Since 1800 to now Dow (or Dow type) stocks with dividends have provided a 8% annual return. That means every nine years the savings doubles. Here's how stocks have compared to T-bills, corporate bonds, and inflation:
8% isn't too shabby either. But I would not recommend a retirment account be invested in stocks, or at least no more than 10% be invested in stocks. Go with quality proven funds that widely diversify the risk. We have a bit in metal funds that are quite volatile but have been in an overall upward trend for decades now; some in the DOW, some NASDAQ, large caps, small caps, domestic and foreign. When one tanks another is likely to be up.
The long term real return to commodities has been 0%. However, the return to gold since the government suspended the last vestiges of the gold standard in 1971 has been 9%. Stocks have returned 10%. Since 1913, gold has returned 4-5%. Since 1928, stocks have returned 10%.
Stocks can go out of favour for 10-20 years but over very long periods of time, they are the best investment. Since SS is a long lived institution that will be around for generations, SS should have a large investment in stocks. Stock returns over very long periods of time grow with the economy. Betting against stocks over very long periods of time, ie 50-100 years, is essentially a bet against the American economy. That doesn't apply to 5-10 or even 20 years, but it does over the very long term.
$100 billion invested today earning 4% per year over 50 years, which is about what government bonds in the SS trusts will earn, will yield $460 billion. The same amount earning 8% over 50 years, which is about the return a portfolio of 60% stocks and 40% bonds, will yield $4.6 trillion, or 10 times the amount compared to solely in government bonds. SS should invest in stocks.