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1 " I define game theory as the study of how rational individuals make choices when the better choice among two possibilities, or the best choice among several possibilities, depends on the choices that others will make or are making. "
― Thomas C. Schelling , Micromotives and Macrobehavior
2 " let me remind you of the particular characteristics of all of these behavior systems that I am trying to focus on. It is that people are impinging on other people and adapting to other people. What people do affects what other people do. "
3 " The generic name for behaviors of this sort is critical mass. Social scientists have adopted the term from nuclear engineering, where it is common currency in connection with atomic bombs. "
4 " If a model meets the criterion of simplicity it will often, like the thermostat-controlled heating system, describe physical and mechanical systems as well as social phenomena, animal behavior as well as human, scientific principles as well as household activities. An example is “critical mass. "
5 " If the option of taking the course pass-fail (without a letter grade) is available to all students, it is usually observed that there are some who will elect pass-fail no matter how many others do, some who will elect letter grades no matter how many elect pass-fail, and an intermediate group who will elect pass-fail if enough do but will choose letter grades if pass-fail is uncommon. Notice that the first and second groups’ behavior is independent of how the third group chooses, but not vice versa; the people whose behavior is uninfluenced nevertheless influence others. "
6 " Because people vary and because averages matter, there may be no sustainable critical mass; and the unravelling behavior, or initial failure to get the activity going at all, has much the appearance of a critical mass that is almost but not quite achieved. This is therefore a kindred but separate family of models. "
7 " Models tend to be useful when they are simultaneously simple enough to fit a variety of behaviors and complex enough to fit behaviors that need the help of an explanatory model. "
8 " The principle of critical mass is so simple that it is no wonder that it shows up in epidemiology, fashion, survival and extinction of species, language systems, racial integration, jaywalking, panic behavior, and political movements. "
9 " Measles vaccination shares some crucial features with the thermostat system but differs in important respects. A measles-epidemic model without vaccination will be different but recognizable as a member of the family. And "
10 " Models often overlap. The measles epidemic is usually a critical-mass process.4 A succession of epidemics, with intervening periods in which the pool of susceptibles renews itself, corresponds to a cyclical model. "
11 " At some point several appear to decide that the flow of pedestrians is large enough to be safe and they join it, enlarging it further and making it safe for a few who were still waiting and who now join. Soon, even the timid join what has become a crowd. The drivers see they no longer have any choice and stop. "
12 " I stay in line if everybody is standing politely in line, but if people begin to surge toward the ticket window I am alert to be—though never among the first—not among the last. "
13 " If there is enough uranium so that half the neutrons produce two others, the process is self-sustaining and a “critical mass” of uranium is said to be present. "
14 " Furthermore, mass is proportionate to the number of atoms, and critical number could have been equally apt. "
15 " But whether the measure is the number of people engaged, or the number times the frequency or the length of time they engage in it, or the ratio of the number who do to the number who do not, or the amount of such activity per square foot or per day or per telephone extension, we can call it a “critical-mass” activity and a lot of people will know what we mean. "
16 " The observed outcome may be one that everybody prefers, it may be one that nobody prefers, or it may be one that some prefer and others deplore. "
17 " Two special terms have begun to come into currency to distinguish subclasses of critical-mass phenomena. One is tipping, and the other is lemons. "
18 " The tipping model is a special case—a broad class of special cases—of critical-mass phenomena. Its characteristics are usually that people have very different cross-over points; that the behavior involves place of residence or work or recreation or, in general, being someplace rather than doing something; that the critical numbers relate to two or more distinct groups, and each group may be separately tipping out or tipping "
19 " Numerous social phenomena display cyclical behavior, either in wave motion or in surges. The thermostat reminds us to look for the time lag, or for an accumulated inventory like the hot water. "
20 " When the supply at last gets back to normal, some years hence, replenishment will be at an abnormally low level, with a “famine” that will get worse for at least six years before it can begin to get better. And then the cycle can start over. "