The biggest investing failing is not saving any money – so failing to invest. But once people actually save the next biggest issue I see is people confusing the investment risk of one investment in isolation from the investment risk of that investment within their portfolio.
It is not less risky to have your entire retirement in treasury bills than to have a portfolio of stocks, bonds, international stocks, treasury bills, REITs… This is because their are not just risk of an investment declining in value. There are inflation risks, taxation risks… In addition, right now markets are extremely distorted due to the years of bailouts to large banks by the central banks (where they are artificially keeping short term rates extremely low passing benefits to investment bankers and penalizing individual investors in treasury bills and other short term debt instruments). There is also safety (for long term investments – 10, 20, 30… years) in achieving higher returns to gain additional assets – increased savings provide additional safety.
Yes, developing markets are volatile and will go up and down a lot. No, it is not risky to put 5% of your retirement account in such investments if you have 0% now. I think it is much riskier to not have any real developing market exposure (granted even just having an S&P 500 index fund you have some – because lots of those companies are going to make a great deal in developing markets over the next 20 years).
I believe treating very long term investments (20, 30, 40… years) as though the month to month or even year to year volatility were of much interest leads people to invest far too conservatively and exacerbates the problem of not saving enough.
Now as the investment horizon shrinks it is increasing import to look at moving some of the portfolio into assets that are very stable (treasury bills, bank savings account…). Having 5 years of spending in such assets makes great sense to me. And the whole portfolio should be shifted to have a higher emphasis on preservation of capital and income (I like dividends stocks that have historically increased dividends yearly and are likely to continue). And the same time, even when you are retired, if you saved properly, a big part of your portfolio should still include assets that will be volatile and have good prospects for long term appreciation.
Related: books on investing – Where to Invest for Yield Today – Lazy Portfolios Seven-year Winning Streak (2009) – Fed Continues Wall Street Welfare (2008), now bankers pay themselves huge bonuses because the Fed transferred investment returns to too-big-to-fail-banks from retirees, and others, investing in t-bills.