The comment about human behavior not being reducible to smooth bell curves is not true. Actually, most measures of human behavior fit bell curves very well when the sample of individuals is large and randomly collected. It is not the variability in human behavior that is a problem.

The claim that human behavior is unpredictable (at the level of the individual) is too broad a brush. Some aspects of human behavior can be predicted with great accuracy, at the individual level. For example, in the US, were I predict that drivers of cars will drive on the right side of the road when they pull out of their driveways, I would be right almost 100% of the time. Whether human behavior is predictable depends on the particular behavior, the nature of the measures used on which to base predictions, and the precision of the prediction. A common demonstration in psychology classes to prove how easy it is predict human behavior at the individual level is this: When taking about prediction and control of behavior, I would right down the following predictions, “Walk to front of room”, “Sit on the table in front of class”. I would then ask students whether they thought human behavior was predictable. Many would say they didn’t think so. I would then say I would like prove them wrong. I would pick a student at random and say I want to do a demonstration that proves I can control your behavior. I would ask the student to come up in front of the class so we could run the test. I would ask them to sit on the table while I prepared the test. Every student in every class did as I requested. After they sat on the table, I would show the class my written predictions. I was right 100% of the time. Behavior can be predicted when researchers have knowledge of the specific situation and know the effect of that situation on people. The situation has to be one that controls specific behaviors. College classrooms exert great control over people. Students are all sitting. They are all dressed. There are clear consequences for violation of the social rules. Under these specific conditions behavior can often be predicted. Performance on final examples is often highly predictable from performance on midterms (the correlations often are .7 and above).

With that said, however, it should be noted that most research conducted by psychologists and psychiatrists is NOT designed to make point predictions. The research not only deal with averages over many “equivalent” examples (different people), something that is common in all sciences (let’s see a physicist predict the trajectory of a particular particle), it also is rarely parametric (unlike physics). Thus, when studying say, the relationship between drugs and future crime, researchers almost never attempt to fit particular theoretically derived functions relating the amount of drug use (but how measured?: frequency of use, dosage per use, type of drug, mixture of drugs, etc.) with the “amount” of crime (but how measured?: frequency of arrests, most serious crime committed, number of times convicted, mix of crimes, etc.). The lack of parametric experimental designs and the lack of agreement about how to measure key conceptual variables is a major reason more aspects of human behavior is not more predictable. Not the fact that human behavior is variable.

Another reason human behavior is difficult to predict is the fact that the situations in which humans find themselves is highly variable and the fact that a good portion of human behavior is under the control of situations in which we find ourselves. Crimes occur in many different specific situations. If the situations do not reoccur, the behaviors controlled by those situations will not reoccur. This is why simple measures (e.g., drug use, family structure, etc.) do not do a very good job of predicting. They ignore the specific criminal settings.