Genetics are wonderful to study, well done OP.
I was going to mention that India has lots of inbreeding too of that type and north africa but your image covers it.
The problem with inbreeding is it is almost universal not to marry to 1st degree inbreeding, kids, siblings, etc. However it is not unusual for everyone except Europeans to pick first cousins specifically to keep wealth in the family and also because Islam encourages no dating because how can a man meet a woman if he can't see her. So people go for the second best thing which is financial planning. You know for a fact what your family has for finances, you can never be sure about the honesty of strangers with arranged marriages. These practices are done despite relatively high population densities. These are not pioneers settling sparse lands on the frontiers with max 48 neighbors within 2 days horse ride.
Genetically 3rd cousins are the best compatibility. However if you do the math if first cousins are 25% on average similar and second cousins are 12.5% then third cousins are 6.25% similar.
On average, people from your nation, such as bavarians with bavarians or londoners with londoners, are about 3% exactly same to you. Third cousins therefore are almost like coupling with a stranger of your very own nation.
The problem with anything closer than 3rd cousins coupling, especially repeatedly within 1-2 generations is that just because someone is on average 12.5% similar to you does not mean they are only that similar to you. Third cousins have the potential to be 0-25% similar to each other, first cousins potential to be 0-50% similar to each other and siblings have the potential of being 0-100% similar to each other. What can happen from repeatedly couplings then of 2nd degree cousins or closer is you end up coupling with people that have the potential to be as related to you as your siblings would be had there been no inbreeding of such close degree.
The other problem, racemixing, or reproducing with organisms so far away from you as possible while still creating viable offspring, is that many genetic mechanisms you have are more complicated than 3 or 4 genes working together. Entire genetic networks that modify and create particular kinds of proteins to particular, possibly rare kinds of, stimuli could involve dozens of genes, on different chromosomes, inherited completely separately. This means if you reproduce with someone of your nation, on average 3% exactly the same as you then you are likely to inherit beneficial genetic networks. With different races evolving to different environments for thousands, perhaps longer, years the chances of those genetic networks, that are inherited completely separately, by chance recombining in some racemixed offspring in 2 generations or later are nearly none. You could need 23 genes for a particular genetic network and if you are missing just 1 it works less than 50% as well and if you are missing 2 genes it will simply not function at all. Only from very high degrees of inbreeding, specifically purebred dogs, do you get any benefit from mating with the most distant relatives. And even then, for anyone that has tried to learn about dogbreeding, you will not mate a jack russel terrier with a labrador retriever to improve health.
Hopefully OP has enlightened you all about why Europeans are so healthy, beautiful, and intelligent compared to all other races, on average. Even if our environments didn't drive us to be so great, our social habits of not dating our cousins would raise us above the rest.
