Point to a group of toddlers in an upper-middle-class neighborhood in America, and it’s a good bet that they will go to college, buy nice houses and enjoy white-collar careers.

Point to a group of toddlers in a low-income neighborhood, and — especially if they’re boys — they’re much more likely to end up dropping out of school, struggling in dead-end jobs and having trouble with the law.

Something is profoundly wrong when we can point to 2-year-olds in this country and make a plausible bet about their long-term outcomes — not based on their brains and capabilities, but on their ZIP codes. President Obama spoke movingly in his second Inaugural Address of making equality a practice as well as a principle. So, Mr. President, how about using your second term to tackle this most fundamental inequality?

For starters, this will require a fundamental rethinking of antipoverty policy. American assistance programs, from housing support to food stamps, have had an impact, and poverty among the elderly has fallen in particular (they vote in high numbers, so government programs tend to cater to them). But, too often, such initiatives have addressed symptoms of poverty, not causes.

Since President Lyndon Johnson declared a “war on poverty,” the United States has spent some $16 trillion or more on means-tested programs. Yet the proportion of Americans living beneath the poverty line, 15 percent, is higher than in the late 1960s in the Johnson administration.

What accounts for the cycles of poverty that leave so many people mired in the margins, and how can we break these cycles? Some depressing clues emerge from a new book, “Giving Our Children a Fighting Chance,” by Susan Neuman and Donna Celano.

Neuman and Celano focus on two neighborhoods in Philadelphia. In largely affluent Chestnut Hill, most children have access to personal computers and the shops have eight children’s books or magazines on sale for each child living there.

Take a 20-minute bus ride on Germantown Avenue and you’re in the Philadelphia Badlands, a low-income area inhabited mostly by working-class blacks and Hispanics. Here there are few children’s books, few private computers and only two public computers for every 100 children.

On top of that, there’s a difference in parenting strategies, the writers say. Upper-middle-class parents in America increasingly engage in competitive child-rearing. Parents send preschoolers to art classes and violin lessons and read “Harry Potter” books to bewildered children who don’t yet know what a wizard is.

Meanwhile, partly by necessity, working-class families often take a more hands-off attitude to child-raising. Neuman and Celano spent 40 hours monitoring parental reading in the public libraries in each neighborhood. That was easy in the Badlands — on an average day “not one adult entered the preschool area in the Badlands.”

When I was a third-grader, a friend struggling in school once went with me to the library, and my mother helped him get a library card. His grandmother then made him return it immediately, for fear that he would run up library fines.

The upshot is that many low-income children never reach the starting line, and poverty becomes self-replicating.

Maybe that’s why some of the most cost-effective antipoverty programs are aimed at the earliest years. For example, the Nurse-Family Partnership has a home-visitation program that encourages new parents of at-risk children to amp up the hugging, talking and reading. It ends at age 2, yet randomized trials show that those children are less likely to be arrested as teenagers and the families require much less government assistance.

Or take Head Start. Critics have noted that the advantage its preschoolers gain in test scores fades by third grade, but scholars also have found that Head Start has important impacts on graduates, including lessening the chance that they will be convicted of a crime years later.

James Heckman, a Nobel Prize-winning economist, argues that the most crucial investments we as a country can make are in the first five years of life, and that they pay for themselves. Yet these kinds of initiatives are underfinanced and serve only a tiny fraction of children in need.

We don’t have any magic bullets. But randomized trials and long-term data give us a better sense of what works — and, for the most part, it’s what we’re not doing, like improved education, starting with early childhood programs for low-income families. Job-training for at-risk teenagers also has an excellent record. Marriage can be a powerful force, too, but there’s not much robust evidence about which programs work.

So, President Obama, to fulfill the vision for your second term, how about redeploying the resources we’ve spent on the war in Afghanistan to undertake nation-building at home — starting with children so that they will no longer be limited by their ZIP codes.