It would be foolish to claim that income and price are the only things that ever affect social behavior, Decisions about fertility, marriage, living arrangements, health, and the like are affected by other factors, just as the amount of coffee demanded also depends upon one’s taste for coffee, the weather, one’s sleeping habits, and so on. Moreover, there may be irrational, subconscious forces at work that conflict with the model of a rational decision maker who is trying to maximize his or her well-being. The economic perspective does not assert that these other factors and forces are never present. It simply says that it is useful to analyze the effects of changes in constraints and prices under the assumption that the other factors are constant. Sometimes this is a reasonable assumption; sometimes it is not. To the extent that we know about changes in the other variables, we can try to control for them. For instance, in studying the influence of income and price on the demand for medical care, it is possible to take into account changes in health which we know also affect this demand.