How just five stocks drive so much of the market's growth

Research shows that investors with diversified portfolios, or for those who use a passive strategy of investing in index funds, the poor performance of nearly 96 percent of stocks is not a big deal, as the diversified portfolio will contain the few stocks that turn out to be big winners. On the other hand, investors who are not well diversified or who bet heavily on small-cap stocks are taking risks similar to those taken by venture capitalists, hoping for those few home runs.

In this article on Forbes June 25, 2018:

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. 

— Professor of Finance Hendrik Bessembinder, who is the Francis J. and Mary B. Labriola Endowed Chair in Competitive Business