small enough to be masked by confounders There are an extremely large number of companies. Unrelated effects should average out.
Regarding statistics: http://thinkprogress.org/economy/2014/07/08/3457859/women-ceos-beat-stock-market/ links to quite some.
Regarding statistics: http://thinkprogress.org/economy/2014/07/08/3457859/women-ceos-beat-stock-market/ links to quite some.