Rasmus Lerdorf, the originator of PHP (a coding language used to create Wordpress, among other stuff) has been giving talks in which he says the reward of oxytocin is a big reason programmers contribute to open source projects.
In the open source world, anyone can initiate a project: let's build a new programming language, a new tool, whatever. Then he or she must recruit other volunteers to help out. These are very ad hoc groups, and very egalitarian. The leaders are the ones who contribute the most.
At any rate, I've been trying to find out more about his talk. Ronald, of the blog Technical Notes and Articles of Interest, provides the best precis I've found so far, plus a link to Lerdorf's slides!