COLUMBUS, Ohio -- The more brothers and sisters that children have, the lower their grades are in school, a new nationwide study shows.

The results, which include data from 24,599 eighth graders, suggest that academic achievement drops as families grow because parents have less time and economic resources for each child.

As family size increases, parents talk less to each child about school, have lower educational expectations, save less for college, and have fewer educational materials available.

These are the factors that had the largest effect on grades, the study found.

"Parents only have so much time and money, and the more children they have, the more those resources are diluted," said Douglas Downey, author of the study and assistant professor of sociology at Ohio State University.

The research will be published in the next issue of the American Sociological Review.

The results are not surprising, Downey said, but the theory that family size affects academic achievement had never before been tested in a nationwide sample. The results hold up even after taking into account parents' education levels and income, race, and other socioeconomic variables.

Data from the study came from the National Education Longitudinal Study of 1988, collected by the National Center for Education Statistics. Downey examined how the number of siblings affected student-reported grades and test scores on standardized math and reading tests.

He also looked at how parents' economic and interpersonal resources were affected by increasing family size, and the impact declining resources had on academic achievement.

The results showed that parental resources don't all decline in the same way as families grow.

Economic resources -- such as educational materials, money for college and a computer in the home -- declined more rapidly than other resources as the number of children increased. Interpersonal resources, which include time spent talking to each child about school and knowing each child's friends by name, didn't decline as quickly.

"It's easier for parents to increase the total amount of time they spend with their children than it is to increase their economic resources," Downey said.

Downey also found that some resources benefited each child less as family size increased. For example, a computer in the home was more beneficial to one child than it was to two or more.

"Although children may not be competing for the use of books or a computer, they don't get as much out of them in larger families because parents can't spend as much time helping each child individually to use these resources," Downey said.

The results suggest that the decline in the number of children born per woman in the last 30 years from 3.6 to 1.9 has probably benefited today's children, according to Downey.

"In today's smaller families, children are more likely to receive undiluted resources than children from 30 years ago," he said.

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Contact: Douglas Downey, (614) 292-6681

Written by Jeff Grabmeier, (614) 292-8457