The paper also has some more detailed observations that come out of the experimental work; among them that voluntary cooperation is fragile; group composition matters (i.e., groups with more conditional cooperators will be healthier); and that ‘belief management’ maters- i.e., if people think that they are in a group with more conditional cooperators, that group will be more robust. None of these will come as a huge surprise to anyone who has been involved with volunteer communities, but still interesting to see it experimentally confirmed. I’ve always suspected that something like this is the case, and that it explains in part why the GPL is so successful, since it uses copyright to force cooperation and penalize defection, and (importantly) makes a clear public statement that that is the case, which serves a signaling function (everyone in the community knows these are the ground rules) and a filtering function (people who aren’t interested in collaborating don’t join as much as they join other groups.)
I think this is the key explanation for the outrage over the MS-Novell deal a couple of years back. By signing on to the GPL, Novell had signaled that it intended to honor the free software community’s principle of reciprocity. Then, it signed an agreement with Microsoft that looked like an attempt to skirt the GPL in a way that gave Novell an unfair advantage over other members of the Linux ecosystem. People who weren’t steeped in the ethos of the free software community saw it as a simple business deal, and objections to it as some kind of knee-jerk reaction to profit-making. They didn’t realize the extent to which the community is made up of “conditional cooperators” whose participation is contingent on everyone else in the community following the rules. When Novell “defected” from the community’s expectations, the rest of the community felt a need to ostracize it to ensure that no one else would be tempted to similarly defect.
Luis also linked to this old post of his which has more interesting citations on intrinsic motivations.