Why do we help each other out

I am always interested in why people help each other out and have used the concept of reciprocal altruism in my talks on knowledge management.
I've been reading over my lunch today an article by Martin Nowak about co-operation. He views co-operation as interesting as it means that you help someone who is a possible competitor and that you reduce your own success in order to increase the success of someone else.

He highlights some types of reciprocity that knowledge managers might find interesting.

1. Direct reciprocity - individuals interact repeatedly - if I help now you may help me later.
2. Reputational (my phrase) Indirect reciprocity which takes place in groups - where people see you interact with another person and reach the conclusion that you are a helpful person.
3. Spatial selection - where neighbours help each other - they survive by being in clusters this could be of interest in communities of practice but also in organisations especially departments which consider themselves in danger.
4. Finally Group selection and he says' it maybe that our group of co-operators is better off than another group of defectors, here selection acts on two levels because in our group there is more co-operation