Jonah Lehrer provides a theory:
While kids exposed to preschool got an initial bump in general intelligence, this dissipated by second grade. Instead, preschool seemed to improve performance on a variety of “non-cognitive” abilities, such as self-control, persistence and grit. While society has long obsessed over raw smarts just look at our fixation on IQ scores Heckman and Cunha argue that these non-cognitive traits are often more important. They note, for instance, that dependability is the trait most valued by employers, while “perseverance, dependability and consistency are the most important predictors of grades in school.” Of course, these valuable skills have little or anything to do with general intelligence. And that’s probably a good thing, since our non-cognitive traits are much more malleable, at least when interventions occur at an early age, than IQ. Preschool might not make us smarter our intelligence is strongly shaped by our genes but it can make us a better person, and that’s even more important.