Pure time preference is simply myopia--and not something any utilitarian should assume. John Quiggin:
“Future generations” are already here: Much of the debate on the question of whether a pure rate of time preference can be justified is concerned with determining the appropriate way to balance the interests of “current” and “future” generations…. The central point of this note is to observe that this way of posing the problem is invalid, because members of different generations are alive at the same time…. [A] social allocation rule that incorporates pure time preference… 3%/year… and 25 years between generations… those aged 50 or more is valued twice as highly as the welfare of their children, and four times as highly as the welfare of their grandchildren…. This is obviously inconsistent with any form of utilitarianism in which all those currently alive are valued equally.