David Leonhardt writes for the The New York Times in "The Morning Newsletter," 10 May 2021, that "the results from the Boston pre-K study" are "being released this morning by three economists, from the University of Chicago, the Massachusetts Institute of Technology and the University of California, Berkeley." It's the result of a long-term observation of some students who, at age 4 in the late 1990s, were randomly chosen to attend a pre-kindergarten program in Boston.
The study found that, while those who attended pre-K "did not do noticeably better on standardized tests in elementary school, middle school or high school...More important than the scores are concrete measures of a student’s well-being. And by those measures, the students who won the lottery fared substantially better than those who lost it."
Leonhardt writes that thoe who won the lottery and thus were able to attend pre-K
were less likely to be suspended in high school and less likely to be sentenced to juvenile incarceration. Nearly 70 percent of lottery winners graduated from high school, compared with 64 percent of lottery losers...The winners were also more likely to take the S.A.T., to enroll in college and — though the evidence is incomplete, because of the students’ age — to graduate from college. ... How could pre-K have these positive effects without lifting test scores? It seems to improve children’s social and emotional skills and help them mature more than it helps in a narrow academic sense, the researchers told me.