There are two reasons that you might see common deleterious recessive alleles in a population.
Founder effects or other bottlenecks: the population is so small that some ungood mutations have become unusually common, just by chance. The total number of bad mutations is generally not higher, but particular ones are. Since they’re usually recessive, a 10% frequency of one bad gene causes more trouble [ 1% homozygotes) than 5% frequency of two different bad genes [ 0.5% homozygotes] .
If you mix a lot with neighboring populations, gene frequencies tend to even out. But endogamy is not the cause: it’s going through a time of very small population size. You’re not going to see a lot of unusually common Mendelian diseases driven by mutation in a population that has been large for a long time, even if they are endogamous.
Second reason is that an allele has heterozygote advantage, increases the fitness of heterozygotes, like sickle-cell or cystic fibrosis. These tend to cluster in metabolic paths related to the selective agent: in areas with lots of malaria, you see a number related to hemoglobin or the red cell. We have fairly recently found a couple that defend against sleeping sickness: likely we will find more.
Common deleterious mutations due to bottlenecks are random: they don’t cluster in metabolic paths.
Most deleterious mutations are rare, haven’t been around for long. If you marry your first cousin, the odds of a kid being a homozygote for some such go way up, since the cousin has a fair chance of sharing that rare mutation. On the other hand, if the mutation is common in your population, due to chance or selection, your children have a noticeable risk for being homozygotes even when you and your mate are not closely related. In other words, Tay-Sachs in Ashkenazi Jews is not a product of inbreeding.
Since the average person has many rare deleterious mutations, with effects ranging from lethal to being a little subpar, children of such consanguineous unions have a spectrum of problems, ranging instant death ( rare) to ” not very healthy”, not so rare. On average, kids of first cousins seem to have have about twice as many serious genetic problems, average about 5 iq points lower & 0.7 cm shorter.
Where is inbreeding a problem? Much of the Moslem world.
Acceptance of consanguineous unions is increasing in liberal circles, since A. It’s an Islamic thing and B. It’s obviously stupid.