What factors affect people's academic and career success in life? Asked to make a list, most of us would mention their parents' income and education level, their family structure and functioning, the quality of schools they attend, their racial or ethnic background, and perhaps the area where they grew up. But one surprising factor that few recognize as important is the number of siblings a person has: the more siblings, the lower a person's educational outcomes.
Scholar Douglas Downey calls this inverse relationship "one of the most consistent findings in the status attainment literature." Citing Judith Blake's 1989 study Family Size and Achievement, he adds that "sibship size" (the scholarly term for the number of siblings) is of such significance that in multiple data sets, the only variable that consistently exerts a stronger effect on educational attainment than sibship size is father's education. When other characteristics (such as socioeconomic background) are controlled for, the effect of sibship size shrinks, but persists.
One dominant explanation of this finding is the "resource dilution" model, which posits that more children means each child receives a smaller share of parents' limited amount of time, energy, and money. That seems intuitively plausible, and it's also consistent with available data. Douglas Downey tested this theory about parental resources using a large sample of students who were in eighth grade in 1988, using measures like how often parents talk to their child, parents' educational expectations for their children, the money parents are saving for the child's college education, the cultural classes and activities a child attends, and so forth. He found that children with many siblings had fewer parental resources than children with few or no siblings, and that controlling for parental resources "greatly reduced" the negative correlation between sibship size and educational outcomes. Though he warns that his data are not 100% conclusive, his findings suggest that resource dilution does indeed exert negative effects on the education of children from larger families.
Nevertheless, large families are certainly not doomed to fail (and they can offer some obvious intangible benefits). In the same study cited above, some parents of multiple children---particularly highly educated parents---still provided above-average parental resources to their kids. Moreover, having siblings may provide advantages in another area: social skills. Examining a large and nationally representative sample of kindergartners, Douglas Downey and Dennis J. Condron found "consistent evidence that children are rated as exhibiting better social and interpersonal skills when they have at least one sibling." (This contradicted older and more limited studies that discovered no such difference.)
And other studies suggest that there are exceptions to the rule about sibship size and educational outcomes: in "highly integrated communities with norms supporting large families," such as Mormons or Muslim Arabs, studies find only "weak relationships between sibship size and educational outcomes," Downey writes. "One reason for this pattern," he suggests, "may be that a child in this type of community has a larger group of adults nearby who have an interest in the child's well-being—aunts, uncles, older cousins, grandparents, and other adults—and this feature buffers the dilution process occurring within the nuclear family." For large families, it takes more than just parents to raise a child.