In the days of old, deafness was arguably a fate worse than death, since it left you unable to produce or understand speech. It also must have reduced fitness to near zero, for those who care about such things.
Today, with deaf education and sign language, it’s a lot better. But it’s still not good.
Deaf adults have poor reading levels (~4th grade, on average), are far less likely to graduate from high school and college, and have a tough time making a living. Their unemployment rate is at least 40%, and seems to be getting worse with time. There used to be some blue-collar jobs that they fit pretty well, but those have become scarcer.
Severe congenital deafness hits about 1 in 1000 children. Some cases are caused by various environmental insults, but most are genetic. One particular mutation (GJB2 35delG) accounts for a fair fraction of genetic deafness (and almost certainly confers some kind of heterozygote advantage), but mutations in many different genes account for the majority of cases. Prenatal rubella used to cause a lot of cases, but that ended when there was an effective vaccination campaign back in the 1970s.
Because of the diverse mutational spectrum, > 90% of the children of deaf couples can hear. Many deaf people wish that their children were deaf like them, but they’re wrong, of course.
I was wondering about the language skills of those hearing children with deaf parents. It seems as if it might well be a disadvantage, but then kids are flexible and robust. We know that typical between-family variation doesn’t have much effect on adult IQ: but on the other hand, parents who can’t talk are pretty far from typical. I dug around, looked at a number of studies, but I’m not sure what the real story is. A lot of them were small-N, statistically useless – someone wrote about three kids. Sheesh. I got the strong impression that some of the articles were motivated: they didn’t want to give the impression that having deaf parents was bad for a kid, even if it were true. Still, that doesn’t mean that they’re necessarily wrong.
Then the light dawned. Or, more accurately, the sun set. If the hearing kids of deaf parents don’t suffer a disadvantage in language development, it’s hard to see how the currently fashionable idea that lower verbal skills in black kids are caused by parents who don’t talk or read to them enough can be true. Yet it must be. And if they are disadvantaged, then that disses deaf parents – and that can’t be right. Does not Compute!
This happens a lot.