How scientific is the five-factor model of personality psychology? On a scale of 1 to 10, where 1 is not scientific at all and 10 is very scientific, I would place the five-factor model at 4. In this case, I use scientific to mean the ability to make useful, reliable predictions. By that standard, IQ, much as some people hate it, is scientific. Astrology, much as some people like it, is not scientific. To be useful, a psychometric instrument has to be predictive in two ways. First, the measure taken on one person using one instrument should be useful in predicting that same measure on that person at a different time or using a similar instrument. That indicates robustness or reliability. Second, personality psychology also must be able to make predictions about differences in behavior and life outcomes. That would make it useful, for example, in assigning tasks in business or in improving online dating services. To understand the requirements for prediction, consider the famous . The first question we should ask about this experiment is how consistent the results would be with repetition. Suppose that you give the test on Monday, and Alice delays gratification but Bobby eats the marshmallow right away. On Tuesday, will the result be the same, or will it be Bobby who delays gratification and Alice who eats the marshmallow? Does a child’s performance on the marshmallow test predict what the child will do with a different treat? If not, then the marshmallow test is not a reliable instrument for measuring a personality trait. Stanford marshmallow experiment What made the marshmallow test famous was the follow-up work which suggested that a child’s ability to defer gratification on the test helped predict future outcomes, such as SAT scores. These correlations with longer-term outcomes speak to the usefulness of the test in revealing some important trait. IQ vs. Astrology Using prediction as a standard, IQ is a scientific measure. Measuring IQ at different points in time or using different instruments tends to yield very similar results. Also, IQ is useful in predicting outcomes in life. On the other hand, some people use astrology to predict life outcomes. But are astrological predictions accurate? Quickly consulting Google, I found . an article by Kelly Katera There haven’t been many studies that investigate the science behind astrology, but of the few that have, the results have failed to support the validity of astrological views. For instance, tested the accuracy of astrological charts in describing the personality traits of 193 study participants, and the results indicated that the scores were at a level consistent with chance. a study This would tend to confirm my view. Astrology is not scientific. The popularity of astrology nonetheless shows that we are inclined to see people as having innate personality differences. We think that we can model these personality differences and use those models to predict behavior and outcomes. Moreover, we are capable of over-estimating the ability of a model, such as astrology, at making such predictions, perhaps because we are guilty of confirmation bias. That is, we are inclined toward noticing when the model succeeds and ignoring or rationalizing instances where the model fails. Personality psychology as a science We know that people differ in their brain chemistry. There is a field known as personality neuroscience that links brain chemistry to personality traits. . Timothy A. Allen, the author of the survey article, concludes that the research shows promise yet leaves many important questions unanswered. The five-factor model has been examined from this perspective Personality psychology will always have scientific limitations. The traits measured in the five-factor model are not nearly as robust as IQ. The traits are measured using self-assessment questionnaires, and they are not as consistent over time or across measurement instruments as what one finds with IQ testing. The five-factor model also has limits in predicting behavior and life outcomes. Those are determined by many factors other than innate personality characteristics: environmental influences, life history, and cultural context. I expect that rigorous studies might find small correlations between measured personality traits and behavior, but it is unlikely that we will find that any single personality trait overwhelmingly determines particular behaviors. Some of us may be guilty of over-estimating the value of personality psychology, just as many people seem to over-estimate the value of astrology. But personality psychology is not total BS.