It's not foreclosures or bad paperwork—it's being stuck in slow-growth economies while the developing world and its banks are booming. So, what's an investor to do?

Here's my big insight for today: US banks have a big long-term problem.

But not the one you're thinking of right now. (Yes, I do have secret powers and can tell what you're thinking—although only about bank stocks.)

The headlines are full of news on robo-signers, mortgage foreclosure moratoriums, and put-backs that will cost banks billions over the next five years and drag out any resolution of the US mortgage crisis.

But that's not the biggest problem facing US banks and not the one that in the long term will doom them to second-class global status unless they can fix it.

What's that problem? The big US banks—JPMorgan Chase (NYSE: JPM), Bank of America (NYSE: BAC), Wells Fargo (NYSE: WFC), and Citigroup (NYSE: C)—are locked out of the world's fastest-growing banking markets (emerging economies, especially in Asia) and are locked into some of the slowest-growing (the United States and Europe). And I don't see an easy way in the long term for these US banks to break out of their slow-growth trap.

And that—if you agree with the argument I lay out below—should determine how you invest in these stocks, if you invest in them at all.

The Current Slump

Let me pick on JPMorgan Chase, the best managed of the big US banks and the one that came through the financial crisis in the best shape, to illustrate the problem. In its October 13 third-quarter earnings report, JPMorgan Chase announced disappointing revenue results: A drop of 15.4% from the third quarter of 2009.

Some of that decline was because of the widely anticipated drop in trading volumes, and therefore, in trading revenue in the quarter. Revenue from JPMorgan Chase's investment banking business fell to $5.4 billion, down from $7.5 billion in the third quarter of 2009 and $6.3 billion in the second quarter of 2010.

But let's look at the rest of the company's core businesses. In retail financial services, revenue fell 7% from the third quarter of 2009. The part of that larger unit called retail banking showed a 3% decline in revenue from a year earlier. Credit services net revenue fell 18%, and mortgage banking net revenue fell 7%.

Now, you can say that those drops in revenue were the result of a slowing US recovery and assume that revenue will grow again when the US economy comes out of this slump in the recovery—in, economists now project, the second half of 2011.

And I'd totally agree. That's why a stock such as JPMorgan Chase is a good medium-term buy and why I added it to my Jubak Picks portfolio on September 16 with a target price of $55 a share. (To see more on this pick and my entire Jubak's Picks portfolio, go here.)

Buying a good US bank stock in anticipation of a stronger US economy in the second half of 2011 is a good idea.

The Longer View

But that's just the medium term. Remember I said that US banks have a big long-term problem?

They do. It's called slow growth in the world's developed economies, where these banks do the bulk of their business. Much slower growth than in the world's developing economies.

Look at the latest economic projections from the International Monetary Fund (IMF) to get a feel for the growth disparities.

On October 6, the IMF released its forecast for global economic growth in 2010 and 2011. Developed economies are projected to grow, on average, 2.7% this year and 2.2% in 2011. The deceleration will begin in the second half of 2010 and continue into the first half of 2011, before growth picks up again in the second half of 2011. (With the IMF looking for just 2.2% growth for the entire year, the first half of 2011 could be very rocky indeed.)

The IMF projected that the US economy will grow 2.6% in 2010 and 2.2% in 2011. The economies of the euro zone are expected to grow an average of 1.7% in 2010 and 1.5% in 2011.

Developing economies, on the other hand, are projected to grow, on average, 7.1% in 2010 and 6.4% in 2011. China's growth likely will slow to 9.6% in 2011 from a projected 10.5% in 2010. India will grow 9.7% this year, the IMF projects, and 8.4% in 2011.

Add in the contrast between a saturated banking market in the US and a huge unbanked population in the developing world, and the gap in relative growth potential gets even larger. Consulting company McKinsey estimates a global count of 2.5 billion people who aren't served by either formal or informal banking institutions. About 2.2 billion of these live in Africa, Asia, Latin America, and the Middle East.

Not all of these unbanked people will become banking customers anytime soon. Many are too poor or live in remote areas. But the figures do give a general feel for the potential in developing economies.

You can get a more precise measure by looking at financial products such as credit and charge cards. In 2005, credit and charge cards per capita in the US stood at 2.53, according to the International Trade Administration. That same year, credit and charge cards per capita stood at just 0.02 in India and 0.38 in Brazil. No wonder consulting company RNCOS projects compound annual growth in credit and charge card numbers in developing countries of 12% from 2010 to 2013.

NEXT: Who's Getting in This Game?

Tickers Mentioned: Tickers: JPM, BAC, WFC, C, SCBFF