[billmalarky]

At the most basic, yes.

Take a look at this chart, it explains it way quicker than words could.

http://cdn.financialsamurai.com/wp-content/uploads/2016/06/T...

Note dropping from 100% equities to 70% equity and 30% bonds drops your return by about a percent while dropping your risk (variance) by just over 4%.

Taking on additional risk leads to a higher expected return, but you can see that more and more risk leads to increasingly marginal amounts of additional return in exchange for large amounts of risk.

The comment I was responding to felt distraught that he had to be exposed to the heavy risk provided by only investing in equities, when he could sleep better at night using investment diversification (ie different types of investments, not different stocks) while still maintaining a great ROI (certainly better than the negative ROI of just hoarding cash).