In Professor's Model, Diversity = Productivity
After reading some of his book, it seems that cognitive diversity (defined as group differences in perspective, heuristics, interpretation, and prediction) generally lead to improvement in overall productivity. There are other kinds of group differences, of course...e.g. what Page calls identity differences (e.g. racial, gender, and social group), and other sorts of differences, but Page identifies cognitive differences as key.
Increased productivity is dependent upon the problem/task at hand, of course. Group tasks/problems that hinge on questions of representation and interpretation get greater value from diverse perspectives, since such diversity allows more alternatives to be generated and considered.
My thought on reading this was that many sw design tasks are like this (i.e. hinge upon how the problem is represented) and so should be particularly good candidates to benefit from teams with high cognitive diversity...and the many perspectives that come with such groups.
Interestingly, Page also identifies a type of diversity that seems to result in lower group productivity...I'll talk about that after I've read a little more.