This article originally appeared in American Banker

News about JPMorgan Chase this year has swirled around the huge fines for the bank's alleged sale of faulty mortgage-backed securities during the credit boom. Yet behind the headlines, Chase has steadily been increasing its retail business by earning stronger loyalty from its customers.

Of all US national banks, Chase posted the biggest gains in Net Promoter Score—a key metric of loyalty, according to new Bain & Company research. Chase also tied with BB&T for overall loyalty leadership among retail banks in the South. That's due to such factors as select investments in mobile technology, a concerted effort to improve the customer experience and effective marketing on how the bank can simplify consumers' financial lives. Those factors also combined to help Chase perform well above average in winning new customer relationships and cross-selling to existing customers.

Customer loyalty and advocacy form the engine of organic growth in banking today, as other routes, such as fees, have diminished. Most US, banks made progress in earning loyalty during 2013, as indicated by their rising NPS. But Bain's online survey of 77,000 consumers in the U.S. shows banks are far from exploiting the full potential of loyalty to improve the economics of their business.

US banks formed new relationships at a rate of just 3.6% over the past year. Loyalty is half the battle here: A bank's NPS relative to peers in its market explains roughly half of the variation in its relative win rate (a metric that shows whether a bank is winning more or less than its fair share of new customers). Other factors influencing the win rate include the level of fees, convenience of branch locations and product features.

Given the low rate of new relationship formation, customer acquisition alone will not anchor a growth strategy.

More significant is the opportunity for cross-selling to the existing customer base. Some 41% of customers opened a new banking product over the past year, and half of those people bought from their primary bank.

Digital channels make it easier to purchase products from a new bank, but the unbundling of services should not limit a bank to a certain share of its customers' wallet. Product win rates – the share of products bought by respondents at their primary bank—range from 38% at the worst performer to 63% at Huntington National. Several years ago, Huntington began to consolidate customer data. It replaced its manual sales process with an automated one that made it easier for employees to manage cross-selling and upselling opportunities. These moves helped Huntington gradually grow the number of products held by customers.

Here again, loyalty plays a key role. The difference in product take-up between customers who are promoters of their primary bank (those who give an NPS of 9 or 10) and customers who are detractors (NPS of 0 to 6) is a healthy 12 percentage points. Indeed, loyalty leader USAA also leads U.S. banks in the average number of all products owned by its customers.

The most effective route to sustained growth, then, comes from joining up customer loyalty with several specific capabilities:

Decide where you must win and where you're willing to lose. Catering to the average customer means catering to no one in particular. Each customer segment has different priorities, expectations and lifetime value for a bank. Our survey finds, for instance, that older customers care more about branch location and quality of branch staff, while younger customers place greater value on mobile device interactions and rely heavily on recommendations from family, friends and colleagues.

Leading banks not only know such preferences and behaviors well, they act on that knowledge by taking distinctly differentiated tacks for each segment.

Accelerate the digital transformation. Digital technologies, when fast and easy to use, can delight customers. This year's survey tracks the ascendance of online and mobile channels: 45% of customers used smartphones or tablets to do their banking, up from 32% in 2012.

But many banks could do more to steer customers to the technology. Mobile usage varies from 28% for the worst-performing bank to 68% for USAA. Lagging banks should be concerned, because mobility continues to spur customers to recommend their bank to others. Mobile users rank their bank with an NPS 14 points higher on average than people who don't use mobile devices.

Loyalty gives you the right to win more business—but you do have to ask for the sale. We find that promoters buy more products with their primary bank than detractors do. That doesn't mean the business comes effortlessly – banks have to ask for it. Our survey finds, for example, that about one-third of banking products in the U.S, are sold, not bought. Customers did not plan to buy a particular product, but they received an offer and then decided to get it.

Some banks that are loyalty leaders score low on the sales metric. They haven't been forced to develop their selling muscles. The good news is their customers will accept the right kind of sales overtures. Other banks have high selling scores but lag on NPS. These players will want to concentrate on adjusting the tone of their selling and delivering a better experience after the sale.

Winning new customers, limiting attrition and seizing the cross-sell opportunity all start with a bank's ability to earn loyalty. But the banks that add these other capabilities to a foundation of loyalty will realize the greatest payoff.

Gerard du Toit and Maureen Burns are partners with Bain & Company's financial services practice.