Today I am going to channel my inner Cliff Asness and demonstrate why it is more or less irrelevant whether you own a Total US Market index fund or an S&P 500 index fund. Intuitively, the reason for this is straightforward:
Since a Total US Market index fund is market capitalization weighted, it is dominated by the largest companies. The largest US companies are all included in the S&P 500. Hence, S&P 500 stocks drive the overwhelming majority of the return of the total market portfolio.
If you trust me, you can stop reading here. If you would prefer to see some supporting data, read on. Fair warning: it gets wonkish rather quickly.
I used Portfolio Visualizer to run regressions on two widely held index funds using the Fama-French Three Factor Model. The model fits the index funds extremely well as evidenced by the respective R^2 values of 99.7% and 99.99% (this means the model explains over 99% of the variation in returns over the time time period analyzed). One regression was for VTSMX and the other for VFIAX. Below is the output:
The key numbers are in the Loading column. Do a quick visual compare/contrast. See how they are almost identical? That is because at the end of the day, when you own the market portfolio, most of your money is invested in S&P 500 stocks (you can verify this using the actual portfolio holdings if you want).
This is further underscored by Portfolio Visualizer’s performance attribution analysis:
In the attribution table, SMB means “the return you got from investing in smaller companies” and HML means “the return you got from investing in “cheap” (value) stocks versus “expensive” (growth) stocks. The total market fund earned basically no return from exposure to small companies over this time period, while the value/growth stock exposures are so similar as to be irrelevant.
The market cap weighted total US market portfolio does not provide a meaningful exposure to small company stock returns (or a meaningful tilt to value or growth stocks–a non-issue for the purposes of this post).
Put another way, in statistical terms, the total US market index behaves nearly identically to an S&P 500 index fund.
For people who are knowingly overweight US large cap stocks in the form of the S&P 500, I’ve got nothing to argue with you over. This bet has worked out pretty well over the last couple of decades. Maybe it will keep working (there are a lot of great businesses in the S&P 500). Maybe it won’t keep working (a lot of those companies are richly valued). Anyone who claims he can handicap future market returns with any degree of accuracy is an idiot or a liar (possibly both).
For people who are naively overweight US large cap stocks in the form of the S&P 500, I have this to say: like it or not you have made a bet on a particular market segment. Admittedly, these are high quality companies and the underlying revenue sources are globally diversified. However, the valuation risk is not necessarily very well diversified. Something like 20% of the portfolio is invested in FANG stocks (that’s an off-the-cuff number).
My point here is not to say definitively that the S&P 500 is a bad place to be invested. No one knows what the next 30 years will look like.
Rather, I am making a philosophical point about asset allocation. Namely, when you “passively” allocate assets predominantly to a market cap weighted US total market portfolio, you have implicitly made an active decision to concentrate your risk exposure in US large cap stocks. Only about 50% of global equity market capitalization is located in the US. If you truly believed in the logic behind a capitalization weighted total market portfolio, you would obtain all your equity exposure via something like ACWI.
However, I have yet to meet anyone who does this. Or any financial advisor who recommends it.
(a good subject for another post)