Psychological egoism is the view that all individuals in fact do what they believe is in their own self-interest. This is *not* synonymous with the claim that all individuals in fact do what they want. ‘Want’ is not ‘self-interest’ by definition, contrary to what one of my furry-faced colleagues thinks. For it to be the case that these two concepts are essentially related the following bi-conditional would have to be true: if you want X, then X is in your own self-interest and if X is in your own self-interest, then you want X. While it is arguable that the second half of the bi-conditional is true, the first half of the bi-conditional is not true. I only need one example to show this: I want to smoke a cigarette right now. Smoking is not in my own self-interest, but I want it nevertheless.
Perhaps my furry-faced-friend does not have psychological egoism in mind, but a popular theory developed in the literature on moral psychology and moral motivation. One view suggests that in order to move someone all the way to action a *desire* must be present in the explanation of the individual’s behavior. This is not the thesis of psychological egoism and even the defender of this view must explain why some desires are different than others as in weakness of the will (she might, for example, appeal to higher-order desires).
Now of course ‘wants’, ‘desires’, ‘self-interest’, etc. are all terms of art and can be filled out in various ways. The key is to avoid rendering one’s own thesis trivially true. If you think ‘self-interest’ just means doing what you want, then you need to rephrase psychological egoism to say something like all individuals in fact do what they believe is in their own best-interest. In other words, if psychological egoism is just the view that individuals in fact do what they want, then you are not telling me anything substantive. For example, a prominent psychological egoist like Hobbes will say that an individual *in so far as she is rational* will want to get out of the state of nature.
Finally, it may be the case that psychological egoism is often in danger of collapsing into a trivially true thesis, but this is something the defender of the view is going to want to avoid, not embrace contra Mr. Furry Face himself.