Companies are right to worry that customers will defect to the competition, but it makes more sense to concentrate on the changing spending patterns of those who don't.

Companies spend millions on loyalty programs to prevent customers from defecting to the competition. Yet some of that money would be better invested in efforts to track the spending habits of the remaining customers, since many more people change the amount of money they commit to a brand than wholly abandon it. At one retail bank, for example, the 5 percent of checking-account customers who defect each year take with them 10 percent of its checking accounts and 3 percent of its total balances. But the 35 percent of customers who reduce their balances substantially during the course of a year cost the bank no less than 24 percent of its total balances. What's more, the 35 percent who put more money into their accounts raise its total balances by 25 percent. This effect showed up in 16 industries—and did so prominently in two-thirds of them.

To understand better why customers behave as they do, McKinsey segmented them into six categories: three comprising people who remain active and loyal as a result of rational decisions, emotional ties, or plain old inertia, and three who spend less because of fiscal need, competitive switches, or dissatisfaction. For more information about these types of customers and how to influence their purchasing decisions, read "Customer retention is not enough."