Are people always selfish? Event

It's clear that people sometimes do things for their own benefit: you might push ahead of other people to ensure you get a good seat on the train. Some philosophers have made the bolder claim that we always act out of self-interest. But is that true? What are the arguments for it? It might seem obvious that sometimes people act for the sake of others: for example, someone might offer to donate blood for the benefit of a complete stranger. But can such examples be explained away as ultimately self-interested?