Kasey Buckles’ kids are two years, two months and two weeks apart. The timing is significant because Buckles, an assistant professor of economics at the University of Notre Dame, has just officially determined that the ideal spacing between an older and younger sibling is at least, you guessed it, two years — at least when it comes to intelligence.
Older children who are born at least two years before a younger sibling’s debut are smarter, according to research that is due to be published in an upcoming issue of the Journal of Human Resources. They score higher on math and reading tests than children born closer together.
The findings are likely to be of interest to parents who are thinking about expanding their family. As it is, all you have to do is mention you’re considering having another baby, and the advice starts pouring in: some people recommend having kids close together so they can be playmates; others suggest spacing them out so that the older child is more independent and able to pitch in and help by the time an infant arrives on the scene.
“There’s a lot of conventional wisdom out there about what is best, but there’s not a lot of actual evidence about what is good for either parents or children,” says Buckles. “This is a first piece of evidence that suggests a benefit to increasing the spacing between the siblings.”
When spacing is under two years, it stands to reason that an older sibling loses parental time and attention. With two in diapers, parenting is more about damage control than enrichment. Indeed, Buckles found that older siblings are read to less and watch more television between the ages of 3 and 5 than kids who are spaced at least two years apart.
Although there appears to be a particular benefit to waiting more than two years, greater spacing at any point seems better for the older sibling. Conversely, there was no evidence of any effect for younger siblings. “It doesn’t seem to help or harm them,” says Buckles.
Buckles’ findings are magnified for familes with lower socioeconomic status; parents with fatter pursestrings can afford to compensate for their lack of parental doting by enrolling their children in quality daycare.
To reach these conclusions, Buckles and co-author Elizabeth Munnich, a graduate student, looked at a data set of about 3,000 women who gave birth to 5,000 sibling pairs. The children took achievement tests in reading and math between the ages of 5 and 7. Buckles found that expanding spacing by one year increases older siblings’ reading scores on the Peabody Individual Achievement Test by .17 standard deviations. The average child in the study was able to read 22 of 84 words; the older siblings in question could read 24.
Buckles zoomed in on the 10% of women who’d suffered miscarriages between births because she was especially interested in looking at randomized spacing in which the duration between births was affected by something beyond a woman’s control. If a woman had intended to have children two years apart, for example, but then had a miscarriage, the spacing between children would automatically have been extended.
The 300 women whose spacing was increased by miscarriage had older children who did better on tests than those of the 2,700 women whose birth spacing was not disrupted by miscarriage.
“There are only so many hours in the day, and the longer that period can be when a child is the only child, the greater the investment they are going to receive,” says Buckles, talking like a true economist.
In case you’re wondering what sibling spacing has to do with a discipline that analyzes the distribution and consumption of goods and services, Buckles has a ready explanation: everything. Economists, she points out, are social scientists at heart. “Really in economics, we are studying how people make decisions. Most people think about it as what to do when interest rates are high, but it also affects issues like when do you marry and have children,” she says. “This [research] is about human capital and what determines test scores for young children because that’s what determines productivity down the road.”