You’re sticking with predominant egoism! This is a very controversial view, since it concedes the possibility of altruism and yet still makes a lot of human behaviour very hard to interpret.
One might also wonder what reason there is to suppose that altruism is rare, given that it appears to be very common.
For more on this topic, you might read Chapter 5 of Rachels’
The Elements of Moral Philosophy
. At Southampton, we run various modules that address these and similar issues: Ethics, Political Philosophy, Moral Philosophy, Happiness and Wellbeing, and Action, Reason, and Ethics. Many of our staff undertake research in this area, such as Dr. Alex Gregory, Prof. Chris Janaway, Dr. Sasha Mudd, and Dr. Jonathan Way.