What does one "invest" in ?
In our "career" years, in order for an investor to maximize the accumulation of retirement assets in an optimal way, we need to use equity assets ( stock market assets ) that have shown highest performance, as proven by academic investment science. Investment in a portfolio of small capitalization sized "value" company stocks, large capitalization "growth" stocks, and water utility / water industry related stocks has produced the highest performance over long time horizons. A read of the E-Book "A Three Asset Class Portfolio For Accumulation Stage Growth" ( in the E-Books tab ) will provide the overview of this portfolio construction.
For the investor in their "income" stage years, investment in a portfolio of large capitalization "value" stocks has produced a respectable income stream for almost a century. A read of the E-Book "A Study of Income Withdrawal Sustainability using Large Cap Value Portfolio" ( again in the E-Books tab ) will provide the overview of portfolio construction.