I've never seen market valuation expressed as market cap as % of GDP. I'm not an economist, so I'll leave the detailed arguments to them. But it would be at least useful to explain why you think this is a meaningful metric as compared to those typically used to measure market valuation (e.g. P/E ratios etc.).
Your graph also ties your valuation metric to the 2000 peak and the 2008 peak. However, there were crashes in 1990 and 1987 as well. Should readers conclude that the 1987 peak level was also too high, and that therefore the last ~30 years have also been too high as well? (Abstaining from investing in the stock market at levels above the 1987 crash would have resulted in the loss of tremendous opportunity for wealth creation.)
There are a lot of opinions implicitly expressed in this site; it would be good to try to make those explicit.
> market valuation expressed as market cap as % of GDP
This metric makes little sense for this use case. Consider two countries. They are identical in every way except in Country A 90% of the companies are publicly-traded while in Country B 10% are. Country A will have a market cap to GDP 9x Country B's. Does that mean Country A is 9 times overvalued relative to Country B?
The objection works in-country, too. Saudi Aramco is going public in New York or London [1]. This will lift one of those market's aggregate capitalisation by up to $1 trillion. Does this mean that market will necessarily become overpriced?
The answer to both question is of course not. Market cap to GDP tells you the degree to which a country has developed public markets. Not anything interesting about the levels in those markets.
So I've had a fundamental complaint about Market Cap to GDP at least since 2005, which I've never gotten a good answer to:
There's this expectation that the market returns 7-10%... a number much in excess of the actual rate of GDP growth (over any significantly long period, anyway)
That can't continue forever. Especially in aggregate across the world. At some point the public market has captured substantially all the economic activity - after that there's no way for it to grow in excess of GDP without things like PE increasing (and again, that can't go FOREVER, regardless of what the "true" PE should be).
This is assuming all numbers are inflation-adjusted "real" numbers, of course, because the money supply CAN grow forever.
I'll probably get some of this wrong, but I read up on these arguments back when Piketty was in the news w/ his book:
Yes, it can go on forever -- the rates of retun in the stock market are based, theoretically, on the changing expectations about the future and not based on current income.
Thought experment: 100 of us live in small society producing widgets, we each make a widget a day at the factory. GDP is 36500 widgets/day. We also spend some time researching a way to make widgets faster. Yesterday we found a breakthrough that made it 50% likely that in 5 years we'll each be making 10 widgets a day.
It would be reasonable for the valuation of our widget company to go up something like 40% on that news, right? But GDP next year is stlil going to be 36500 widgets/day.
Since the stock market bakes in all optimistic expectations, then in the eras it that it outpaces GDP it could be the case that there remains unrealized optimism for the future.
If the question is "but where is the capital coming from that flows into the stock market?" The answer is that it can be created via credit, or it could be created via appreciation in assets not captured in the stock market (like housing, the major one).
Your example anticipates a huge growth in productivity, which is a factor in GDP; in that case, the stock market is a forward indicator of anticipated GDP.
Which, if so, still doesn't allow it to grow infinitely out of proportion to GDP, unless the time horizon keeps changing or the anticipation of the future gets steadily farther away from reality.
So you mention in the eras it that it outpaces GDP... I've consistently had the message drilled into me that in the long term (20+ year horizon), stocks will return something like 7-10%, while of course, nobody would expect GDP growth anything like that (outside of a rapidly industrializing environment where productivity is rocketing upward, like China).
Nit pick: only about 5% of Saudi Aramco is going public, the total company is worth $1-2T (although this is a matter of some debate, as you might imagine), so aggregate market cap will only increase by $50B.
You sound knowledgable about these matters, and I am not. In my ignorance, "agg. market cap" sounds like an easily gamed number. For example, I could start an exchange and ask every company in the world to list 10^-10 of their shares, and so become the largest exchange in the world by agg. market cap. (Perhaps this doesn't come up in practice because most firms list in only one market, and so agg. market cap becomes a useful measure of how much of the economy a given exchange touches, in some sense.)
Here is the case for looking at Market Cap / GDP: Warren Buffet looks at the total market cap vs GDP because it is free from the influence of corporate accounting, unlike the more traditional P/E ratio.
The case against: economic activity in the US has been concentrating in larger companies (less small companies being started for example) and there is more "financialization" in general. So you would expect the ratio of market cap / GDP to go up for those reasons as well.
One issue I see here is how much of the S&P500 (or general market cap) are we attributing to a US-centric view of global companies? That is, if a company is "in the US" but economically are not.
The ratio has units of years - how many years of GDP does the value of the stock market represent? How long will it take the economy to produce the value assumed by the stock market valuations? Sure, that's not a pure number, but it's also not a totally nonsense number. At a minimum, it can be compared to historical values of the same number.
It makes some sense if you consider market caps to be expectation of returns. All other things being equal (like the percentage of the GDP made by publicly traded companies) it makes sense to expect the market to expand at about the same speed as the GDP.
That this ratio doubled in 8 years is worrisome.
The thing is that an alternative analysis is to look at the linear regression since 1980 and it seems clear that a background upward tendency does seem to exist, so we can see 2009 as an adjustment to the 2008 crisis and 2017 as only slightly above the expected trend.
In particular, people's evaluation of a company are not (wholly) based on its current annual output, whereas the GDP is exactly the amalgam of all companies' output. People evaluate a company at least in part on the assets it holds, intellectual property, and future earning power, none of which are captured by the GDP.
Your graph also ties your valuation metric to the 2000 peak and the 2008 peak. However, there were crashes in 1990 and 1987 as well. Should readers conclude that the 1987 peak level was also too high, and that therefore the last ~30 years have also been too high as well? (Abstaining from investing in the stock market at levels above the 1987 crash would have resulted in the loss of tremendous opportunity for wealth creation.)
There are a lot of opinions implicitly expressed in this site; it would be good to try to make those explicit.