For the history buffs, is there an example of a genuinely "diverse" population building and enjoying a successful civilization? From what I've seen, when there's a critical mass of diversity things tend to get worse and less efficient rather than better. DIE departments in both government and corporations seem to be unproductive tumours that leech resources and time from departments with purpose.

What I want to know is if "diversity is our strength" is an assertion with no evidence.