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THE MICRO-MACRO PROBLEM

The circularity evident in commonsense explanations is important to address because it derives from what is arguably the central intellectual problem of sociology—which sociologists call the micro-macro problem. The problem, in a nutshell, is that the outcomes that sociologists seek to explain are intrinsically “macro” in nature, meaning that they involve large numbers of people. Paintings, books, and celebrities can only be popular or unpopular to the extent that large numbers of people care about them. Firms, markets, governments, and other forms of political and economic organization all require large numbers of people to abide by their rules in order for anything to actually happen. And cultural institutions such as marriage, social norms, and even legal principles have relevance only to the extent that large numbers people believe that they do. At the same time, however, it is necessarily the case that all these outcomes are driven in some way by the “micro” actions of individual humans, who are making the kinds of choices that I discussed in the previous chapter. So how do we get from the micro choices of individuals to the macro phenomena of the social world? Where, in other words, do families, firms, markets, cultures, and societies come from, and why do they exhibit the particular features that they exhibit? This is the micro-macro problem.

As it turns out, something like the micro-macro problem comes up in every realm of science, often under the label “emergence.” How is it, for example, that one can lump together a collection of atoms and somehow get a molecule? How is it that one can lump together a collection of molecules and somehow get amino acids? How is it that one can lump together a collection of amino acids and other chemicals and somehow get a living cell? How is it that one can lump together a collection of living cells and somehow get complex organs like the brain? And how is it that one can lump together a collection of organs and somehow get a sentient being that wonders about its eternal self? Seen in this light, sociology is merely at the tip of the pyramid of complexity that begins with subatomic particles and ends with global society. And at each level of the pyramid, we have essentially the same problem—how do you get from one “scale” of reality to the next?

Historically, science has done its best to dodge this question, opting instead for a division of labor across the scales. Physics, therefore, is its own subject with its own set of facts, laws, and regularities, while chemistry is a different subject altogether, with an entirely different set of facts, laws, and regularities, and biology is a whole new ballgame all over again. At some level the laws that apply at different scales must be consistent—one cannot have chemistry that violates the laws of physics—but it is not generally possible to derive the laws that apply at one scale from those that govern the scale below it. Knowing everything about the behavior of individual neurons, for example, would be of little help in understanding human psychology, just as a complete knowledge of particle physics would be of little use in explaining the chemistry of synapses.6

Increasingly, however, the questions that scientists find most interesting—from the genomics revolution to the preservation of ecosystems to cascading failures in power grids—are forcing them to consider more than one scale at a time, and so to confront the problem of emergence head-on. Individual genes interact with each other in complex chains of activation and suppression to express phenotypic traits that are not reducible to the properties of any one gene. Individual plants and animals interact with each other in complex ways, via prey-predator relations, symbiosis, competition, and cooperation, to produce ecosystem-level properties that cannot be understood in terms of any individual species. And individual power generators and substations interact with each other via high-voltage transmission