A great book on this topic is from one of my favorites--Mark Twain. It's called
What is Man? You can get a free ebook version
here. An example of a seeminly selfless act from the book is: If a man jumps into the water to save another, he's really acting selfishly because he couldn't live with himself if he didn't. So, he's essentially acting to please his own nature. Twain's version is more eloquent, of course, so I encourage anyone interested in this subject to give it a read.