What The Number Of Siblings You Have Says About You, According To Science

Have you ever thought about how the number of siblings you have affects you? It’s common to hear negative stereotypes about kids who are only children, but what about those of us who have one, two, or three siblings? What can our siblings — specifically, the number of siblings we have — predict about our lives?

We've all heard about the influence siblings might have on our personalities when it comes to birth order. Those stereotypes are so common, they’re easy to rattle off: the oldest child tends to be the authoritative, controlling one; the middle child is the overlooked quiet one; the youngest child is the spoiled baby. (I have an older brother and two younger sisters, and yes, there may be some truth to this.) But while these birth order theories are mostly based on how parents treat their children — parents pay a lot of attention to their first-born, are more lax with the middle child, and then baby the last-born — sibling number research focuses primarily on how your relationship with your siblings themselves affects you.

So what does science have to say about family sizes? While there isn't as much research into sibling number as there is about birth order, what we know so far about the influences of your sibling number is pretty interesting. Let's take a closer look.

If You Have No Siblings...

Pro: You’re More Likely To Be Confident And Smart

A 20-year study on about 3,000 high school students found that only children are more confident than other children, and with good reason: they had higher IQs than children with just one sibling and better vocabularies than children from families with seven or more kids. With that winning combination of confidence and intelligence, only children are also more likely to pursue a college education and get medical and law degrees than other kids.

Con: You’re More Likely To Get A Divorce

While most of the negative stereotypes about only children have been debunked, there are, of course, some negative side effects to growing up without siblings. According to a study conducted by Ohio State University, one of these is that only children are more likely to get divorced than children who have more than one sibling. Doug Downey, co-author of the study and a sociology professor at the university, says that “[G]rowing up in a family with siblings, you develop a set of skills for negotiating both negative and positive interactions.” Downey added that these skills are a “good foundation for adult relationships, including marriage.”

If You Have Just One Or Two Siblings...

Pro: You’re More Altruistic And Sympathetic

This only holds true if the relationship between siblings is a healthy and affectionate one. Fights and sibling rivalry are unavoidable (again, I’m speaking from personal experience here), but as long as the overall relationship is positive, siblings are good influences — or so says the study conducted by Brigham Young University. Jim Harper, a professor at the university and researcher found that “Having a sibling you can count on seems to make a difference especially for prosocial behavior.”

Prosocial behavior is behavior that's meant to benefit others, like donating and volunteering. Harper found that having even just one sibling can help make you a more considerate and giving person. The study also found that sisters especially help promote their sibling’s mental health. Does my brother know how lucky he is to have three sisters? (You’re welcome, bro.)

Con: You’re More Susceptible To Your Sibling’s Negative Habits

Looks like bad behavior is contagious. Katherine Jewsbury Conger, an associate professor at the University of California-Davis, found that having siblings who engage in “delinquency” puts you at a higher risk of engaging in the same activities. And according to Patricia East, a researcher at the University of California San Diego School of Medicine, girls with sisters who are teenage moms are four times as likely to also have babies while they are teenagers.

If You Have Three or More Siblings...

Pro: You’re More Likely To Stay Healthy And Married

Each additional sibling you have reduces your chance of divorce by two percent, an Ohio State University study found. (In case you were curious, this benefit seems to cap at about seven siblings.)

Another study from the University of Cincinnati found that having lots of siblings is also good for your immune system: The more siblings an infant has, the more they're exposed to infections that can boost their immune system and make them better equipped to handle allergies and illness in the future. Though, since only one of us doesn’t suffer from terrible allergies and asthma, I’m pretty sure my youngest sister is the only one who benefited from this. (You’re welcome, sis).

Con: You Have A Higher Chance Of Developing Brain Tumors

In order to find the connection between the number of siblings a person has and brain tumor development, researchers looked through the family histories of over 13,000 people with brain tumors and discovered that “people with four or more siblings were twice as likely to have developed brain cancers" than only children. The study's findings suggest that a type of polyomavirus could be involved in the development of brain cancers. (A polyomavirus is a virus that is latent, but that can cause illnesses in times of immune compromise.) But even Andrea Altieri, an epidemiologist who led the study, stated that researchers still know "very little about why people develop brain tumors," so there's still more to understand about this link.

At least we know for sure now that siblings are both a gift and a curse — even when you don't have any. But we didn't need science to tell us that.

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