Here’s a fact that may not surprise you: the children of the rich perform better in school, on average, than children from middle-class or poor families. Students growing up in richer families have better grades and higher standardized test scores, on average, than poorer students; they also have higher rates of participation in extracurricular activities and school leadership positions, higher graduation rates and higher rates of college enrollment and completion.

Whether you think it deeply unjust, lamentable but inevitable, or obvious and unproblematic, this is hardly news. It is true in most societies and has been true in the United States for at least as long as we have thought to ask the question and had sufficient data to verify the answer.

What is news is that in the United States over the last few decades these differences in educational success between high- and lower-income students have grown substantially.

One way to see this is to look at the scores of rich and poor students on standardized math and reading tests over the last 50 years. When I did this using information from a dozen large national studies conducted between 1960 and 2010, I found that the rich-poor gap in test scores is about 40 percent larger now than it was 30 years ago.

To make this trend concrete, consider two children, one from a family with income of $165,000 and one from a family with income of $15,000. These incomes are at the 90th and 10th percentiles of the income distribution nationally, meaning that 10 percent of children today grow up in families with incomes below $15,000 and 10 percent grow up in families with incomes above $165,000.

In the 1980s, on an 800-point SAT-type test scale, the average difference in test scores between two such children would have been about 90 points; today it is 125 points. This is almost twice as large as the 70-point test score gap between white and black children. Family income is now a better predictor of children’s success in school than race.

The same pattern is evident in other, more tangible, measures of educational success, like college completion. In a study similar to mine, Martha J. Bailey and Susan M. Dynarski, economists at the University of Michigan, found that the proportion of students from upper-income families who earn a bachelor’s degree has increased by 18 percentage points over a 20-year period, while the completion rate of poor students has grown by only 4 points.

In a more recent study, my graduate students and I found that 15 percent of high-income students from the high school class of 2004 enrolled in a highly selective college or university, while fewer than 5 percent of middle-income and 2 percent of low-income students did.

These widening disparities are not confined to academic outcomes: new research by the Harvard political scientist Robert D. Putnam and his colleagues shows that the rich-poor gaps in student participation in sports, extracurricular activities, volunteer work and church attendance have grown sharply as well.

In San Francisco this week, more than 14,000 educators and education scholars have gathered for the annual meeting of the American Educational Research Association. The theme this year is familiar: Can schools provide children a way out of poverty?

We are still talking about this despite decades of clucking about the crisis in American education and wave after wave of school reform.Whatever we’ve been doing in our schools, it hasn’t reduced educational inequality between children from upper- and lower-income families.

Part of knowing what we should do about this is understanding how and why these educational disparities are growing. For the past few years, alongside other scholars, I have been digging into historical data to understand just that. The results of this research don’t always match received wisdom or playground folklore.

The most potent development over the past three decades is that the test scores of children from high-income families have increased very rapidly. Before 1980, affluent students had little advantage over middle-class students in academic performance; most of the socioeconomic disparity in academics was between the middle class and the poor. But the rich now outperform the middle class by as much as the middle class outperform the poor. Just as the incomes of the affluent have grown much more rapidly than those of the middle class over the last few decades, so, too, have most of the gains in educational success accrued to the children of the rich.

Before we can figure out what’s happening here, let’s dispel a few myths.

The income gap in academic achievement is not growing because the test scores of poor students are dropping or because our schools are in decline. In fact, average test scores on the National Assessment of Educational Progress, the so-called Nation’s Report Card, have been rising — substantially in math and very slowly in reading — since the 1970s. The average 9-year-old today has math skills equal to those her parents had at age 11, a two-year improvement in a single generation. The gains are not as large in reading and they are not as large for older students, but there is no evidence that average test scores have declined over the last three decades for any age or economic group.

The widening income disparity in academic achievement is not a result of widening racial gaps in achievement, either. The achievement gaps between blacks and whites, and Hispanic and non-Hispanic whites have been narrowing slowly over the last two decades, trends that actually keep the yawning gap between higher- and lower-income students from getting even wider. If we look at the test scores of white students only, we find the same growing gap between high- and low-income children as we see in the population as a whole.

It may seem counterintuitive, but schools don’t seem to produce much of the disparity in test scores between high- and low-income students. We know this because children from rich and poor families score very differently on school readiness tests when they enter kindergarten, and this gap grows by less than 10 percent between kindergarten and high school. There is some evidence that achievement gaps between high- and low-income students actually narrow during the nine-month school year, but they widen again in the summer months.

That isn’t to say that there aren’t important differences in quality between schools serving low- and high-income students — there certainly are — but they appear to do less to reinforce the trends than conventional wisdom would have us believe.

If not the usual suspects, what’s going on? It boils down to this: The academic gap is widening because rich students are increasingly entering kindergarten much better prepared to succeed in school than middle-class students. This difference in preparation persists through elementary and high school.

