I was recently asked if infrastructure investing should be considered its own asset class. The building and renovation of highways and utilities is going to be an important theme for the next decade, boosting the stocks and exchange traded funds that track related industries.

Put simply, a middle class is slowly emerging in places where there was no middle class and these people will need running water, electricity and cars. Money will be spent to build roads in developing nations and update old infrastructure in the U.S.

Most airport and toll-road stocks are foreign and trade on the pink sheets, which makes them difficult to research and access. However, you can find information if you spend enough time looking.

There are many ETFs and stocks that allow investors to tap into infrastructure spending, each with its own pluses and minuses. There's the SPDR FTSE/Macquarie Global Infrastructure 100 Fund ( GII), which invests in foreign utilities; the iShares S&P Global Infrastructure Fund ( IGF), a mix of utilities and industrial companies; and the PowerShares Emerging Markets Infrastructure ETF ( PXR), which buys mostly materials and industrial stocks. There's also the First Trust ISE Global Engineering and Construction Fund ( FLM), which has a very narrow focus on engineering and construction.

Projects like toll roads, airports and railroads usually don't influence the performance of these funds. The Macquarie Infrastructure Trust ( MIC) also covers this space but it's an actively managed fund that relies on transactions that require financing. Those conditions caused the fund to implode, along with several other Macquarie funds, during the financial crisis.

The SPDR FTSE/Macquarie Global Infrastructure 100 and iShares S&P ETFs are down 8.1% and 1.9% this year, respectively. The PowerShares and First Trust funds, introduced in the past year, have gained 45% and 13%. The S&P 500 Index has advanced 3.8%.

I'm not sure whether infrastructure can be viewed as a separate asset class. After all, plenty of mining and industrial stocks have outperformed the broad market, not just infrastructure names.

As you consider the validity of infrastructure is an asset class, it's worth looking at real estate investment trusts, or REITs. Can they be considered an asset class?

REITs have offered no protection from falling stocks during the past two years. In fact, the segment fared much worse than the broad market. Defenders might point out that real estate was at the heart of the crisis. While that might be true, the argument for considering REITs as a distinct asset class is weak.

Even if infrastructure isn't an asset class, it remains a compelling investment theme. Economic growth in emerging countries should benefit the stocks operating in this space for years to come.
At the time of publication, Nusbaum held shares of the iShares S&P Global Infrastructure Fund, although positions may change at any time.

Roger Nusbaum is a portfolio manager with Your Source Financial of Phoenix, and the author of Random Roger's Big Picture Blog. Under no circumstances does the information in this column represent a recommendation to buy or sell stocks. Nusbaum appreciates your feedback; click here to send him an email.