In its 1969 song "Spinning Wheel," the rock group Blood, Sweat & Tears immortalized the phrase "what goes up, must come down."
I cite this bit of music trivia in an investment story for two reasons: First, the name of the band fairly well describes the conditions attached to the stock market's historically sad performance over the just-completed decade. Specifically, a lot of blood, sweat and tears - thankfully followed by a tiny bit of healing in the final eight months of 2009.
Second, for those more attuned to mathematical theory than music, the market results for the 10 years from 2000 to 2009 provide the basis for a prediction for the coming decade - one essentially the inverse of the band's famous phrase. To be more precise, "what went down, must now come up."
That forecast is based on a fairly simple principle: Things that move in wave patterns - such as the stock market - have an overwhelming tendency to "revert to the mean." In other words, when stocks perform well above their long-term historical norm (known as the "arithmetic mean") for an extended stretch, they usually have to underperform for an extended period to get back in line with that long-term mean.
Conversely, when stock-market performance is well below the norm for a long stretch, it theoretically should enjoy an extended run well above the historical mean in order to bring the market's performance back in line with the long-term norm.
And the decade we've just completed was definitely far below that norm.
The prediction is also based on historical precedent, as demonstrated by past per-decade performances of the major stock market indexes.