My research suggests that one part of the explanation for this is rising income inequality. As you may have heard, the incomes of the rich have grown faster over the last 30 years than the incomes of the middle class and the poor. Money helps families provide cognitively stimulating experiences for their young children because it provides more stable home environments, more time for parents to read to their children, access to higher-quality child care and preschool and — in places like New York City, where 4-year-old children take tests to determine entry into gifted and talented programs — access to preschool test preparation tutors or the time to serve as tutors themselves.

But rising income inequality explains, at best, half of the increase in the rich-poor academic achievement gap. It’s not just that the rich have more money than they used to, it’s that they are using it differently. This is where things get really interesting.

High-income families are increasingly focusing their resources — their money, time and knowledge of what it takes to be successful in school — on their children’s cognitive development and educational success. They are doing this because educational success is much more important than it used to be, even for the rich.

With a college degree insufficient to ensure a high-income job, or even a job as a barista, parents are now investing more time and money in their children’s cognitive development from the earliest ages. It may seem self-evident that parents with more resources are able to invest more — more of both money and of what Mr. Putnam calls “‘Goodnight Moon’ time” — in their children’s development. But even though middle-class and poor families are also increasing the time and money they invest in their children, they are not doing so as quickly or as deeply as the rich.

The economists Richard J. Murnane and Greg J. Duncan report that from 1972 to 2006 high-income families increased the amount they spent on enrichment activities for their children by 150 percent, while the spending of low-income families grew by 57 percent over the same time period. Likewise, the amount of time parents spend with their children has grown twice as fast since 1975 among college-educated parents as it has among less-educated parents. The economists Garey Ramey and Valerie A. Ramey of the University of California, San Diego, call this escalation of early childhood investment “the rug rat race,” a phrase that nicely captures the growing perception that early childhood experiences are central to winning a lifelong educational and economic competition.

It’s not clear what we should do about all this. Partly that’s because much of our public conversation about education is focused on the wrong culprits: we blame failing schools and the behavior of the poor for trends that are really the result of deepening income inequality and the behavior of the rich.

We’re also slow to understand what’s happening, I think, because the nature of the problem — a growing educational gap between the rich and the middle class — is unfamiliar. After all, for much of the last 50 years our national conversation about educational inequality has focused almost exclusively on strategies for reducing inequalities between the educational successes of the poor and the middle class, and it has relied on programs aimed at the poor, like Head Start and Title I.

We’ve barely given a thought to what the rich were doing. With the exception of our continuing discussion about whether the rising costs of higher education are pricing the middle class out of college, we don’t have much practice talking about what economists call “upper-tail inequality” in education, much less success at reducing it.

Meanwhile, not only are the children of the rich doing better in school than even the children of the middle class, but the changing economy means that school success is increasingly necessary to future economic success, a worrisome mutual reinforcement of trends that is making our society more socially and economically immobile.

We need to start talking about this. Strangely, the rapid growth in the rich-poor educational gap provides a ray of hope: if the relationship between family income and educational success can change this rapidly, then it is not an immutable, inevitable pattern. What changed once can change again. Policy choices matter more than we have recently been taught to think.

So how can we move toward a society in which educational success is not so strongly linked to family background? Maybe we should take a lesson from the rich and invest much more heavily as a society in our children’s educational opportunities from the day they are born. Investments in early-childhood education pay very high societal dividends. That means investing in developing high-quality child care and preschool that is available to poor and middle-class children. It also means recruiting and training a cadre of skilled preschool teachers and child care providers. These are not new ideas, but we have to stop talking about how expensive and difficult they are to implement and just get on with it.

But we need to do much more than expand and improve preschool and child care. There is a lot of discussion these days about investing in teachers and “improving teacher quality,” but improving the quality of our parenting and of our children’s earliest environments may be even more important. Let’s invest in parents so they can better invest in their children.

This means finding ways of helping parents become better teachers themselves. This might include strategies to support working families so that they can read to their children more often.. It also means expanding programs like the Nurse-Family Partnership that have proved to be effective at helping single parents educate their children; but we also need to pay for research to develop new resources for single parents.

It might also mean greater business and government support for maternity and paternity leave and day care so that the middle class and the poor can get some of the educational benefits that the early academic intervention of the rich provides their children. Fundamentally, it means rethinking our still-persistent notion that educational problems should be solved by schools alone.

The more we do to ensure that all children have similar cognitively stimulating early childhood experiences, the less we will have to worry about failing schools. This in turn will enable us to let our schools focus on teaching the skills — how to solve complex problems, how to think critically and how to collaborate — essential to a growing economy and a lively democracy.

Sean F. Reardon is a professor of education and sociology at Stanford.