To begin with, recall that egoism is a relativist theory of ethics that maintains that right and wrong are relative to self-interest. One way to support egoism is to argue that self-interest is central to our lives in various ways. If it is central to our lives, then you might think that this makes it a good candidate to evaluate ethical action.
One way that self-interest is central is that we all must act in our own interests just to survive.
The fact that we do act in our own interests (at least some of the time) can be offered as a reason for why we should act in our own interests. But you need to be careful with this kind of argument. All it says is that acting self-interestedly comes naturally to us. But just because something comes naturally to us doesn’t mean we should do it.
The fact that we act self-interestedly does offer some support for the egoist, though. It at least shows that its ethical ideal (of acting in your own interests) is possible for humans since we do it anyway. What’s more, if your ideals or goals are self-determined, then you’re also more likely to achieve them.
Let’s consider another way that self-interest might be central to our lives. There are points in history where self-interested action has brought about development in society.
Of course, a scientist could also be motivated by their desire to help humanity. After all, if you’re trying to find cures for diseases, then you probably care about other people. But you might also get a kick out of your work or enjoy your achievements being recognized.
We can see the importance of self-interested action in areas other than science as well. For instance, slaves weren’t helped out of their condition purely by the charity of other people (such as the Abolitionists). In parts of the world, slaves liberated themselves.
The slaves must have been at least partly motivated by their own interests: their interest in being free and no longer suffering at the hands of their oppressors. The actions of these self-liberated slaves not only produced social progress by getting rid of slavery in Haiti, but by influencing the rest of the world.
For instance, their actions disproved the racist idea (common at the time) that black people weren’t capable of organizing and ruling themselves. What’s more, this would go on to inspire many more movements against slavery and oppression, especially in the Americas.
Another reason to think self-interested action is desirable is that action done in the service of others sometimes stops societal progress. For instance, think back to our example about the scientist that works for the love of the subject. Throughout history, scientists working freely according to their interests produced groundbreaking insights and important innovations. But when they’re forced to work in the interests of others, their work often stagnates.
A consequence of this is that less development goes into radical new drugs that progress society, and more development goes into less risky drugs, which are more likely to make money (e.g. variations and rebranding of existing drugs).
Here’s a very different example of social stagnation being produced by working in the interest of others. Many communist states became obsessed with the interests of the country or the collective. They ended up spending their resources on detaining and spying on their own people, who they suspected of being enemies or traitors to their country.
Their paranoia and pride about the perception of their country meant they wasted time and money on gathering private information about its citizens and censoring them. Thus energies were taken away from more productive activities.
Source: For drug development statistics see http://bit.ly/2ceGFKa