People don’t mate randomly ...

The idea that correlation does not imply causation is a fundamental caveat in epidemiological research. A classic example involves a hypothetical link between ice cream sales and drownings – instead of increased ice cream consumption causing more people to drown, it’s plausible that a third variable, summer weather, is driving up an appetite for ice cream and swimming, and hence opportunities to drown.

People don’t mate randomly – but the flawed assumption that they do is an essential part of many studies linking genes to diseases and traits