This is not a randomized trial. They cite work based on arbitrary school year cutoff dates, but parents do take action based on those dates, e.g. by holding the

This is not a randomized trial. They cite work based on arbitrary school year cutoff dates, but parents do take action based on those dates, e.g. by holding their kids back one year or choosing a different school with different cutoff dates. The result is that due to parent intervention, more intelligent kids are in higher grades (i.e. have more schooling) relative to their age.