People in a genetic database have segments of DNA in common unexpectedly often.
13 points
Could it be that people who have a higher suspicion they are at risk for a hereditary illness will participate? Then the DNA they share is actually to do with the illnesses and not directly coding for a trait that makes them want to participate. This assumes the higher suspicion correlates to a higher history of hereditary illness which I feel would check out.
6 points
