The mothers of those in the experimental program earned more when the children were in preschool, and the difference was still there two decades later.

When the boys reached age 30, they earned an average of $19,800 more a year than those in the control group and had half a year more education. (The small sample size — 37 boys in the programs who stayed in the study — means the difference was not very precisely estimated.) When the girls reached 30, they had two more years of education and earned about $2,500 more, the study found.

In their mid-30s, men who attended the program were 33 percent less likely to be drug users, had fewer misdemeanor arrests and were less likely to have high blood pressure.

The conclusion that boys benefited more than girls meshes with other research findings that boys are more sensitive to disadvantage and responsive to intervention.

The program was expensive — $18,514 per student a year — but after calculating effects like the cost to society of unemployment, crime and poor health, the researchers concluded that it returned $7.30 for every dollar spent. In addition to Mr. Heckman, the researchers were Jorge Luis García of the University of Chicago and Duncan Ermini Leaf and María José Prados of the University of Southern California.

High-quality preschool showed multigenerational benefits in another new study, which traced nearly one million children in Denmark until old age. Children who attended the high-quality programs had slightly more years of schooling and an increased likelihood of living beyond 65 — and their children also got more education.