Even as two-thirds of Americans now say that there are strong conflicts between rich and poor, another great American division is slowly healing.

As National African-American History Month begins tomorrow, we should celebrate the decline of racial segregation in America for the fourth consecutive decade. While there are far too many children -- of all races -- who are raised in the midst of poverty and desperation, the lessening of segregation reminds us that our nation continues to have a great ability to fight even its worst problems.

The attached chart, from a new Manhattan Institute report I co-wrote with Jacob Vigdor of Duke University, shows the long history of segregation across U.S. metropolitan areas from 1890 to 2010.

The figure displays two standard measures of segregation that are typically called dissimilarity and isolation. The dissimilarity index measures how uneven the black population is within a metropolitan area. It asks what share of either blacks (or non-blacks) would have to change neighborhoods to ensure that each neighborhood had exactly the same racial composition. The isolation index measures the share of the average African-American’s neighborhood that is also African-American.

Rural Exodus

From 1890 to 1960, as blacks fled southern farms, seeking a freer and more prosperous life in urban areas, our cities became increasingly segregated. Across cities and metropolitan areas from 1910 to 1970, the growth in segregation was larger where there was a more rapidly growing black population.

Whites often bitterly opposed the influx of blacks into their communities, sometimes violently, as with the Chicago Race Riot of 1919, which started with “the drowning of an African-American teenager who had crossed an invisible line at 29th Street separating customarily segregated ‘white’ and ‘black’ beaches.” For many blacks, segregation was the only way to find safety from white gangs and violence.

During the first half of the 20th century, segregation was also enforced with legal tools. Some cities, such as Baltimore and Louisville, Kentucky, passed explicit racial zoning ordinances, formally restricting the areas where blacks could live.

In my book, “Triumph of the City,” I relate the battle against racial zoning in Baltimore, led by two pioneering black lawyers, W. Ashbie Hawkins and George W.F. McMechen, which resulted in the 1917 Supreme Court ruling against explicit racial zoning, Buchanan v. Warley.

Cities couldn’t zone by race, but developers could still insert restrictive covenants into new subdivisions that prevented units from being sold to minorities. Helen C. Monchow’s 1928 study of deed restrictions found that these racial caveats were widespread in subdivision units built after World War I. Only after World War II did Supreme Court Justice Thurgood Marshall and the National Association for the Advancement of Colored People knock the teeth out of restrictive covenants in the landmark case -- Shelley v. Kraemer -- which is a wonderful fusion of civil rights and libertarianism.

The case didn’t ban restrictive covenants as such -- private persons could still write whatever they wanted -- but it banned the use of governmental resources, such as police and courts, to enforce racial restrictions on resale. Without public enforcement, the covenants became irrelevant.

In 1957, a New York coalition of blacks, Jewish groups and labor unions helped create the nation’s first fair housing ordinance, which made it illegal to refuse to sell or rent on the basis of “race, color, religion, national origin or ancestry.” A decade of civil-rights activism led to the federal Fair Housing Act of 1968, which made racial discrimination in housing illegal nationwide.

Legal Groundwork

These political and legal victories didn’t magically erase racism or change America overnight. They didn’t automatically integrate neighborhoods, but they set America on the road toward a more integrated society.

My past work with David Cutler and Vigdor found that in 1960, blacks made up less than 1 percent of the population in 56 percent of city census tracts (tracts are census-defined units with about 5,000 inhabitants). Almost one in five tracts had no black Americans. By 1990, only about 7 percent of city tracts were 100 percent white.

As the figure shows, as of 1970, almost 80 percent of either whites or blacks would have had to move neighborhoods in order to achieve an even distribution of whites and blacks within the average metropolitan area. By 1990, that dissimilarity measure had dropped to 66 percent; it is 54 percent today. We are very far from living in a perfectly integrated society, but our nation is far more integrated than it was 40 years ago.

The progress over the last decade has been particularly dramatic. Every one of the 10 largest metropolitan areas experienced drops in both dissimilarity and isolation of 3.6 points or more. The isolation index is below 45 percent in every one of those 10 largest areas, except for Chicago. Long among the most segregated places in America, the Windy City has experienced a particularly dramatic decline in segregation since 2000.

The general decline in segregation has also been accompanied by a change in its nature. Before 1968, segregation is best understood as the result of hard, if often informal, barriers against black mobility. There were neighborhoods that were simply off-limits. The effect was that blacks paid more for housing, especially in more segregated cities.

Limited Choices

A core economic idea is that people pay higher prices when their choices are limited, whether those limits come from tariff barriers or racial restrictions. Thirty-five years ago, the economists John Kain (one of my mentors) and John Quigley found that blacks paid more than whites for comparable housing. My work with Cutler and Vigdor found that before 1970, the price and rent premium paid by blacks increased in more segregated areas, as you would expect of a group that had a more limited set of housing choices.

After 1970, however, that pricing pattern switched. By 1990, blacks were paying less for housing than whites, especially in more segregated metropolitan areas. This switch can be explained if segregation, post-1970, reflects white preferences rather than barriers preventing black mobility. If the segregation that remains is the result of whites liking to live in primarily white neighborhoods, then we should expect whites to pay a price for limiting their own choices, and that is exactly what the data show.

The decline in segregation hasn’t been uniform across the black population. Much of the decline reflects relatively well-educated black Americans moving into white districts. While that freedom is something to celebrate, the exodus of the more skilled left many urban neighborhoods behind, and the effect of growing up in a segregated community appears to have gotten worse over time.

During the 20th century, blacks moved from being an overwhelmingly rural community to having a large presence in cities. They moved for economic opportunity and for freedom from the Jim Crow laws of the South. Unfortunately, black migrants were too often prevented from enjoying the full benefits of freedom that city air can bring. Gradually, those barriers have fallen and American cities are more integrated than they have been since 1910. That is a triumph for blacks and whites alike.

(Edward Glaeser, an economics professor at Harvard University, is a Bloomberg View columnist. He is the author of “Triumph of the City.” The opinions expressed are his own.)

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To contact the writer of this article: Edward Glaeser at eglaeser@harvard.edu.

To contact the editor responsible for this article: Max Berley at mberley@bloomberg.net.