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The latest issue of Nature carries a Letter on social networks and the
evolution of cooperation.

Here's the citation plus abstract.

Ohtsuki, Hisashi, et al.
 2006	A Simple Rule for the Evolution of Cooperation on Graphs
and Social Networks. Nature 441(7092):502-05.


A fundamental aspect of all biological systems is cooperation. Cooperative
interactions are required for many levels of biological organization
ranging from single cells to groups of animals. Human society is based to
a large extent on mechanisms that promote cooperation. It is well known
that in unstructured populations, natural selection favours defectors over
cooperators. There is much current interest, however, in studying
evolutionary games in structured populations and on graphs. These efforts
recognize the fact that who-meets-whom is not random, but determined by
spatial relationships or social networks. Here we describe a surprisingly
simple rule that is a good approximation for all graphs that we have
analysed, including cycles, spatial lattices, random regular graphs,
random graphs and scale-free networks: natural selection favours
cooperation, if the benefit of the altruistic act, b, divided by the cost,
c, exceeds the average number of neighbours, k, which means b/c > k. In
this case, cooperation can evolve as a consequence of 'social viscosity'
even in the absence of reputation effects or strategic complexity.

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