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A libertarian think tank, reckon that eliminating all travel visas to the United States would add between $90 billion and $123 billion in annual tourist spending. By one estimate, introducing visa restrictions can lower trade and foreign direct investment between a pair of countries by as much as 25% .

For any service company that bills on a recurring basis, a key variable is the rate of churn: How many customers cancel? In many competitive industries, churn can be substantial — some wireless carriers, for instance, lose 3% of subscribers each month. Companies with high churn typically spend vast sums on marketing to try to replace all those defectors. New research shows that they might be better served by smart strategies aimed at getting lost customers to come back to the fold.