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.
Hmmmm this is actually a good hypothesis! my grandfather died of dementia and part of the reason i got my DNA tested was to see if i had the gene for that (thankfully I do NOT!)
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