As seasonal aisles are taken over by backpacks and Elmer’s glue, there’s no denying the start of a new school year. For parents of preschoolers whose birth dates are on or near the cutoff, thinking about school means deciding how early their child should begin kindergarten. While there is lots of evidence that children who are old for their grade tend to have better long-term academic outcomes, Fabrizio Bernardi’s new study shows that this is not necessarily true for everyone. It turns out that the importance of a child’s age relative to his or her classmates’ depends on the family’s socioeconomic status.

Using data on elementary school children in France, where about 20 percent of students have to repeat a grade in primary school, Bernardi investigates who is getting held back. By looking at how likely children born in different months are to be successfully promoted every year in primary school, he determines that indeed, the older students have the upper hand. However, when Bernardi compares the patterns for children of different social classes, there are stark differences among the groups. For the children of university- educated parents, there is almost no difference between being older or younger at the start of school. For the children of less educated parents, however, relative age matters significantly.

Bernardi hypothesizes that upper class children who experience an early disadvantage are more likely to catch up because they benefit from compensatory advantages. One such advantage may be in the way upper-class parents react to their children’s setbacks. For example, upper class parents might invest more resources to help a son who fails, whereas, in contrast, lower-class parents might respond by redirecting their scarce resources to his siblings, resulting in a smaller investment in him.

Looking at the big picture, this means that compensatory advantage contributes to vast educational inequalities among children from different social classes. Understanding how it operates may be a step in a journey of a million miles, but it is a step in the right direction.