Regarding J.P. Rushton's argument that we assortatively mate with those most like us genetically, I want to suggest a distinction that might help us better understand mating patterns. While people do indeed tend to marry people who are similar to them on a number of traits, people do not want to marry people who are exactly like them.
Most obviously, they want someone of the opposite sex, with all those highly opposite physical and mental sexual secondary characteristics. Even homosexuals show this tendency: judging from Personal Ads, most gay men want a man who is more masculine than themselves, and most lesbians want a woman who is more feminine.
Thus, it's useful to distinguish between social attraction (which urges us toward mating with those like ourselves) and sexual attraction (which urges us toward mating with those not like ourselves). Social attraction is when your family owns a chain of supermarkets in Ohio and your beloved's family owns a chain of supermarkets in Pennsylvania; you both "summer" in Maine; have mutual friends, etc. What would make more sense than for you two to get marriage?
Of course, sexual attraction does not always align with social attraction. One of the most common storylines in movies is this: the blonde debutante gets engaged to the blonde fraternity president, but then she falls hard for the tall, dark, and handsome boy from the wrong side of the tracks.
Why have I seen a hundred movies with this basic plot? Why in roughly 90-95% of the sex scenes in big budget Hollywood movies is the man darker than the woman? (One reason, among many, that I so anticipated the great Ron Shelton's "White Men Can't Jump" is that I wanted to see how Hollywood handled a love scene between brown-skinned Rosie Perez and The Whitest Man in Hollywood, Woody Harrelson. Would they break the pattern and have a man who was lighter colored than his woman? Well, with enough beige lighting and body makeup, they made both come out an even butterscotch color, so the streak continued.)
Peter Frost has explained why this is so. All else being equal, women really are fairer than men. This was extremely obvious in monoethnic societies, although in modern America few people consciously notice this because of the wide racial variations in coloring. But we unconsciously notice this.
That's why Hollywood likes its women blonde and its men tall, dark and handsome. And it's not just Hollywood. Try watching Mexican TV. It's wall to wall blonde babes. The only time you see Mexican-looking girls is in commercials made by American firms like Busch and Miller.
Peter also argues that it's also likely that women from icy climes are sexually selected more for sex appeal because wives are very expensive in regions where hunting husbands have to feed them over the winter. Conversely, West African males like Michael Jordan have become icons of masculinity possibly because in climates where women can feed their families, husbands are expensive and need to earn their dinner by looking sexy.
Thus, we see patterns in interracial marriage that can't be explained by traditional assortative mating theory. Since African-American women tend to be more middle-class than African-American men (e.g., 80% more black women than black men in grad schools), assortative mating theory should predict that there should be more white husband-black wife marriages. But, of course, in 1990 there were 2.5 times more black husband-white wife marriages. Why? Because social attraction (homogeneity) and sexual attractions (heterogeneity) are frequently opposed.