A small number of stocks drive the returns of the market, according to research by Professor of Finance Hendrik Bessembinder, who is the Francis J. and Mary B. Labriola Endowed Chair in Competitive Business.

This article on Forbes June 25, 2018, explains why this means indexing — and not using a single-stock strategy — makes sense:

The implications of Bessembinder's research are that, as we’ll see, owning stock in your employer may be more risky than you think, and that, tracking a large index of stocks may be a far better idea than it first appears.