[This was an answer to the 2013 edge.org question, What should we be worried about? That link is the official page. And this is an archive.org capture of that page, What should be worried about?
Scroll down about a third, and right between Naughton and Scheiner, the archive has answer by mathematician Steven Strogatz, which is missing from the newest version, even though the count at the top still says there are 155 responses. This is the missing response in full.]
In every realm where we exist as a collective -- in society, in the global economy, on the Internet -- we are blithely increasing the coupling between us, with no idea what that might entail.
"Coupling" refers to the ability of one part of a complex system to influence another. If I put a hundred metronomes on the floor and set them ticking, they'll each do their own thing, swinging at their own rhythm. In this condition, they're not yet coupled. Because the floor is rigid, the metronomes can't feel each other's vibrations, at least not enough to make a difference. But now place them all on a movable platform like the seat of a child's swing. The metronomes will start to feel each other's jiggling. The swing will start to sway, imperceptibly at first, but enough to disturb each metronome and alter its rhythm. Eventually the whole system will synchronize, with all the metronomes ticking in unison. By allowing the metronomes to impose themselves on each other through the vibrations they impart to the movable platform, we have coupled the system and changed its dynamics radically.
In all sorts of complex systems, this is the general trend: increasing the coupling between the parts seems harmless enough at first. But then, abruptly, when the coupling crosses a critical value, everything changes. The exact nature of the altered state isn't easy to foretell. It depends on the system's details. But it's always something qualitatively different from what came before. Sometimes desirable, sometimes deadly.
I worry that we're playing the coupling game with ourselves, collectively. With our cell phones and GPS trackers and social media, with globalization, with the coming Internet of things, we're becoming more tightly connected than ever. Of course, maybe that's good. Greater coupling means faster and easier communication and sharing. We can often do more together than apart.
But the math suggests that increasing coupling is a siren's song. Too much makes a complex system brittle. In economics and business, the wisdom of the crowd works only if the individuals within it are independent, or nearly so. Loosely coupled crowds are the only wise ones.
The human brain is the most exquisitely coupled system we know of, but the coupling between different brain areas has been honed by evolution to allow for the subtleties of attention, memory, perception, and consciousness. Too much coupling produces pathological synchrony: the rhythmic convulsions and loss of consciousness associated with epileptic seizures.
Propagating malware, worldwide pandemics, flash crashes -- all symptoms of too much coupling. Unfortunately it's hard to predict how much coupling is too much. We only know that we want more, and that more is better... until it isn't.