From the opening of the article:

“While early formal instruction may appear to show good test results at first, in the long term, in follow-up studies, such children have had no advantage. On the contrary, especially in the case of boys, subjection to early formal instruction increases their tendency to distance themselves from the goals of schools, and to drop out of it, either mentally or physically.”

—Lilian G. Katz,
Professor Emeritus, U. of Illinois