Mainly, however, excuses for poverty involve the assertion that the United States is a land of opportunity, a place where people can start out poor, work hard and become rich.

But the fact of the matter is that Horatio Alger stories are rare, and stories of people trapped by their parents’ poverty are all too common. According to one recent estimate, American children born to parents in the bottom fourth of the income distribution have almost a 50 percent chance of staying there  and almost a two-thirds chance of remaining stuck if they’re black.

That’s not surprising. Growing up in poverty puts you at a disadvantage at every step.

I’d bracket those new studies on brain development in early childhood with a study from the National Center for Education Statistics, which tracked a group of students who were in eighth grade in 1988. The study found, roughly speaking, that in modern America parental status trumps ability: students who did very well on a standardized test but came from low-status families were slightly less likely to get through college than students who tested poorly but had well-off parents.

None of this is inevitable.

Poverty rates are much lower in most European countries than in the United States, mainly because of government programs that help the poor and unlucky.

And governments that set their minds to it can reduce poverty. In Britain, the Labor government that came into office in 1997 made reducing poverty a priority  and despite some setbacks, its program of income subsidies and other aid has achieved a great deal. Child poverty, in particular, has been cut in half by the measure that corresponds most closely to the U.S. definition.

At the moment it’s hard to imagine anything comparable happening in this country. To their credit  and to the credit of John Edwards, who goaded them into it  both Hillary Clinton and Barack Obama are proposing new initiatives against poverty. But their proposals are modest in scope and far from central to their campaigns.