Equities “are now barely through an 18-year secular bear market,” says our favorite underground analyst David Rosenberg. As illustrated by the nearby chart, US stocks have a historical tendency to move in 18-year cycles.

This chart blew us away. Stock prices are supposed to move at random. So how do we explain such a seemingly orderly pattern? To answer this question you have to also consider the cyclical movements of commodities prices. Take a look at the chart below. It shows the trend in commodity prices between 1982 and 2000 – an 18-year upswing in equity prices.

As you can see, commodities entered a secular bear market just as equities entered a secular bull market. And vice versa.

The reason for this is actually relatively simple. As costs for raw materials increases corporate profits decrease. Eventually, the decrease in profits causes demand to fall for commodities… and prices fall.

This fall off in prices then reduces investment in the acquirement and production of raw materials, which in turn reduces supply. As supply gets tighter prices begin to rise again. Investment in commodities becomes once again profitable, and the cycle completes itself.

This story gets really interesting when you consider that during the vicious sell off in commodities last year prices bottomed far higher than in previous recessions.

According to Rosenberg:

In the 2001 recession, the oil price bottomed at $19.33/bbl; in 1990, it bottomed at $16.81/bbl; in 1982 at $28.48/bbl; and in 1975 at $10.11/bbl. We bottomed this cycle at levels that were peaks in prior cycles. The same holds true for copper – it hit its trough at $1.39/pound this time around versus $0.630 in 2001 and $1.00 in 1992. Ditto for the ‘softs’ – soybeans bottomed at $8.48/bushel this time, compared with $4.15 in 2001, $5.42 in the recession of the early 1990s and $5.32 in the early 1980s downturn.

What does this mean for your investments? Put simply, this implies that “the floor is in” for commodities. Consider adjusting your portfolios accordingly.