Proof? Ha...all they can offer is “diversity is our strength” pablum. There’s not one whit of empirical evidence that this is true. Yet the rubes swallow it hook, line and sinker (non-lead, of course).
Why do you suppose everyone in the military wears the same clothes? Eats the same meals? Carries the same weapons? Speaks the same language? Because there is strength in UNITY, not diversity. A unit is stronger when it is UNIFORM, not diverse.
Diversity is the enemy of unit cohesion. You can't function efficiently if you have everyone doing his own thing.
And nobody ever charged up a hill in the face of withering gunfire, planted a flag, and yelled "Divided we stand!"