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There is a popular belief that the behavior of children in a family depends a lot on their . First-born children are supposed to be fairly conformist, because they do not have to compete for their parents’ and resources at the start of their lives. They presumably have a favored status that leads them to identify with authority. Middle children have to fight hard for attention, and so they may rebel as a way of getting noticed. This effect should be mixed for the youngest in a family: On the one hand, they also have to compete for attention. As the youngest, however, they may get some amount of attention just for being the last child.

This idea has also been put forward within the psychology community. For example, Frank Sulloway's popular book, Born to Rebel, essentially made this case.

Some data have been collected that support this idea: In typical analyses, researchers find large-scale surveys that measure rebellious behaviors in teens (like drinking, use, and nonviolent ) and then look to see whether birth order in families predicts the delinquent behavior. The typical finding is that middle children are the most likely to exhibit these behaviors, and that first-born children are least likely to display them.

The problem with these analyses is that they are typically done between families. That is, the children all come from different families, and so it is hard to know for sure whether birth order is the true cause of the effect or whether some other variable, like parental involvement in the family, is causing the observed relationship.

A paper in the August, 2013 issue of Personality and Social Psychology Bulletin by Patrick Cundiff found a data set that allowed him to tease apart these effects. He analyzed the Add Health survey that was collected on school children from 1994-2008. This survey collected information about a variety of children’s behaviors. The survey also contained a lot of data about siblings within a family—and it had information about potential confounding factors like socioeconomic status, grade point average in school, and parental involvement in the home.

Cundiff did both a between-family and a within-family analysis of the data. The between-family analysis used all of the children in the sample (over 14,000), while the within-family analysis used only the data from the 3,800 children whose entire family was observed.

The between-family analysis showed the same effects as previous research—that middle children were about 33% more likely to exhibit delinquent behaviors than first-born children. Last-born children were about 20% more likely to exhibit these behaviors than first-born children. Examination of a variety of other factors, like grade point average, and aspects of families, like parental closeness, did not eliminate the effect of birth order.

However, when the data set looked only within families, the relationship between birth order and delinquent behavior was sharply reduced in size and was in fact no longer statistically significant. In this analysis, only and parental involvement had a reliable effect on behavior. That is, boys were more likely to engage in delinquent behavior than girls, and children with a low level of parental involvement were more likely to engage in delinquent behavior than children with a high level of parental involvement.

This paper is consistent with a growing body of evidence suggesting that there are few reliable effects of birth order on children’s behavior. Thus, while it is intuitively reasonable to think that first-born and later-born children differ, it does not look like that is really happening.

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