Let me give you an example. William Hamilton's wonderfully named paper "Geometry for the Selfish Herd" imagines a group of frogs sitting at the edge of a circular pond, from which a snake may emerge - and he supposes that the snake will grab and eat the nearest frog. Where will the frogs sit? To compress his argument, Hamilton points out that if there are two groups of frogs around the pool, each group has an equal chance of being targeted, and so does each frog within each group - which means that the chance of being eaten is less if you are a frog in the larger group. Thus if you are a frog trying to maximize your choice of survival, you will want to be part of the larger group; and the equilibrium must involve clumping of all the frogs as close together as possible. - economistsview.typepad.com