I learned / was taught that large cap = less risk, mid cap = meh risk, small cap = wtf risk, meaning I'm investing in the smallest risk possible
If we're talking about individual stocks, I agree. If we're talking about large cap
index funds vs. mid or small cap
index funds, I'm not so convinced. After all, the probability of one "upstart business" going belly-up might be high, but the probability of
all of them doing so at the same time is low.
For example, consider the Vanguard Extended Market fund (VEXMX), which is basically total stock market minus the S&P 500. According to
Morningstar, its 15-year standard deviation is 18.02 vs. 14.71 for the S&P 500. That's still higher, but not all that much higher. More importantly, its Sortino Ratio is 0.69 vs. the S&P 500's 0.54, which suggests that small caps have better risk-adjusted return.