On the Fact that (According to the APA; American Psychological Association) the Heritability of Intelligence Rises from .45 in Childhood to .75 in Adulthood

It probably goes a long way toward explaining why the gains from such early intervention programs as Head Start (a topic that I did numerous papers on in college and which could never find compelling evidence for) are so damned transitory and why they eventually disappear by the latter teens.