When it comes to banking, culture matters. But simply having a good culture is not enough. Great banks also work hard to ensure that it is passed down from one generation of leaders to the next.

By "good culture," I mean a relentless focus on expense and risk management. And not just paying lip service to these things on conference calls and in the narrative of annual reports. There must be evidence of them in the financial statements themselves.

As I've discussed before, the only way for a bank to consistently earn superior returns is to keep expenses low and write good loans. This isn't complicated, but it's hard to do. So hard, in fact, that we can trace the cultures of the two best big banks today back to a single individual: Carl Reichardt, the CEO of Wells Fargo (WFC -6.02%) from 1983 to 1994.

Reichardt was a relentless cost-cutter. He remains infamous today for doing away with the Christmas tree in the executive suite three decades ago and refusing to approve the installation of draperies in an executive's office. His obsession became known as the "Wells' way."

Just as importantly, Reichardt was good at instilling these values in subsequent generations. While Bank of America and Goldman Sachs have built and moved into elegant office towers in New York and New Jersey, Wells Fargo's current CEO John Stumpf refuses to update the "shabby" decor and rust-red carpet of its aging executive suite. "That's not us," says Stumpf.

Even more telling is the fact that Reichardt's influence found its way to U.S. Bancorp (USB 0.41%). The current CEO of the Minneapolis-based bank, Richard Davis, formerly worked under Jerry Grundhofer, who, in turn, once reported to Reichardt during a stint at Wells Fargo.

Grundhofer, like his 61-year-old brother, rose through the ranks of Wells Fargo bank in California. Both took to heart the 1980s-era cost management principles instilled by former Wells chief executive Carl Reichardt. Jerry would later adapt what became known as the "Wells say" -- an intense focus on efficiency balanced by strategic investments and generous employee incentives -- into what might be terms a "Firstar way."

That was Jack Milligan, the managing editor of Bank Director Magazine, writing in Nov. 2000 about the acquisition of U.S. Bancorp (which was run at the time by Jack Grundhofer, Jerry's older brother) by Firstar Corp., the Milwaukee-based bank run by Jerry.

Given this, it should come as no surprise that Wells Fargo and U.S. Bancorp both shine at expense management. As you can see in the chart below, they had the two best efficiency ratios among the nation's 10 biggest commercial banks in 2014 -- and, for the record, the same could be said for virtually any other year over the past few decades.

The point is that the culture of a bank can't be emphasized enough. If you want to make money with bank stocks over the long run, the two most important things to check are that a bank's culture revolves around controlling expenses and prudent risk management, and that its culture has been successfully transmitted over multiple generations of leaders.