Siblings are both similar and different. When you look at your friends, you may notice family resemblances. Friends may look like their siblings, or sound like them, or even have similar interests. At the same time, they are unique individuals.
One particularly interesting aspect of the differences among siblings is that their position in the family may affect who they are. There has been a lot of interest in the ways that birth order may affect personality, achievement, and behavior. Research suggests that there are some systematic influences of whether you are a first-born or later-born sibling on aspects of your behavior.
In a 2001 paper in American Psychologist, Robert Zajonc reviewed a number of studies on effects of the order of birth on performance on tests of intelligence and academic ability. He focuses on data suggesting that both birth order and the size of a family affect academic performance. That is, first born children tend to do better on intelligence tests and tests of academic performance than later-born children. But, the larger the family, the worse that everyone in the family does overall.
There are lots of reasons why birth order might affect academic performance. Zajonc argues that when families have fewer kids, they can spend more time with each child, which tends to help them academically. In addition, older children help to teach younger ones, which tends to help them as well. If you have ever tried to teach someone, you know that you have to learn it well yourself in order to be helpful. So, older siblings end up learning material better just because they have to help their younger siblings.
Birth order also affects risky behavior. Frank Sulloway and Richard Zweigenhaft discuss this issue in a paper in a November, 2010 paper in Personality and Social Psychology Review. They looked at birth order and sports. First, they reviewed a number of studies looking at the kinds of sports that people end up engaging in. They found that older siblings are generally less likely to participate in risky contact sports than younger siblings.
Then, the authors analyzed major league baseball players. Over the years, there have been many sets of siblings who have both made it to the majors as baseball players. Overall, there was a tendency for younger siblings to be more successful overall than older siblings. Younger siblings tended to have longer careers and to play more games overall.
The biggest differences between older and younger siblings, though, were in risky behaviors. For example, one of the riskiest behaviors in baseball is the stolen base. Players try to move ahead on the base paths by running from one base to another while the pitch makes its way from the pitcher to the catcher. Once they were on base, younger siblings were far more likely to attempt to steal bases than their older siblings. Interestingly, younger siblings were also more likely to be hit by a pitch than their older siblings.
It wasn't that younger siblings were just better athletes, though. Older siblings tended to be better pitchers than younger siblings. Older siblings struck out more batters and walked fewer than their younger siblings. These differences were small and not statistically reliable. The important thing is that this result suggests that younger siblings are not just better at baseball overall.
Why do older and younger siblings differ? Psychologists have given many explanations for these differences, and chances are there are a lot of factors that contribute to the differences. One prominent explanation has to do with differences in family dynamics for older and younger siblings. Older siblings get a lot of their parents' attention. First children have a few years alone with their parents. After the first child, each child after that has to do something to get some attention from the rest of the family. The argument is that later children have to be more extraverted and more likely to take risks to get attention. And these sibling rivalries may continue into adulthood. The behaviors that people use to get the attention of their families as kids may continue to affect the way they act long after they leave home.
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