Something I have wondered in passing, but perhaps should now explore more carefully, is McDougall's relation to Oxford idealism and to the political theory of the early 20th century known as pluralism.

Pluralism was an outgrowth of Oxford political idealism. Where the former had conceived of the higher self of the individual in terms of a relationship to the State (capital deliberate), pluralism maintained that individuals belong to many different groups, the state being but one.

I suspect that McDougall adopted much of this - ultimately idealist - philosophy while at Oxford, and that this explains why his The Group Mind (1920) was so unpalatable to practicing social psychologists like the Cambridge F.C. Bartlett. Both Bartlett and McDougall were extremely interested in the psychology of groups. But where Bartlett treats ultimately only of indivduals, but posits that the nature of any one individual is determined in part by the groups he or she is a member of. McDougall, by contrast, seems to identify an actual collective mind - a notion reminscent of Jung, but I think perhaps ultimately indebted rather to Oxford idealism.