I consider both a natural human trait. Some may exhibit one more than the others, but all around humans naturally exhibit both selfishness and altruism. We have stolen and killed for our own gain and we have given up our possessions and lives for eachother.

Whatever side takes more mental effort is all dependent on the individual. It doesn't take much mental effort to be kind and it doesn't take little effort to be cunning and ruthless. Hell, some of our most evil deeds are some of the most complicated and thought out, while some of our best or "good" deeds have been extremely simple, like doing unto others as you'd have done to you.

I think it is morally weak to totally deny one side of a two sided coin. We humans are not black and white creatures and if we are so selfish then how did we get to where we are today? You don't evolve into an animal that survives best in a group while also being completely selfish. In order for us to be social animals there must be an inherently selfless and altruistic aspect, otherwise how would a group survive if everyone is looking out for themselves?

Even if altruistic actions are for self preservation, that still makes the altruism a core part of ourselves as it is the way in which we are able to realize our selfish agendas.