... based on the research, holding the two in a 70/30 up to 50/50 allocation lowers the volatility of the portfolio so it's less risk than owning VTSAX by itself.
In recent years, international stock returns correlate pretty closely with US stocks. E.g., over last decade, a 70/30 portfolio of US and International stocks shows .99 correlation with US stocks. (Source Portfolio Visualizer's Backtest Portfolio Asset Allocation tool.)
BTW, I share this comment not to say we shouldn't diversify... we should. But it seems to be getting tougher. And international stocks don't seem to help as much as one would hope.
SeattleCPA, then, how would you propose diversifying outside of International Equities (other than Bonds)?
I started to write up a good answer to this question... and soon found my "answer" totaling 700 words. So I'm going to make it a little more thorough and in a few weeks post at my blog...
But two half baked answers... first, if you have choice, a cheap target index fund would be my choice were I doing this all over again. (These didn't exist during my accumulation phase. But these should always give you a pretty textbook asset allocation if you're working with some big well-known investment firm: Vanguard, Fidelity, etc.)
Second, regarding bonds, I earlier noted that Portfolio Visualizer says a 100% US stock market portfolio earned roughly 9.5% over last quarter century while causing investors to bear risk of a 15%-ish standard deviation.
I then pointed out that by splitting your money evenly between US stocks and the REIT index, you got a 10% return and still only needed to bear a 15%-standard deviation... and this, oddly, even though both US stocks and the REIT index returned roughly 9.5% annually.
But here's another thing to ponder. If you say you're okay with a 9.5%-ish return that US stocks returned over the last quarter century, you could have put 30% of your money into long-term treasuries... and then split that remaining 70% into even shares with half going into the US stocks bucket and half going into the REIT bucket... and here's the shocker: Had you done this, you would have gotten a 9.5% return (so same return as from 100% US stocks) but with a 10%-ish standard deviation.
FYI, I am not saying someone should use the above asset allocation or that one can predict future from past... only that you can very possibly dial up return and dial down risk by diversifying across uncorrelated asset classes... including riskless bonds.