While mutual funds by themselves offer tremendous diversification potential within sectors, industries, market capitalization and so on, unless you invest in balanced funds, proper asset allocation is usually only achieved by holding different types of asset classes. Understandably, giving up the potential for aggressive gains in growth funds (which are usually suggested through historical return data) in favor of more stable returns in bond funds is not an easy thing  for most investors to do. However, while those historic gains might seem attractive at first sight, proper asset allocation is paramount if one wishes to achieve long-term growth and income.

All investors need to do is look back to 2009 to see just how powerful proper asset allocation can be. For example, the S&P 500 returned roughly 20% throughout all of 2009 (over 60% since its impossible-to-predict low in March 2009). Investing in a 30-year Treasury by comparison would have returned roughly 65% a staggering amount of return for that same period.

While 2009 was undoubtedly one of those “off” years, the data from 2008 tells a similar story about the importance of proper asset allocation through asset class diversification. Arguably a devastating year for equities, 2008 was also a painful (a much more painful in fact) period for long-term Treasury bonds. For that year, equities gave up a touch more than 35% while 30-year Treasury gave up marginally more. While both were evidently “losers” in terms of total return, equities actually outperformed the 30-year Treasuries.

And one year earlier, 2007, also teaches a valuable lesson. While equities ended the year up nearly 5%, 30-year Treasuries were down nearly that same amount.

By incorporating an asset allocation model that touches all asset classes (we are simply looking at the S&P 500 and 30-year Treasuries here), investors will not only sacrifice some of the “wild” gains by being 100% invested in equities, but they will mute those losses in one classes by offsetting them with investments in another.

To illustrate this, consider that same period from 2007 through to 2009. While the year-by-year playbook shows favorably for equities, in fact the 2009 gains in 30-year Treasuries would help produce positive (or less negative) returns for an investor who purchased an S&P 500 Index Fund as well as a long-term bond fund. With a long-term bond fund that imitates the returns on 30-year Treasuries returning nearly nothing over that same period, the other fund, which follows the S&P 500, would still be down roughly 20%. In other words, an asset allocation model that splits the assets 50/50 between an index fund and a long-term bond fund would have only lost a little more than 10% (instead of the full 20%).