Research from Arizona State professor Hendrik Bessembinder looked at how equities performed versus treasuries from 1926 to 2016. Bessembinder found that only ~4% of companies made up virtually all value created (and returns generated) over the 90-year period. In each group of 25 stocks, roughly 24 (96%) did not outperform treasuries.
This work effectively invalidates the idea that stocks broadly outperform government bonds. Only a very select number of stocks outperform. The study also gives ammunition to evangelists on both sides of the passive-active debate. One could say it makes little sense to index into a large, broadly diversified set of equities when only a small portion will do well over time. Another might rightly retort that one must be able to select ex-ante which those four percent of outperforming equities.