Really not an issue with second cousins. Second cousins are genetically different enough that issues with the children are just barely more common in non-related couples.
Yep. That's the kind of thing you look up at a young age when you discover you're the recent product of second cousins. Some interesting quotes from the "Cousin Marriage" Wikipedia article:
"According to Professor Robin Fox of Rutgers University, it is likely that 80% of all marriages in history may have been between second cousins or closer."
"Worldwide, more than 10% of marriages is between first and second cousins."
"In some countries it is seen as incestuous and is legally prohibited: it is banned in China and Taiwan, the majority of U.S. states, North Korea and South Korea."
Well Darwin had 10 children, but only 9 grandchildren. So maybe it didn't turn out so well. Not from a personal or moral perspective mind, just from strictly Darwinian perspective.
Darwin's family history is fascinating -- the intertwining with the Wedgewoods (as you note, Charles and Emma were first cousins), but doubtless exacerbated by the frequent first-cousin marriages in Emma Wedgewood's recent ancestry.
Darwin was aware of the risks of inbreeding, and this marriage in particular, expressing those concerns in various writings.
They had 9 or 10 children, IIRC, with 3 or 4 of them dying in childhood, and only a couple going on to produce a further generation. (Not so uncommon these days, but at the time I believe this was quite noteworthy.)
> Such a rate of childhood death would be high even for sibling parents.
Really? In the mid-19th century?
https://ourworldindata.org/child-mortality/ says "in 18th century Sweden every third child died, and in 19th century Germany every second child died" (that's before the age of 5; the usual definition of child mortality). The "England and Wales" chart on that site shows that in the mid-1800s child mortality was about 250-300 per 1000.
So out of 10 children you would expect 2-3 to die before reaching age 5 at the time.
A dude with whom I went to elementary school married his first cousin here in USA. This was in a red/"statute bans" state by the map at your link, but I never heard they had any problems from it.
From a genetic perspective, even occasional first-cousin marriage is not really a problem. The problems start when you have several generations of cousin and sibling marriages, allowing homozygous recessive alleles to concentrate.
What jacobolus said. First cousins are also genetically different enough that issues with the children are just barely more common than in unrelated couples. (The relative risk adjustment is quite high, but the absolute risk is still low.)
If marrying your first cousin is a culturally accepted practice, it happens all the time and people get much more inbred than you might expect from looking at, or calculating from, a stylized 8-person cousin marriage family tree. And that goes for second-cousin marriages too, to a lesser degree.
But first-cousin marriages generation after generation are not a great idea, as happened in various royal houses of Europe in order to make sure both parents were of noble blood. In fact, it has been estimated that the last Hapsburg ruler of Spain, Charles II, ended up with more homozygous alleles than someone who was the offspring of siblings would. And he had numerous physical and mental problems that were probably the result of it.
Your reply starts with "but", but I don't see the contrast with anything I said? My comment specifically notes that cousin marriage as a recurring practice causes much bigger problems than a single case of cousin marriage in isolation. That note is more than half of the entire comment.
Fun fact: people like to present Charles II as the inevitable result of ongoing inbreeding, but he was noticeably less inbred than more competent historical rulers. Amenhotep I and his great wife had, between them, two parents, two grandparents, and two great-grandparents. Unsurprisingly, he and his great wife turned out not to produce any viable children. Unsurprisingly, that line also experienced early death outside of combat. But surprisingly, they don't seem to have been plagued by mental difficulties at all; as best we can tell from the record, they were very strong kings.
It wasn't first-cousin marriages that screwed up the mainline Habsburgs; it was generation after generation of uncle-niece marriages that did it in. The current branch of the family (Habsburg-Lorraine/Hohenberg) was fortunate enough to avoid that.
"According to Professor Robin Fox of Rutgers University, it is likely that 80% of all marriages in history may have been between second cousins or closer."
"Worldwide, more than 10% of marriages is between first and second cousins."
"In some countries it is seen as incestuous and is legally prohibited: it is banned in China and Taiwan, the majority of U.S. states, North Korea and South Korea."
https://en.wikipedia.org/wiki/Cousin_marriage