A $10 trillion underground street economy that will employ two-thirds of the world's workers by 2020 has been uncovered.

Robert Neuwirth, in his book "Stealth of Nations: The Global Rise of the Informal Economy," treks through the developing world and finds small, informal -- sometimes illegal -- enterprises are changing the shape of the global economy.

I believe him because I have seen it myself. And if you have ever traveled in developing nations, you have too. Those kiosks and small stores selling the basics, where you can find pretty much everything and anything from cigarettes to pirated DVDs, employ a large chunk of the world's population -- half at the present time, according to Neuwirth -- and outsell Wal-Mart Stores (WMT, news). Indeed, big brands know this.

He cites numerous brands, including Procter & Gamble (PG, news), Colgate-Palmolive (CL, news) and Unilever (UL, news), among providers that distribute their products via this network -- sometimes knowingly and sometimes not.

Neuwirth calls this network of small enterprises "System D" -- taken and shortened from a French term.

"System D is growing faster than any other part of the economy, and it is an increasing force in world trade," Neuwirth writes. "What's more, after the financial crisis of 2008/2009, System D was revealed to be an important financial coping mechanism. A study by Deutsche Bank, the huge German commercial lender, suggested that people in the European countries with the largest portions of their economies that were unlicensed and unregulated -- in other words, citizens of the countries with the most robust System D -- fared better in the economic meltdown of 2008 than folks living in centrally planned and tightly regulated nations."

Moreover, given that the population of the developing world is growing right along with those nations' economies, it makes sense that the sales outlets people are used to shopping in would grow, too. And smart brands, such as those mentioned, should benefit. It's easy enough to spot big brands even in remote places. U.S. soda producers, snack makers and bathroom product distributors are seemingly omnipresent the world over.

This should give investors pause as to which companies to invest in for the future. Small growth companies don't have the distribution systems that big, old-school consumer brands do.

To be sure, big brands often don't want the association with street stalls and the like, especially if these outlets are selling illegal items, which many do. Neuwirth points this out as well and explains that third-party distributors are often involved.

In any event, the power of the street economy should be taken seriously.

If you need a prepaid phone card in Addis Ababa, you won't find a pleasantly lit convenience store chain; you'll likely end up shopping at a muddied hut near the Merkato, Africa's largest market.

Neuwirth describes that market in "Stealth of Nations." Alien perhaps to us, with items strewn along tables and on the ground to be bartered over, but common enough to most people in the world. Such commerce is the future.

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From micro-loans that support these street vendors to big brands that stock the shelves, such as they are, there are many ways for investors to play System D for profit.

It's well worth reading about and investigating these shadow markets. They will add up to what Neuwirth alludes to in his book's title: the wealth of nations to come.

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