Time heals all wounds, but the wounds of the recent market drop as evidenced by elevated option premia are still fresh. The so-called “Fear Index,” the VIX, traded down to 28 at yesterday’s close, compared to the sub-10 it was recording just a month ago.

As the market dropped, pundits rushed into TV studios to opine on causal ties between stock market prices to fluctuations in the bond market, consumer expectations for inflation, and worries about productivity of US workers.

CNBC’s coverage of recent market action

Before you, gentle reader, fall prey to these facile explanations, let me introduce you to two very accessible academic papers that study the precise question “What Moves Stock Prices.”

The first is a paper written in 1988 by a David Cutler, James Poterba, and former Treasury Secretary, Lawrence H. Summers entitled “What Moves Stock Prices.” The second is an update to the 1988 paper, written in 2012 by Cal Tech professor Bradford Cornell.

Both papers find that while some extreme market moves have clear causal relationships to external factors (e.g., Japanese planes bombing Pearl Harbor, 9/11 attacks), most large stock price movements have no such causal connection.

After Monday’s price fall, people started writing me asking why the drop was happening. My reply: I have no idea.

There are a few things that we can say about conditions in the market at present:

Animal spirits have been running high. In my mind, Bitcoin and other crypto-currencies should be exhibit A here.

Investors are crowded onto one side of the volatility investment boat. Volatility has been low for so long and “short vol” strategies have been successful. Humans tend to crowd into successful strategies (not only for investments, as a student of Easter Island would attest), so the “short vol” trade is crowded – the popularity of risk parity and various option-selling strategies are a mark of this trend.

Volatility clusters. This simply means that low volatility environments tend to beget more instances of low volatility and high volatility ones, more instances of high volatility. Like all outputs of complex systems, it is very hard to tell when one “regime” is going to flip over to the opposite one, but that flip can occur very quickly.

Investors are crowded onto one side of the yield investment boat. Multiple rounds of QE on several continents has suppressed interest rates worldwide, and fixed income investors are aching for yield. This is driving them to pay up for dividend stocks and reduce the quality of bonds in which they will invest.

Equity prices are toppy. Framework members know that I do not believe that popular measures like the Cyclically-Adjusted P/E Ratio (CAPE) say much about relative valuations. My “toppy” comment is based on my own experience – every firm we are picking to value these days seems to be straining the upper limits of what we consider a reasonable valuation range based on observable operating metrics.

The global growth engine is at risk. The expansion of international trade has been responsible for an average rise in global wealth. The effects have not been uniform; developed country workers are being displaced by cheaper developing country ones. This has resulted in knee-jerk political reactions – Brexit, Trumpism, and far-right movements throughout Europe. As politicians move to protect their respective electorates, the system as a whole slows down. Lakshman Achuthan at the Economic Research Cycle Institute recently warned that the first signs of both a domestic and international slowdown have started to appear in his data.
In my mind, the presence of these background factors helps to make the likelihood of future falls in equity prices higher.

ECRI’s Lakshman Achuthan

The drop in Bitcoin prices by about half might be a signal to all investors (including those who wouldn’t touch an investment in Bitcoin) that the general speculative environment is becoming riskier. Increasing long-term bond yields might be a signal to equity investors that required rates of returns must head higher (making fewer equity investments relatively attractive). As data showing slowing growth here and overseas becomes more visible, equity investors may start to realize that sunny assumptions for future cash flows may be too optimistic.

When (I won’t say “If and when”) equity investors’ perceptions of riskiness and future growth change – for what could be a trivial reason or for no reason at all if the academics are right – the volatility and yield trades that now seem crowded will seem painfully more so. As investors rush to move from the side of the boat they find themselves crushed into, the boat runs the real risk of capsizing.

Intelligent investors should have their life vests securely fastened by that time.

This article originally appeared on Forbes.com