by Fareed Zakaria.

America has long been seen–by its citizens and the world–as the place where anyone can make it. And yet studies from the past two decades all point to a different reality. Rich people rarely become poor in a generation–and the poorest seldom get rich. Despite the rags-to-riches myth, such stories are the exception. A comprehensive study by the Pew Economic Mobility Project documents that in the U.S. today, few poor people become even upper middle class.

That’s why President Obama’s proposal to expand early-childhood education is vitally important: the idea is to provide high-quality pre-K for 4-year-olds from families whose incomes are at or below 200% of the poverty line–that is at or below $47,000 for a family of four.

Critics share his goals, but worry about the government’s track record in the area. Specifically, they point to Head Start, the long-standing program that provides early education to disadvantaged children. The Department of Health and Human Services released a study of Head Start in 2010, updated in 2012, that concludes that the program’s positive effects begin to fade within a few years.

But critics are jumping to conclusions about a very complicated subject.

It is premature to reject Head Start. They note that many factors may have intervened to erode the early gains in scores, including sharp rises in single-parent families, non-English-speaking households and severe health problems like childhood obesity and diabetes. They also noted that early education in public schools has been getting better, a trend that might explain why Head Start kids lose their advantage over non–Head Start kids.

Early-childhood education “improves children’s cognitive abilities, helps to create a foundation for lifelong learning, makes learning outcomes more equitable, reduces poverty and improves social mobility from generation to generation.”

From a study of 34 rich countries, 90% of 3-year-olds get early-childhood education. The OECD average for 4-year-olds is 81%. In the U.S., it is only 69%, and those children tend to be from middle- and upper-middle-class families.

Head Start should be reformed to ensure its effectiveness. But Obama’s proposals will help the U.S. start to catch up in the great human-capital struggle that will define the new century even more dramatically than it did the last.

Read more: Upward Mobility