Neibuhr is an American Protestant Christian theologian and he, like Weber, is writing in counterpoint to Karl Marx.

Marx believes that by tweaking the economic structures of human societies we can achieve Utopia, the perfect human society.

Neibuhr holds this to be naive silliness -- human societies are inevitably imperfect by virtue of the structural immorality of group process.

In general, this structural immorality of group process results from the fact that the best (that is, the most moral) people in a group are always better than the group as a whole.  In other words, groups of people tend to function at the "lowest moral common denominator."

Beyond this, however, there are six reasons Neibuhr adduces for the structural immorality of society:

1. Social distance limits the moral imagination.
2. Group selfishness is a channel for the loyalty and altruism of the individual.
3. Large groups can too easily develop rationalizations for their own self-interest.
4. Struggles for power which involve threats of defeat, loss or destruction.
5. The internal political situation in groups promotes irresponsibility
    A. The quick fix is often easier, for instance, than addressing long term goals.
    B. Passing responsibility on to others is easier and less risky than assuming responsibility and doing the right thing.
6. Finiteness and sin
    A. The average person is morally not too alert
    B. Because we are finite -- embedded in a particular time, place and culture -- we all too often become morally
         compartmentalized and less concerned for others unlike ourselves.