Why are so many people so desperate to hold onto the idea that America is as racist as it has ever been?
The phenomenon is apparent in much of the commentary on the George Zimmerman case. Facts were blithely ignored -- the fact that Zimmerman is Hispanic, not white, by current standards; the evidence that he and not his victim, Trayvon Martin, was pummeled and wounded; the failure to find any hint of anti-black bias in Zimmerman's past.
Instead there was a desperate longing to see this unhappy incident as a case of a white racist hunting down and murdering an innocent black -- with a view to establishing that this kind of thing happens all the time.
It isn't. Yes, young black men are homicide victims in large and tragic numbers. But the perpetrators are almost always other young black men, as in President Obama's hometown of Chicago, where almost every weekend there are multiple such murders.
Nevertheless, journalism is full of opinion articles, many written by people who should know better, likening the death of Trayvon Martin to the murder of Emmett Till in Mississippi in 1955.
Till was a 14-year-old black boy raised in Chicago who, on a summer trip to his native Mississippi, "wolf-whistled" at a white woman. Two white men abducted and brutally murdered him.
They were tried, and the all-white jury acquitted them after deliberating 67 minutes. Months later, the defendants told Look magazine's William Bradford Huie that they had indeed killed the young man.
The Emmett Till case attracted national attention, with heavy media coverage. Rep. Charles Diggs, one of three blacks in Congress, attended the trial. National magazines ran pictures of the grinning defendants.
In the process, Northerners were forced to confront the brutality with which white Southerners enforced the subjection of blacks.
This went beyond the laws requiring segregated schools, buses and drinking fountains. Also in place was an unwritten code of behavior, breach of which could result in violent retaliation.
Blacks were called by their first names and could approach whites' houses only by the back door, and black men could never, never ogle white women.
This was unknown to most Northerners. As I explain in my forthcoming book, Shaping Our Nation: How Surges of Migration Transformed America and Its Politics, there was almost no migration between South and North in the years between the Civil War and World War II.
Southern mores were so unknown in the North that Yale psychologist John Dollard's 1937 book Caste and Class in a Southern Town, based on five months' field work in Indianola, Miss., was hailed as a great revelation, akin to Margaret Mead's writing on Samoa.
Yet everything in it was common knowledge for every 10-year-old, black or white, in Indianola.
The great genius of the civil rights movement was to make Northerners face the reality -- and the violence -- of the segregation system. The Emmett Till case was one of the first incidents that forced them to do so. It was followed a year later by Rosa Parks' refusal to move to the back of the bus in 1955 and Martin Luther King's resulting Montgomery bus boycott.
It is sometimes said that laws cannot change customs. But the Civil Rights Act of 1964, banning racial discrimination in hiring and public accommodations, did in fact change behavior in the South. It not only ended legally enforced segregation but effectively ended the unwritten code of black subjugation.
Which is to say that the America of our time -- the America of Trayvon Martin and George Zimmerman -- is hugely different from and hugely better than the America of Emmett Till.
Back in the 1950s, most Americans -- not just in the South but across the nation -- opposed interracial marriages. As blacks were migrating in large numbers to Northern cities, whites moved out of neighborhoods when they moved in.
Today things are different. Our President, twice elected with majorities of the vote, is the product of a mixed-race marriage. Black presence in neighborhoods no longer results in rapid white flight.
Yet many Americans have a desperate need to believe nothing has changed. They yearn for the moral clarity that enables almost all Americans today to retrospectively condemn the old Southern code.
The irony is that those who claim they lead the civil rights movement today have a vested psychological interest in denying its great triumph.