Performance indices, which rank social issues or policy outcomes in different countries by combining related measures into a single score for each, are enjoying a boom. Their number has soared over the past two decades. They are popular, influential—and sometimes flawed.

Close to 90% of Swedish fathers take paternity leave. In 2013, some 340,000 dads took a total of 12 million days’ leave, equivalent to about seven weeks each. Women take even more leave days to spend time with their children, but the gap is shrinking. Why do Swedish dads take so much time off work to raise their children?