In the early 1980s, a Taiwan steel company accidentally mixed some highly radioactive cobalt-60 into a batch of steel rebar. The radioactive rods were then used in the construction of 1,700 apartments. As a result, people living in these buildings were subject to radiation up to 30 times the normal amount received from the natural background.

When dismayed officials discovered this enormous error 15 years later, they surveyed past and present apartment dwellers expecting to find an epidemic of cancer. Normal incidence would...