Family size study used reliable data but got murky results

A recent research study in which the authors stated children from large families suffer declines in cognitive functioning and increases in behavioral problems with the birth of each younger sibling has stirred much controversy, particularly among those who either grew up in large families or are the parents of several children.

A detailed analysis of the study, which was published by the National Bureau of Economic Research and conducted by researchers from the Department of Economics at the University of Houston and the Department of Management at the London School of Economics, demonstrates that the methodology appears scientifically sound, but there are some ambiguities in its interpretation.

The researchers based their study on longitudinal information the U.S. Department of Labor began collecting in 1979. That data was intended to serve as a research tool for economists, sociologists, and other scientists. The Labor Department gathered the detailed information from 12,686 young men and women in the United States who were 14 to 22 years of age when first surveyed. The subjects were interviewed annually from 1979 through 1994 and every two years after that. The researchers of the family size study did not administer the tests, design the survey questions, or interview the participants; they only analyzed the data to make correlations and comparisons.

The researchers also matched the data from 4,925 mothers who participated in the longitudinal study with data from their 11,464 children, who were surveyed biennially from 1986 to 2012.

Valid, widely used, and accepted tests were employed to assess the home environment, child behaviors, and cognitive functioning of both mothers and children. The study analyzed changes in the behavior and cognitive functioning of children when siblings were born and the ways in which parental involvement changed. For example, they found that even with many children in a family, parents continued to provide their children with the same amount of resources, affection, and home safety. But as the families grew larger, parents had less time to read to their children or have family meals.

The researchers also looked at gender differences and found girls were more likely to experience declines in cognitive scores after the birth of a sibling and boys were more likely to have behavioral difficulties. They also found differences between mothers with higher scores on intellectual testing and those with lower scores. The mothers with higher scores did not show the same declines in parental involvement after the birth of a child as the mothers with lower scores did. Also, the mothers with higher scores had more of a tendency to cut back their hours at work or quit employment to stay home with their children.

The researchers did not analyze other factors such as sibling relationships or social interactions.

When it comes to interpretation of the data, the process becomes a little murky and ambiguous. For example, the researchers said the arrival of a younger sibling reduces the older child’s cognitive score and the parental-involvement score by 2.8 percentile points. That does not seem to be a substantial decline, although the researchers said it is significant, which means it’s unlikely to happen by mere chance. And although they claim each added child causes further decline in the cognitive scores of the older siblings and in parental investment, they do not say whether the regression is linear. It is not clear whether cognitive testing was performed after the birth of each subsequent sibling.

Without such information, it is difficult to draw any conclusions about the validity of the researchers’ interpretations. Perhaps the best that can be said of this research is that it serves as a good reminder that parenting is an extremely important job. Kids not only crave, but need Mom and Dad’s attention. Parents are wise to be intentional about giving their children adequate time whether they have one child or 20.

See “Family size study used reliable data but got murky results” for additional coverage of this study.