We often confuse evidence with proof and overestimate the reliability of our instincts. Bad policy results.
FOR two generations, Americans ate fewer eggs and other animal products because policy makers told them that fat and cholesterol were bad for their health. Now both dogmas have been debunked in quick succession.
How did experts get it so wrong? Certainly, the food industry has muddied the waters through its lobbying. But the primary problem is that nutrition policy has long relied on a very weak kind of science: epidemiological, or “observational,” studies in which researchers follow large groups of people over many years. But even the most rigorous epidemiological studies suffer from a fundamental limitation. At best they can show only association, not causation. Epidemiological data can be used to suggest hypotheses but not to prove them.
Instead of accepting that this evidence was inadequate to give sound advice, strong-willed scientists overstated the significance of their studies.
The challenge, of course, is determining the risks of giving—and following—bad advice. The unknown unknowns are precisely that: unknown.