Exactly: Most studies that I've been referred to are shiny McKinsey pages that boil down to big companies having more diverse boards on average.

Besides what you said, that's not a surprising correlation because it's easy for Multinational MegaCorp Inc. to have a multi-ethnic board after a few international mergers.

And then there are very opinionated shareholders like Vanguard and BlackRock, which receive voting rights for every Euro that I save for old age, who can make this outcome a self-fulfilling prophecy by punishing companies for lack of diversity.

I don't think it's possible to control for all of this.