Just posted the following as a comment to an observation of how low volume has been.
The world stock markets have had a good run in September prior to Sunday evening. Mass x momentum = energy. You are correct in that volume has been declining on this rally. I'll post a volume chart for SPX with a trendline. Technical deterioration is evident in many series. The conclusion is that buying power is waning. However, weak technicals can exist longer than capital can hold out. It still requires a game-changing shift in market psychology to translate weak technicals into price decline. The psychology is the real mass because it tends to become ingrained on different time dimensions. The current short-term psychology has been bullish, but is increasingly less confident. Less willing to buy, more willing to wait-and-see. Fund managers act alike, look at similar metrics, discover and mimic each other's portfolios during uncertain times. So, in July they had record low cash, 3.4%. On this rally they increased their stock allocations. They don't have any more ammunition.