This is not a joke.
Several scientific studies have shown that redheads are resistant to local anaesthetics. This means that when they go to the dentist, it hurts much more than for other people. As a consequence, as is also shown by scientific research, redheads are more afraid to go to the dentist than others, and they often put it off as long as they can, resulting in again an extra painful treatment.
The solution is actually very simple: The dentist just has to give them 20% more aneasthetics than usual. Duh!
When some of these studies were presented, many redheads were insulted and felt that this type of research is stigmatizing and can be compared with blonde jokes. Wake up people, it's simply a scientific study! Nothing to get upset about. Many of the angry redheads also mistakenly understood that they are more sensitive to pain, which is not what the research said.
Maybe as some kind of natural compensation for their pain, many redheads don't have wisdon teeth or have less than four. This has not been confirmed by research that I know of, but I simply noticed that many redheads were saying that in various internet discussions.