Value of cognitive diversity
James Surowiecki - Wisdom of Crowds - pp.31
The reason, March suggested, is that groups that are too much alike find it harder to keep learning, because each member is bringing less and less new information to the table. Homogeneous groups are great at doing what they do well, but they become progressively less able to investigate alternatives.
Or, as March has famously argued, they spend too much time exploiting and not enough time exploring.
Bringing new members into the organization, even if they're less experienced and less capable, actually makes the group smarter simply because what little the new members do know is not redundant with what everyone else knows.