In such studies, researchers must b

In such studies, researchers must be particularly cautious because, in additionto age, the size of one’s family is another factor that’s intertwined with siblingposition. A child from a family of four has a 50 percent chance of being afirstborn; the more siblings, the lower the probability. For example, the factthat many astronauts are firstborns does not necessarily speak to the specialqualities of those born first. It’s likely that many astronauts come from smallerfamilies. To better understand these influences, Rohrer and her teamcontrolled for the number of siblings. That’s because when there are more ofthem, there are more later borns. So the researchers hypothesized later bornsmay more often appear in families of lower socioeconomic classes—whichcould account for differences between children of different-sized families.