People particles

Last week’s Science included an article by Adrian Cho examining the way that social modelers use math to describe human behavior on a large scale (“Ourselves and our interactions: the ultimate physics problem?”). I’m sort of irritated at the way physics shows up in this. I mean, sure if – for the purposes of a model – we can treat people as interacting particles, then that shares a mathematical basis with (some kinds of) physics modeling.

Behind it all lies the assumption that, at least within distinct types, people are like subatomic particles: basically the same. "We like to think that we are unique," says Alessandro Vespignani, a physicist at Indiana University, Bloomington, who works on networks. "But probably for 90% of our social interactions, we are not so unique."

This isn’t a very relevant criticism – some models may assume that every individual is identical, but they need not do so. If there are well-characterized variations in behavior, a model can incorporate them directly. At some level this is what shopping centers do to predict the behavior of teenagers – do you put the pink cell phones across from Hollister, or the blue ones?

In any event, does that mean that every kind of mathematical model should be called “physics”? In practice, it seems to be people trained in physics who carry out this kind of work:

Forays into "sociophysics" began in the early 1970s. Physicists proposed, for example, that individuals interact to form public opinion much as neighboring atoms make a crystal magnetic by aligning their magnetic fields; researchers analyzed the social phenomenon by adapting the Ising model used to describe such magnetic interactions. In the 1990s, many physicists turned to economics in the controversial subfield of econophysics (see sidebar, p. 408). Now, the movement seems to be gathering momentum, as complex-systems researchers have made solid contributions in the study of traffic, epidemiology, and economics. Some are now tackling more-daunting problems, such as the emergence of social norms.
"The problems are more complicated than most natural scientists assume, but less hopeless than most social scientists think," says Dirk Helbing, a physicist-turned-sociologist at the Swiss Federal Institute of Technology Zrich (ETHZ).

Sadly many traditional disciplines are safe harbors for the math-impaired. Disciplinary fence-building happens for understandable reasons – not least, that “interdisciplinary initiatives” often cover administrative efforts to cut faculty or increase courseloads. The route to useful new mathematical models may be easier through cross-disciplinary institutes of various kinds, but even these are often subject to a kind of tunnel vision – the founders of institutes have pretty specific ideas of what they value.

Is there a future in particle models of humans, from an anthropology perspective? There’s no doubt in my mind – several of the high-ranking anthropologists and primatologists I know are deeply interested in network effects, hub/spoke models, and phase transitions. My only hesitation is that the models are being driven mainly by consistency. Models can produce outcomes that look like real social systems, and people who don’t dig into the mathematical details can find this consistency very convincing. But consistency is not enough; untested models may be simpler, more realistic, or consistent with broader observations. So we need more people familiar with social systems to dig into the details of these models.


Cho A. 2009. Ourselves and our interactions: the ultimate physics problem? Science 325:406-408. doi:10.1126/science.325_406