Investors learned difficult lessons as financial markets melted down in 2008.
Some sold in a panic when stocks were at their lows. Others were surprised when fund managers proved just as capable of losing investors' money as investor themselves were.
Now is a good time, with markets swaying again amid uncertainty about global economic growth, to ask yourself how well you learned your lessons from 2008.
"Smart people make mistakes," said Larry Swedroe, the director of research at Buckingham Asset Management in St. Louis. "What separates them from fools is they don't repeat them."
With that in mind, consider some key investing takeaways from 2008 that are fitting now:
1. Diversification isn't a cure-all, but it works
Every major U.S.- and international-stock index lost money in 2008, as did many bond benchmarks. Moreover, assets that were considered less risky than others didn't necessarily lose less.
For example, many investors in 2008 believed that high-quality, dividend-paying large-cap stocks would buffer the shocks they expected would wallop riskier midcap and small-cap shares. They were misguided. That year, the large-cap Standard & Poor's 500 Index ($INX) posted a bigger loss than the small-cap Russell 2000 ($RUT.X).
Disillusioned, some market sages questioned the conventional wisdom that diversifying an investment portfolio will protect you in a down market. They have a point, but only in the extreme.
One lesson for investors is that when pessimism is greatest, many investments that ordinarily go their own way are likely to tumble together. A market in free fall crushes anything in its path, just as positive momentum tends to lift all boats.
Over longer periods, in contrast, you'll get steadier returns with a mix of assets that don't soar and sag in lockstep.
True diversification requires spreading a portfolio widely -- across stocks, bonds, gold, other precious metals, real estate, commodities and the like -- even if this means giving up some gains in bull markets.
So far this year, most stock-fund categories are down, but most bond funds are up, and gold-related investments have been strong.
2. Safe havens are necessary
Going into 2008, U.S. and international stocks had been the top-performing assets for five years running, and portfolio insurance in the form of Treasury bonds hardly seemed worth the bother. Yet once stocks went into a precipitous slide, U.S. government debt played its traditional protective role; Treasury bonds were the best place to make money in 2008.
Treasury trashers were back this year with dire warnings of America's default and demise. Credit-rating company Standard & Poor's even stripped U.S. debt of its coveted triple-A rating, a once-unthinkable action.
Yet Treasurys have outperformed most other investments so far this year, even rallying after the S&P downgrade in early August. Maybe, as some have quipped, U.S. debt is the cleanest dirty shirt in the world's hamper. But prospects for slow economic growth have investors fleeing to the relative safety of federal and municipal bonds, pushing yields down and prices up.
"For fixed-income assets, stick only with Treasurys, bonds of government agencies and the highest-rated municipal bonds," Swedroe said. "Anything else, such as high-yield junk bonds, convertible bonds, emerging-market bonds and preferred stocks, can have the risks show up at the wrong time."
He said fools, like 70% of the commentators on these blogs, that don't know what they're doing shouldn't invest in the markets 
