I tweeted: Should I be worried or reassured that my taxi driver isn’t wearing a seat belt? An econ puzzle.

Most replies said I should be worried. I think that is correct and it reveals something of importance. First note that there is an incentive and a selection effect. All else equal, a driver without a seat belt should drive more carefully—that’s the rational response to increased personal risk. But drivers who forgo seat belts are probably more risk-loving or less safety-conscious across many dimensions. I think the replies were correct, the second effect, the selection effect, dominates: be worried.

What makes this an economics puzzle is that it reveals a failure of the standard adverse selection story. Adverse selection predicts that if someone wants to buy a lot of life insurance, the seller should be suspicious—fearing the buyer knows something about their own health that the seller doesn’t. Unusually healthy people, by the same logic, should buy less life insurance.

Notice the parallel to the taxi driver: the driver is buying less insurance (by not wearing a seat belt) and so, by adverse selection logic, should be the safer type. But that’s exactly backwards.

In reality, people who buy a lot of life insurance tend to be the kind of people who take care of themselves on many margins—they eat well, exercise, go to the doctor. Insurers know this, which is why the per-unit price of life insurance falls with quantity. Big buyers are the good risk, not the bad one.

The taxi driver puzzle is a clean real-world case where the selection effect runs opposite to what adverse selection theory predicts. Adverse selection theory is correct that information asymmetries can challenge markets but it’s often not obvious which way the asymmetry runs (who know more about your life expectancy, you or an insurance company with millions of data points?). Moreover, preferences and norms can make the selection run the opposite way so be worried about the taxi driver without a seat belt and be happy when someone demands a lot of life insurance.