Utilitarianism is an interesting ethical theory, not because of its content, but because it remains appealing despite its flaws. Utilitarianism, for those of you who have forgotten, is the philosophy that those actions that maximize the total happiness are good, or at least that is the Reader’s Digest version.
One selling point of utilitarianism is that it encourages people to make sacrifices for the good of the community. This fits with our intuitions as to what is good, and any moral theory that condemns self sacrifice is would be immediately questionable for contradicting what seems like a universally accepted judgment.
We might also appreciate utilitarianism because it is based on a few simple principles, but at the same time many conclusions can be drawn from them. The best theories in science have this property, and it is no less attractive in philosophy.
Finally utilitarianism is easy to apply to one’s daily life, unlike Kant’s categorical imperatives for example, which might require a laborious philosophical inquiry for each choice we made. In contrast we can tell which actions maximize the total happiness fairly easily, allowing us to actually live by utilitarianism if we so wished.
One flaw of utilitarianism is that to maximize happiness in some we might be forced to act in ways that our intuition tells us are morally despicable, or at least allow such behavior in others. For example consider a sadist, who revels in the pain of others. Utilitarianism would have us believe that the sadist should be allowed to do as he pleases, as long as the happiness that he receives from his activities is greater than the happiness he takes away from others.
Tyranny of the majority
Similarly the argument presented above that lets the sadist do as he pleases can also be used to justify the systematic oppression of a small group of people in order to benefit the rest of society. Given the right numbers it might even be used to justify genocide, for example if a large percentage of the population hated a small minority, and if the extermination of that small minority would make the vast majority happier it is conceivable that the pain and suffering inflicted on the minority would be outweighed by the increased happiness of the majority.
Another problem carrying forward utilitarianism practically is determining whose happiness matters in the final calculation. Does the happiness of animals count? Of unborn infants? Aliens from another world? Does the happiness of some beings count more or less than that of others? Any moral theory that treats people as special runs into these kinds of problems of course, but I would be negligent if I didn’t at least mention them.
Failure to provide for the future
Even more seriously utilitarianism does not take into account the happiness of future generations. Consider utilitarianism as presented, clearly it is conceivable that being wasteful with the available resources might increase the total amount of happiness. Even if we consider the total happiness of the people alive as summed over their lifespans we can still justify wasting resources as long as we leave enough to provide for everyone currently alive until they die. If this means that two or three generations down the road the Earth will be only a smoking husk, so be it. Even if we want to avoid this outcome we cannot rationally modify utilitarianism to state that we should maximize the happiness of everyone, including future people, because such a principle would prevent us from ever using resources. Let me illustrate with an example. Suppose you have a log, which you can either sit on, and get some small amount of happiness from in that manner, or burn and get much more happiness from the heat it provides. If we take into account the happiness of all future generations there is no way we could ever burn the log, for the happiness we would get from burning the log is less than the happiness any future generation would have by possessing the log in order to sit on it and then burning it at a later date. (happiness(burning log) < happiness(owning log) + happiness(burning log))
Of course just because I don’t support utilitarianism doesn’t mean that I have a better moral theory in mind. Some have proposed an equally simple, but opposite, theory, that actions that promote ones personal happiness are good, and that enlightened self interest will result in cooperation and altruism. Unfortunately a little math shows that cooperation would not actually arise from self interest alone without laws to enforce it, and how could one justify laws the prevent people from working towards their own self interest if self interest defines what is good?
If you are interested in utilitarianism you can read all about it here, although be warned that this site seems a little biased in favor of utilitarianism.