At first, this concept appears to be obvious on the face of it. All human motivations come from within the individual's brain. There can be no other source. All our actions are a result of the biochemical and structural functioning of our brains.
I remember dealing with this problem in high school. It was there I first read Ayn Rands' Atlas Shrugged and realized that even altruistic actions nevertheless came from our brains and gave us some sense of reward for having performed them. Of course, Rands so-called philosophy ignored this subtlety. She simply stated you should always do what is selfish, with selfish being synonymous with greedy and self serving. She goes so far as to condemn any altruistic action, declaring that it is, by nature, evil.
But this seemingly obvious factual basis for egoism comes into doubt when the subject is examined more thoroughly. Even acknowledging that the reward center of the brain lights up under functional magnetic resonance imaging when altruistic acts are performed, cannot this really be called selfish? After all, you are making a sacrifice. That sacrifice may cause you serious difficulties. Yet you feel rewarded when you do something which causes yourself discomfort or denial.
It seems to me that the issue here is actually one of definition. Yes, everything we do comes out of the function of our brain and therefore there is some benefit to us in the action, at least in the sense of personal satisfaction or a brief activation of the reward center of the brain. Nevertheless, human beings acting in defense of their children or their country will subject themselves to death, mutilation, and other horrors. They will do so even if they know for a fact those will be the result. How can that possibly be called selfish or self interested?
Those who are advocates of psychological egoism, in my opinion, have confused the basic functions of our bodies with a basic reality of our actions in the broader world. They ignore actual real world consequences to focus upon internal physiological details. As is generally true, the situation is much more complicated than this nice, simple, on-off explanation allows.