Regarding Chris Bertram's ongoing discussion of racial intermarriage rates: it's worth noting that the two groups whose out-marriage rates are being compared ("blacks"-- i.e. African and Afro-Caribbean immigrants and descendants of immigrants-- in the UK and "blacks"-- those two groups plus African-Americans-- in the U.S.) are of different sizes. Blacks in the UK are, I believe, something in the neighborhood of 2-3% of the total population. Blacks in the U.S. are something in the neighborhood of 10-12% of the population.
All else being equal, out-marriage rates tend to be much higher among smaller populations. There are fewer in-group members to choose from, fewer whom one meets at school or at work or at play. Of course marriage isn't a matter of randomly selecting a mate from the population at large. But those elements of it that resemble randomness push in the direction of higher exogamy rates from proportionately smaller populations.
Note too that the way the statistic is being expressed-- proportion who marry out-- may be biased. It's very likely that if you asked what proportion of whites marry blacks, the answer would be higher in the U.S.-- because, again, of the different relative populations. (There are many fewer whites relative to blacks in the U.S. than in the U.K.) Would the answer be as much higher as the black share of the U.S. population is higher than the black share of the British population? I have no idea. I know there are ways to statistically normalize for this sort of thing, but I don't offhand remember what they are...
I doubt that these population-proportion differences are the whole story. But they're undoubtedly part of it. The raw comparison of exogamy rates among British and American blacks doesn't show very much.
FInally, the BBC article Chris draws his inferences from isn't very carfeul about this kind of thing. The claims made on behalf of Britain are:
1) "one of the fastest growing mixed-race populations in the world"
2) "Data from the 2001 census due to be released later this year is expected to confirm that Britain has one of the highest rates in the world of inter-ethnic relationships and, consequently, mixed race people. By 1997 already half of black men and a third of black women in relationships had a white partner according to a major study of ethnic minorities published by the Policy Studies Institute (PSI). It also revealed that other inter-racial relationships were flourishing with a fifth of Asian men and 10% of Asian women opting for a white partner."
I can't find the PSI study. But notice that there's no mention at all of white rates of exogamy. Nor is there any mention of interracial marriages as a share of all marriages. The inference that the UK has an especially high share of mixed race people in its population is therefore especially dubious. It might be true; but I doubt it, and even 100% exogamy rates among blacks wouldn't necessarily make it true.