Genetic Future points to a new paper which suggests that inbreeding is declining over time. Their methodology involved surveying different age cohorts and noting the decreased levels of homozygosity among the youth. Dienekes points out that the use of living people might simply be confounding inferred mating patterns with the fact that homozygous individuals have a higher life expectancy. The plausibility of these two hypotheses varies based on how much weight you put on the shifting of mating patterns. In Consanguinity, Inbreeding, and Genetic Drift in Italy the authors used Catholic Church records to show quite clearly that inbreeding declined rapidly with the rise of modern transportation and communication. But that decline varied by region, with the shift toward exogamy occurring first in the north and moving over time toward the south. In Europe as a whole the rate of consanguinity has not changed monotonically over the past 1,000 years. After the Reformation cousin marriage was sanctioned in many Protestant nations whereas before the Catholic Church had strictly proscribed it. The main caveat to this is that religious regulation of marriage was a much greater concern for elites than it was for common people, as the latter often entered into common law relationships which were never formally solemnized in any case. In any case, after the initial spike in cousin marriages in Northern Europe the same variables which drove up the rates of outmarriage in Italy were also instrumental in driving the proportion back down.
Could this contribute to the Flynn effect (rising IQ scores over time)?
yes, this is a hypothesis that has been mooted....
Americans (now) associate inbreeding with low-status whites in Appalachia. When Americans think of inbreeding, they think of mountain hicks: Cletus from The Simpsons, the lads from Deliverance, the KKK and the rest of it.
Inbreeding is kind of gross anyway, and unnecessary; and with the Church aligned against it too, few people would get tempted today. But I suspect that marrying a third or even cousin might be more acceptable if it weren't for the Jerry Springer associations.
I would expect homozygosity to have a U-shaped relationship with age. Presumably individuals homozygous for deleterious versions of genes die out first. They are followed by heterozygous individuals, finally leaving individuals predominantly homozygous for advantageous versions of genes standing in the oldest cohorts. Or would something else operate for the Runs-of-Homozygousity measure that the paper uses?