One common criticism of utilitarianism is that we have no way of way of trading off utility gains to one person against utility losses to another in order to say whether the net change is an increase or decrease in total utility. That is not a problem for much of economic theory, which can be done not only without interpersonal comparisons but without assuming anything more than individual preferences. But it is a problem for the idea of economic efficiency, a criterion of goodness derived from Marshall’s definition of an economic improvement. Marshall, a utilitarian, offered the concept of an economic improvement as corresponding, albeit imperfectly, to a change that increased total utility.
We can and do make interpersonal utility comparisons, although not very well. A parent making decisions that affect his children is implicitly asking himself whether doing something one child wants to do and the other doesn't will increase the former's happiness more than it decreases the latter. Someone deciding which friend to give a gift to is doing it in part on which he thinks will be made happier by it. I signal my feelings, including preferences, in facial expressions, voice tones, and the like; others appear to do the same, giving me some idea of the strength as well as the ordering of their preferences and comparing to mine.
I cannot know another person’s preferences with certainty but I have no serious doubt that the disutility to a random stranger of being tortured to death is greater than the disutility to me of stubbing my toe.