It's official: You are now out of excuses to ignore foreign stocks for your portfolio.

Why? Because a big, beleaguered bank says so!
The Wall Street Journal reported that Citigroup advised investors to tilt their stock portfolios up to 55% toward foreign exposure, up from their current levels of 30%.

That's even more aggressive than the then-shocking 40% that Wharton wiz Jeremy Siegel recommended in his 2005 book The Future for Investors. Siegel saw a world economy growing much faster than our own: "We must look to these changes taking place in our global economy as opportunities and not as threats. Huge markets await those willing to tackle them."

The evidence, though, shows that not many U.S. investors are willing to actually tackle them. According to the Journal, the typical American mutual fund investor "holds 12% to 15% in foreign stocks -- or half that in a 401(k)."

That's putrid.

Perhaps even more putrid is that some American investors are actually against investing in foreign stocks. Indeed, a reader comment from Tim's recent article -- about a stock posed for extraordinary growth -- is illustrative:

Why do we have to spend so much time discussing non-American stocks? I really don't care if I could make more money investing in Chinese or other foreign companies, because on principle, I refuse to invest in anything but American companies. I can't imagine that I'm the only one who believes it's best to keep American money in America, so it would be really nice to get more articles about American stocks.

Although we understand the gentleman's sense of patriotic pride, xenophobia has no place in investing. The fact of the matter is that we must all adapt to an increasingly globalized world where the line between "ours" and "theirs" has become blurred -- and will only get more blurred.

Though this can seem frightening, we view it as good news ... because it can benefit us all.

For example, just as Germany's Deutsche Telekom (NYSE:DT) and Canada's Research In Motion (NASDAQ:RIMM) have profited by selling mobile devices in the United States, New York-based IBM (NYSE:IBM) did some $15 billion in fourth-quarter sales outside the U.S.!

Then there are iconic "American" companies such as General Mills (NYSE:GIS) and Caterpillar (NYSE:CAT), which are counting on international and emerging markets to deliver future growth. Even Silicon Valley search king Google (NASDAQ:GOOG) is generating half its from foreign markets.

It's a waste of time trying to decide which one of these companies is the "most" American. What is important, however, is that every one of these companies, American or not, is focused on selling more products in rapidly growing emerging economies such as China, India, Chile, and Brazil. That's because, when compared to our domestic market, these underpenetrated markets offer more growth potential over the next decade.

That's also true of their stock markets
As these economies develop, their stock markets will rise. You can go along for the ride if you're ready, willing, and able.

But if you refuse to invest there -- or are too frightened by the volatility to commit to a disciplined, long-term international investment strategy -- then you will miss out.

Another word about international investing
We agree with the emerging consensus that 12% foreign equity exposure is low any way you cut it. Not only do foreign stocks offer heightened growth potential, but they can also give your portfolio some needed diversification from the American economy.

Yes, it's true that Europe and Asia have been hit by the same economic troubles currently causing panic in the United States. But if you live here in the U.S., you probably own U.S. equities, have a U.S. job, get paid in U.S. dollars, and have all of your savings denominated in those dollars. If or when something goes haywire, you're dangerously concentrated in one specific geographic region.

Though we're confident that our country will rebound from the current malaise, it will take time. Stock and real estate markets are down. Interest rates are low. The dollar is volatile. GDP growth ranges from anemic to nonexistent. This terrible confluence of economic weakness should serve as a wake-up call that you need to make foreign stocks a larger part of your portfolio going forward.

In other words, you have no more excuses to ignore these stocks
At our Motley Fool Global Gains international investing service, we believe we're finding some of tomorrow's next great stock market success stories by looking abroad. With the haircuts some of the world markets have experienced in the past eight months, many of our recommendations are trading at extremely attractive prices.

You can see our team's top five foreign stocks, and you can join our growing community of investors with a 30-day free trial. Just click here to get started.

This article was first published Oct. 17, 2008. It has been updated.

Brian Richards and Tim Hanson have no more excuses for ignoring the power of juicing with Jack Lalanne's Power Juicer. Neither Brian nor Tim owns shares of any companies mentioned. Google is a Rule Breakers recommendation. The Fool's disclosure policy regrets that first-round Tom Brady fantasy draft pick.