Huntington's disease is a genetic disorder that invariably leads to death, after causing cognitive, psychiatric, and muscular problems. The onset is usually in the middle of adulthood. There is no cure.
The children of someone with Huntington's disease have a 50% chance of having the disease themselves. A genetic test is available (for about $300) that can tell whether any individual will develop the disease.
Imagine, for a moment, being a 20 year old child of a parent diagnosed with Huntington's disease. Would you want to take that test, and know whether you were going to get Huntington's disease 15 or 20 years down the road?
When I first read that question, I thought “Of course I'd want to know! What idiot wouldn't?!?” I can think of all sorts of life choices I'd make differently if I knew I would die early. Then I read that 95% of people at risk don't get the test. This fascinating Freakonomics podcast explores the reasons why most people at risk don't get the test.
Myself, I think they're all crazy...