Perhaps in the majority groups in the West. Certain communities in the UK still widely practice cousin marriage and suffer hugely in genetic abnormalities. They are not constrained by transport, it is a cultural thing that has not yet been updated.
http://www.dailymail.co.uk/news/article-1392217/Muslim-outra...
It's noteworthy that it's simultaneously socially unacceptable to criticize Pakistanis for their preference for first cousin marriages, despite the undeniable harm caused by it, and yet it's also socially unacceptable for whites to have the much less incestuous preference for just marrying other whites, despite the absence of harm:
http://www.slate.com/articles/news_and_politics/politics/201...
> Baroness Flather said because of 'inbreeding', members of the ethnic group are 13 TIMES more likely than the rest of the UK population to have disabled children.
> The Pakistan-born peer has called on the Government to take steps against the issue of unions between close relatives in Muslim communities, which she called an "outdated, un-British custom".
...
> She also accused Britain of a "cowardly reluctance to tackle damaging social practices within certain ethnic minorities".
> In what has been described as a brave attempt to break a taboo she said there was a fear of "accusations of racial prejudice" which has made the issue "unsayable".
> She added: "That is certainly what happened over the disgraceful sex-grooming scandal in Rotherham, in which the civic authorities, including the police and social services, tried to cover up the systematic abuse of more than 1,400 vulnerable girls by predatory gangs of Pakistani heritage men."
Why that's true: The reason inbreeding is bad is that the child is at risk of inheriting two copies of a nasty recessive mutation, in a region of their genome where both versions came from the same recent ancestor. However, if the child goes on to successfully reproduce, it's more likely that they had zero copies than two--and in that case the grandchildren have zero copies, which is good for them.