Rob Shiller recently waxed lyrical about the fact that - by his own metrics - the markets are overpriced, yet no crash is coming because there is not enough 'leverage in the system' to propagate any shocks to systemic levels.
The indicator Shiller used to define overpricing is his own CAPE - Cyclically Adjusted PE Ratio - and the indicator does indeed flash red:
CAPE is defined by dividing the S&P 500 index by the 10-year moving average of index components' earnings. The long-run average of CAPE is 16, and the index currently sits above 31, making the current markets valuations trailing those of the dot.com bubble peak (using recent/modern comparatives).
So the markets are very expensive. But what Shiller says beyond this mechanical observation is very important. His view is that these levels of valuations are 'sustainable' in the medium term because there is very little leverage used by investors in funding these levels of stock prices. In the nutshell, this says that if there is any major correction in the markets, investors are unlikely to be hit by massive margin calls, triggering panic sell-offs. So any correction will be short-lived and will not trigger a systemic crisis.
All fine with the latter part of the argument, if we only look at the stock market brokerage accounts leverage, ignoring other forms of leverage. And we can only do this at a peril.
Investor is a household. Even an institutional one, albeit with a stretch. When asset prices correct downward, income received by investors falls (dividends and capital gains are cut) and investor borrowing capacity falls as well (less wealth means lower borrowing capacity). But debt levels remain the same. Worse, cost of funding debt rises: as banks and other financial intermediaries see their own assets base eroding, they raise the cost of borrowing to replace lost income and capital base with higher earnings from lending. Normally, the Central Banks can lower cost of borrowing in such instances to compensate for increased call on funds. But we are not in a normal world anymore.
Meanwhile, unlike in the dot.com bubble era, investors/households are leveraged not in the investment markets, but in consumption markets. Debt levels carried by investors today are higher than debt levels carried by investors in the dot.com and pre-2007 era. And these debts underwrite basics of consumption and investment: housing, cars, student loans etc (see https://www.bloomberg.com/view/articles/2017-10-18/don-t-rely-on-u-s-consumers-to-power-global-growth). Which means that in an event of any significant shock to the markets, investors' debt carry costs are likely to rise, just as their wealth is likely to fall. This might not trigger a market collapse, but it will push market recovery out.
An added leverage dimension ignored by Shiller is that of the corporates. During the crises, cash-rich and/or liquid corporates can compensate for falling asset prices by repurchasing stocks. But corporates are just now completing an almost decade-long binge in accumulating debt. If the cost of debt carry rises for them too, they will be the unlikely candidates to support re-leveraging necessary to correct for an adverse asset prices shock.
I would agree with Shiller that, given current conditions, timing the markets correction is going to be very hard, even as CAPE indicator continues to flash red. But I disagree with his view that only margin account leverage matters in propagating shocks to a systemic level.