So, to summarize, stock prices are held up by abnormal profit margins, which in turn are produced mainly by lower real rates, the benefits of which are not competed away because of increased monopoly power, etc. What, we might ask, will it take to break this chain? Any answer, I think, must start with an increase in real rates. Last fall, a hundred other commentators and I offered many reasons for the lower rates. The problem for explaining or predicting future higher rates is that all the influences on rates seem long term or even very long term. One of the most plausible reasons, for example, is the aging of the populations of the developed world and China, which produces more desperate 50-yearolds saving for retirement and fewer 30-year-olds spending everything they earn or can borrow. This results, on the margin, in a lowered demand for capital and hence lower real rates. We can probably agree that this reason will take a few decades to fade away, not the usual seven-year average regression period for financial ratios.

Source: GMO 

‘Nuff said