be more likely to play with dolls as well—regardless of your gender. If you had a brother who played aggressively, you might be more likely to do so too—again, regardless of your gender. Social learning theory would lead us to predict that a girl who has brothers would be less conventional when it comes to traditional, gendered behaviors. We would make the same prediction for a boy with sisters. Makes sense, right?

But we could just as easily tell a story with the opposite conclusion. The theory of sibling differentiation suggests that feelings of rivalry push siblings to distinguish themselves from their brothers and sisters. According to this theory, a boy with a sister might develop fewer stereotypically feminine traits (and vice versa for girls with brothers). If you’re trying to be distinct from your siblings, you reject their traits and interests and head in the opposite direction.

So which theory is correct? Evidence has been mixed, sometimes favoring one theory and sometimes the other. The authors of this new paper suggest two main reasons for these mixed findings.

First, the samples studied by previous researchers were often much too small or too narrow to allow for meaningful conclusions. Second, in previous studies, researchers used vastly different types of questions to assess the potential psychological impact of siblings on personality. Measures have ranged from asking about interests (Do you like hunting or handicrafts?) to focusing on economic outcomes (like wages or career achievements) to focusing on traditional personality traits (like extraversion or agreeableness).

The researchers leading this new project gathered data from 12 different surveys that included representative samples from nine different countries. In total, over 85,000 individuals between the ages of 10 and 60 were included in their analysis. When it came to sibling relationships, the researchers excluded cases in which the age gap between siblings was less than nine months (i.e., in the case of twins or other multiples) and cases in which the age gap between siblings was more than six years. Sibling relationships did not have to be biological; step and adopted siblings were included.

The authors focused on several of the most commonly measured personality traits. These included the Big Five traits as well as risk tolerance, trust, patience, and locus of control. To examine “gender typicality” (the tendency of someone to behave in ways traditionally associated with their gender), the researchers created an index that combined scores on the personality measures that showed the largest differences in scores between men and women.

Researchers cannot run a true experiment on the influence of sibling gender. After all, it would neither be possible nor ethical to randomly assign people to have a sister or a brother.

To get as close to an experimental design as possible, the researchers focused on individuals’ next younger sibling. They reasoned that when people decide to have an additional child, the gender of that child will, in nearly all cases, be random. This approach allowed the researchers to consider whether people who have a sister as their next youngest sibling differed from people who have a brother as their next youngest sibling.

So, what did these researchers find? Drumroll…

Not much at all. They sliced up and analyzed the data in numerous ways, but nothing they did revealed any strong evidence that your personality is shaped by the gender of your next younger sibling. It also didn’t matter what your birth order was, how many brothers or sisters you had, or how far apart in age you were from your sibling(s). None of these factors had much influence on your personality at all.

Normally, researchers might feel disappointed in an outcome that fails to support either of two competing theories. But in this case, not finding an influence of sibling gender is pretty interesting. Our stereotypes about the effects of things like birth order and the number or gender of our siblings tend to be strongly held. In this case, a rigorous scientific test suggested our hunches about the influence of our siblings are likely wrong.

Personality traits influence our academic and career outcomes, our close relationships, and our mental and physical health. It’s natural to wonder about the origins of these traits. But regardless of whether you love, hate, or simply tolerate your particular constellation of personality traits, this new research suggests that on average, your siblings bear none of the credit or the blame.

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