NEW YORK

SEVERAL YEARS AGO, during a class on the history of race in America, I asked a group of students whether white parents should be encouraged to adopt African-American children. Most of the students were white, and most of them said no. “Black and white culture are too different,” one student explained. “The kids would feel out of place.”

I thought of this remark as I watched Barack Obama accept the Democratic nomination for president. In a luminous speech, dazzling with inspiration and intelligence, Obama called upon citizens to “come together as one American family.”

For too many Americans, however, family ends where race begins.

That’s particularly ironic, given Obama’s own biracial background.

But it’s predictable, too. The more that Americans “celebrate diversity,” to quote the popular bumper sticker, the less they seem to have in common. And that’s very bad news for Barack Obama, who needs to persuade millions of white voters that we’re all in this together.

It’s a tough sell. Invoking their own family metaphor, large numbers of whites still say they can’t “relate” to Obama. Although it’s tempting to ascribe that sentiment to simple racism, my students’ remarks point to a more subtle culprit: the ideal of diversity itself.

By underscoring our differences, it drives us apart.

Consider the concept of racial “learning styles,” which former Obama pastor Jeremiah Wright summoned earlier this year. According to this theory, black students learn better in groups, not alone; they prefer to move about the classroom, rather than to sit at their desks; and they are more impulsive — and less cerebral — than other children.

Wright’s comments sparked an inferno of popular ridicule and a small political firestorm for Obama, who quickly distanced himself from his longtime minister. But the idea of racial learning styles remains accepted wisdom at institutions like my own, where students often tell me that Asians, Hispanics, and blacks acquire knowledge differently than white people.

Or consider the continued resistance to white adoptions of black children. According to the National Association of Black Social Workers, interracial adoption threatens nothing less than the “cultural genocide” of African-Americans. “Black children in white homes are cut off from the healthy development of themselves as black people,” the association claims. We are family? I think not.

Beneath all of this talk, of course, lies the fallacy of race itself. Although America is a richly diverse place, we’re told, people in any given race are the same — or should be. That’s why you still hear whispers in the African-American community about whether Obama is “really” black.

He isn’t. And you’re not “really” white, or Hispanic, or Asian, or whatever it is you say you are. We’re all mongrels, each and every one of us. But the concept of race masks the diversity inside of each group, even as it exaggerates the differences outside of them.

Compare that to women, who are now assumed to disagree with each other.

And look no further than Sarah Palin, John McCain’s surprise running-mate selection. Palin is resolutely anti-abortion, bucking the National Organization of Women and other liberal organizations. Heck, she’s even a member of something called “Feminists for Life”!

But race is somehow different from gender. If you’re of the same race, you’re not supposed to be different.

For most of American history, of course, white people presumed that the country was theirs. The ideal of diversity arose to challenge white supremacy, reminding us that Americans come in many colors — and that all of them deserve equal rights and respect.

Along the way, however, it also reinforced the same racial categories that had bedeviled us for so long. And that helps explain why so many white voters — and even a few black ones — see Barack Obama as strange, exotic, or alien. Not evil or threatening, necessarily. Just different. Unfamiliar.

Or, should I say, un-familial?

Don’t get me wrong: there’s still plenty of old-fashioned racism out there. When Obama faced off against Hillary Clinton in my home state of Pennsylvania, one of six white voters told exit pollsters that race influenced his or her decision. Nationwide, five percent of white voters admit that they would never vote for a black candidate. The real number is probably much higher than that.

But the problem isn’t just racism. It’s race, plain and simple, and the doctrine of diversity that we’ve attached to it. If we teach our children to celebrate racial differences, they’ll never see themselves as a single national family. And they won’t elect leaders like Barack Obama, who reminds us about the common humanity that should unite us all.

Jonathan Zimmerman, an occasional contrbutor, teaches history and education at New York University.