The erratic rhythms of human life

Some ideas from a meeting on Complex Networks, 2-6 July, Sardegna, Italy

Most thinking about the rhythms of human life remains fixated on ancient concepts. For most psychologists and social scientists, physics is the physics of the nineteenth century or before -- the physics of Johannes Kepler or Isaac Newton. Mention "mathematical patterns", and they tend to think of regular cycles akin to planetary motion, or simple linear trends. What they don't realise is that modern physics has moved far beyond these very simple concepts, and today works with a much richer view of mathematical pattern, in which even highly irregular and erratic behavior may turn out to reveal hidden regularities.

Send an email, and sometimes you get a quick reply. Sometimes you don't. Naively, you might expect the response times to emails to work pretty much like lots of other random things in our lives; there would be an average time, with some random scatter about that. But in fact this isn't at all true -- emails come back at us with times that fluctuate in a wildly erratic way. Over the past few years, a number of researchers, including many physicists, have scrutinized the data on email responses and found that they don't follow "ordinary" random statistical patterns, but have what physicists refer to as "fat tails". You find, roughly speaking, that while the bulk of emails solicit quite rapid responses, there are many that instead get replies only after very long times, weeks or months.

This implies, among other things, that email correspondence has a naturally "bursty" character, with long intervals of quiet interrupted sporadically by lots of activity. It's not regular and predictable at all.

Curiously, you find the same pattern also in how frequently people visit libraries, in patterns of web surfing and in lots of other individual human activities. This irregular behavior seems to be a kind of universal rhythm that typifies human activity. (In fact, a psychologist from Texas Dave Gilden, has even found very similar irregular patterns in the actions of people who try to do the same task repeatedly, such as tapping their fingers in exact one-second intervals; we seem to do it even if we try not to.)

What's the cause? One might suspect something in the character of the human brain, perhaps, but physicist Laszlo Barabasi and colleagues suggest a much simpler idea -- that much of it comes down to how we prioritize tasks. Suppose that you, like all of us, have lots of tasks you need to take care of -- shopping, doing the dishes, sending a letter and so on. You might just choose items at random off the list, and do them. If you chose which emails to respond to this way, it turns, out then the times for email responses wouldn't be what they are. They'd follow the Bell curve, with a nice average for the time between emails.

But suppose instead that some emails have more priority than others, and you tend to respond to them first, while pushing those of lesser priority down the list. The mathematics shows that in this case, because emails of lesser priority keep getting pushed down the list, what naturally comes out are fat tails -- and an amplified chance for email reponses to take a surprisingly long time. It's not human procrastination, it seems, but the more or less mechanical consequence of how we deal with sequences of tasks.

There are probably many more surprises of this kind of work to come. For one thing, as Laszlo pointed out, it has a curious link to some work of several years ago that found similar fat tails in the way people move around. The way we move from place to place seems to have the same erratic quality as do our email reponse or library visiting patterns. But these fat tails also show up throughout nature, in the way natural disasters and earthquakes strike, in the ups and downs of markets and so on. (I wrote a brief overview a couple years ago for the business magazine strategy + business.) Indeed, explaining these so-called "power-law" patterns in nature has been big business in physics for twenty years, and will continue that way for some time.

This work doesn't actually have any direct link to networks. But it certainly demonstrates how individuals, without knowing it, end up following quite striking and seemingly universal patterns. And how human science, if it is going to understand human dynamics more effectively, is going to have to embrace the irregular mathematics of modern physics.