Critical Mass by Philip Ball is a mind expanding multidisciplinery book.
Its subtitle suggest the nature of it - similar to Gladwell's Tipping Point but much more sciencey, it applies the physics of phase transitions - the sudden shift in state from gas to liquid of millions of molecules of water for example - to other more 'complex' phenomena such as traffic and social networks.
All very interesting but the bit I particularly latched on to is the rule, also referenced in complexity theory, that MORE IS DIFFERENT.
What they mean is you can't know something about how large groups will act by observing individual behaviour because the interactions of large numbers change the output in ways you cannot predict from smaller numbers. Groups are not simply scaled up individuals. They act differently.
To me this suggests [if there was ever any doubt] that Mark Earls was on to something with his brilliant paper Advertising to the Herd [you can download it here - scroll down a bit]:
the most important characteristic of mankind is that of a herd animal, not a lone individual
If we want to understand how herds act and respond we need to study herd behaviour - not individuals.
[Side note: Critical Mass also began the popularisation of Power Law probability distribution - more commonly known now as The Long Tail.]