For the Democrats, “diversity” is one of the most important themes of the presidential campaign. As Democratic National Committee Chair Howard Dean recently boasted, “The Democratic Party’s field of Presidential candidates is not just the strongest but the most diverse ever,” based on the fact that the party’s two leading contenders, Barack Obama and Hillary Clinton, are black and female, and a lesser candidate, Bill Richardson, has Latino roots. In contrast, Mr. Dean characterized the Republicans as embracing “the politics of division, hatred, and fear.”

It is no surprise that Mr. Dean and the Democrats go on in this way about diversity: The political left has traditionally owned this cause, having long made the claim that they celebrate their differences while the right is insular and homogenous. A look at how both the left and right practice diversity in day-to-day life is in order, however. It is one thing to celebrate diversity in theory, but quite another actually to live with people who differ from us.

Does the left live in more diverse circumstances than the right? The data tell an interesting story — one that does not square well with today’s political rhetoric.

In 2006, the General Social Survey asked several hundred Americans about their neighbors. Did people live near others like themselves? When it came to race, the answer on both sides of the political aisle was generally “no.” For example, the survey found that the 59% of white people who called themselves “conservative” or “very conservative” had exactly zero black neighbors; 60% had no Latino neighbors. How much better did “liberal” or “extremely liberal” whites do? Almost no better at all: 55% had no black neighbors and 56% had no Latino neighbors.

Furthermore, if we only look at upper-income and high-education whites, we find that liberals were actually somewhat less likely than conservatives to have racial minorities in their midst. Diversity does not stop with outward appearances, however. Real differences involve attitudes, beliefs, and lifestyles. To embrace true diversity means to live around people who interpret the world in different ways. For example, religious conservatives can benefit by living around secular liberals, and vice versa. Unfortunately, this is often not the case.

We constantly hear from non-religious liberals about the predations of the religious right, and especially the dangerous intolerance of American evangelicals. But do secular liberals actually live close enough to any religious folks to know them personally? You would assume so; after all, the Democratic National Committee has declared that, “The religious diversity of our party reflects the rich religious diversity of our nation.”

But in fact, nearly half (45%) of liberals in the 2006 data who called themselves secular — attending a house of worship seldom or never — admitted to having no church-attending neighbors at all. Meanwhile, only a quarter of religious conservatives had no secularist neighbors — despite the fact that non-attendance is rarer than regular attendance at a house of worship in America today.

Many data sources reinforce this finding. According to the Social Capital Community Benchmark Survey, secular liberals in 2000 were 12 percentage points more likely than religious conservatives to say they had no personal friends with a different religious orientation than their own. In heavily liberal communities like Seattle and San Francisco, this difference opens to about 20 points. Despite the rhetoric about diversity and inclusion, the secular left today is significantly more likely to be cut off from opposing worldviews than the religious right is.

Maybe it doesn’t matter. Just because you don’t see people with whom you disagree religiously and politically, you can appreciate their views, right?

I believe this is wrong. Personal exposure to divergent worldviews in a democratic society pushes us to be more tolerant, because we are thus less likely to dehumanize people who see things differently than we do. It’s easy to see an atheist or an evangelical as the enemy — until you meet at a block party or your kids become friends with hers. Furthermore, it’s simply more interesting to live around different kinds of people than in a group where everyone thinks the same way.

Despite the rhetoric, the unfortunate reality is that there appears to be less actual diversity in our personal lives than we often like to argue. Neither political side has that much to brag about. What is truly ironic, however, is that liberals — who are using the issue so heavily for political gain in the current election cycle — are arguably weaker than conservatives when it comes to diversity of political and religious beliefs. Perhaps the right — particularly the religious right — should not so willingly concede this cause.

Mr. Brooks, a professor at Syracuse University’s Maxwell School and a visiting scholar at the American Enterprise Institute, is author of the forthcoming book “Gross National Happiness.”