And the sum itself is a huge problem. There is no natural scale on which to compare utility functions. Divide one utility function by a billion, multiply the other by eπ, and they are still perfectly valid utility functions. In a study group at the FHI, we've been looking at various ways of combining utility functions - equivalently, of doing interpersonal utility comparisons (IUC). Turns out it's very hard, there seems no natural way of doing this, and a lot has also been written about this, concluding little. Unless your theory comes with a particular IUC method, the only way of summing these utilities is to do an essentially arbitrary choice for each individual before summing. Thus standard total utilitarianism is an arbitrary sum of ill defined, non-natural objects.

This interests me. Do you have any literature I should read on this topic?