Consumers value convenience, and when you provide them with technology that lets them do what they want to do, when they want to do it, on their terms in the channel of their choice, that leads to loyalty and stickiness. So concludes an enormous study conducted by Bain & Company, in which it queried 177,937 banking customers around the world.

"We've seen a very clear correlation between loyalty, digital usage and share of wallet — those three form a virtuous circle," says Gerard Du Toit, partner at Boston-based Bain. "It's hard to perfectly say which is cause and which is effect: if you have the best product, you give the customer a reason to use your digital offering. But there's no doubt that the three go hand in glove. The more people adopt digital technologies in banking, the higher loyalty they have and therefore the more money the bank can coax out of them because they use more products."

Bain measures loyalty in terms of net promoter scores. It asks consumers if they would recommend their bank to others. It adds up all those who give their bank a 9 or a 10 (those with high loyalty), and subtracts those who give it 0 through 6 (the detractors), to come up with a net promoter score. It then correlates the scores with other measures of behavior — retention, number of products owned, actual recommendations and referrals.

Digital-led direct banks like USAA performed extremely well in Bain's study — the direct banks had an average 66 (out of 100) net promoter score, versus 62 for credit unions, 46 for community banks, 19 for regional banks, and 9 for national banks. In all four regions of the U.S., USAA ranked about 80, higher than any other financial institution.

Just the way online bill payment is "sticky," and dramatically improves customer retention rates because it saves customers time and it's a hassle for consumers to unwind all their bill payment setups to switch providers, digital banking forges a kind of loyalty from the sheer nuisance factor of having to un-enroll and re-enroll for the service someplace else.

And as the world moves toward mobile banking, getting that right is at the center of any effort to win customers and loyalty, Du Toit says. "Bank technology has moved from back stage to star billing, because customers are seeing that it makes their lives easier," he says.

Making routine transactions (checking account balance, making a payment) quick, easy and free is important, therefore digital delivery is critical, Du Toit observes.

"Then there's another category that we call the moments of truth, those are things that are a bit more complicated," he says. Examples could include losing a card or needing advice on something like a mortgage. At such times, ease and convenience still matter and digital is becoming more prevalent, but by and large, the physical channel and talking to a person is essential.

The study found that about a third of new product sales at banks are pushed by a bank making an offer, rather than the customer requesting a financial product.

"Increasingly banks are working on doing that effectively through digital channels," says Du Toit. "That's the next frontier for the banks to get right — how to effectively, easily and conveniently sell products through digital channels."

U.S. customers still choose their banks based on convenience, defined primarily by having a branch and ATMs near where the consumer works or lives.

"What's changing is that the banks need to dramatically reduce their cost," Du Toit observes. "There are far too many routine interactions happening in the branch, including people coming to tellers to get cash and do other routine interactions."

Some banks are moving aggressively to take teller functions out of the branch and handle them through ATMs, including video ATMS where the humans sit in a centralized location, he points out. "You can have longer hours, and you don't need offices and a security system," using video ATMs, Du Toit notes. "You then can improve convenience to the customer while at the same time dramatically reducing cost to the bank."

One U.S. bank in the Bain study has a remarkable customer loyalty turnaround story: JPMorgan Chase.

"Chase has been on this journey for a few years, and this year it's really starting to pay off for them," Du Toit says.

In 2010, SunTrust led banks in the South in terms of loyalty, with a net promoter score of 21. Chase had a -7% NPS — in other words, it had more detractors than promoters — in the region. By 2013, Chase had improved to a 29% NPS — a 36 point improvement over three years, which is very impressive, Du Toit notes. "We're seeing similar improvements for Chase in Northeast as well, which is the most important market for them. They've improved by 24% in NPS for the last three years in that region. They're still behind TD, but they closed a 20-point gap to a 7-point gap."

Key to this rise has been Chase's mobile banking strategy, in which it has moved aggressively to add capabilities and drive adoption. "You'll remember the way they were one of the earlier movers to have remote deposit capture," Du Toit says. "USAA did it, but Chase talked about it in their TV campaigns. That's one of those features like online bill pay where when you do it, it's a real delight because it saves so much time and has a wow factor, a cool factor."