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Days after the Federal Reserve approved the merger of Capital One and ING Direct last month, the combined company started announcing changes to ease customers' minds. ING customer accounts wouldn't change, for example, and ING customers would gain access to Capital One's ATMs at no cost. There's no mystery why the bank so quickly made such moves: When mergers occur, customers demand consistent and seamless services across both companies from the start. If they don't get them, they defect.

Customer defections are a major reason why more than half of all mergers fail to deliver the intended improvement in shareholder value, as our own studies and those of others (including KPMG and Watson Wyatt) have shown. The trouble is that merged companies tend to focus primarily on quickly capturing synergies and avoiding major technology disasters. They typically lose sight of customers at the time when they are most likely to bail.

It doesn't have to be that way. The merger and integration process offers natural opportunities to re-evaluate—and even improve—the customer experience. First Union Bank lost 20% of its customer base in the year after purchasing CoreStates Financial in 1997. But in 2008, when Australia's Westpac acquired St. George Bank, an institution one-third its size, Westpac's leaders set an audacious goal of not losing a single customer and largely succeeded.

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