Psychology is a mess. And I don’t say that because of the consequences of the replication crisis.

No, compared to biology and the physical sciences, psychology is a mess because it has no unifying theory. Biology has evolutionary theory as a powerful framework for understanding everything from ecology to genetics. Chemistry has atomic theory, which brought great clarity to the understanding of elements and compounds.

In contrast, psychology has nothing approaching a unifying theory of human thought and behavior. Psychology has, instead, a hodgepodge of theories about different aspects of behavior–some of which are inherently contradictory. Some theories are more popular than others, but none ever seem to discredit or supplant earlier theories. (There are still people who subscribe to psychoanalytic theory!)

Thomas S. Kuhn (1996), in his famous book, The Structure of Scientific Revolutions, described this state of affairs as being typical of the pre-paradigm phase of scientific fields. In this phase, several competing theories vie for scientists’ allegiance, and no theory is dominant. That sounds a lot like psychology.

Kuhn stated that the next phase was normal science, a time when scientists settle on an overarching theory. In normal science, the main goal is to apply the theory, solve practical problems and build up the knowledge base of a field.

Intelligence Research’s Journey to Normal Science

Psychology as a whole is very far from entering the normal science phase, and I’m not sure it ever will. However, the psychological study of intelligence is certainly in the phase of normal science.

I have been pondering this ever since I wrote the introduction section of my upcoming book, In the Know: Debunking 35 Myths About Human Intelligence. In that intro, I have a brief history of intelligence research, and it follows Kuhn’s (1996) outline well. [Update: The book has been released.] Most of the 20th century saw different intelligence theories–often mutually contradictory from one another–fighting for adherents. That was intelligence research’s pre-paradigm phase.

Things changed in the 1990s with the publication of Caroll’s (1993) Human Cognitive Abilities: A Survey of Factor-Analytic Studies and Jensen’s (1998) The g Factor: The Science of Mental Ability. Both these books settled many of the controversies in the pre-paradigm phase of intelligence research, and quickly psychologists who studied intelligence rallied behind a theory of general intelligence as sitting atop of hierarchy of narrower cognitive abilities.

Since the 1990s, the field has progressed well under normal science. Psychologists and other scientists have started answering important questions, such as how the brain produces individual differences in reasoning ability emerged in the 2000s (Jung & Haier, 2007) and why intelligence correlates positively with life expectancy (Arden et al., 2016; Gottfredson & Deary, 2004). Many other discoveries about intelligence have occurred since the 1990s that have greatly enlarged the world’s understanding of intelligence.

Few research fields in psychology reach the stage of normal science. Working in a field that has is pretty exciting.

The Future of Intelligence Research

So what’s next for intelligence research? Probably a lot more time in the normal science phase. There are a lot of questions, and the current theory and way of studying intelligence seem good at finding answers. The pre-paradigmatic stage for intelligence research lasted for about 90-120 years (depending on when marks the exact beginning and end of that stage of the history of intelligence research). In physics, the normal science phase from Newton to Einstein lasted over 200 years. It is possible that the current stage of normal science lasts the rest of my life.

According to Kuhn (1996), though the normal science phase will end eventually as the theory becomes unable to solve problems and answer questions. That will be the start of a crisis phase that forces scientists to re-examine their dominant theory and try to understand why it fails to fully explain the world.

Next comes the paradigm shift, where a new theory replaces the old one in a process that is tumultuous and often difficult for individual scientists who are invested in the old theory. After that comes a return to normal science.


I think that intelligence research shows that psychology as a whole may be stuck in a pre-paradigm stage, but that research communities investigating specific topics in psychology can advance to normal science.

Additionally, a field that has entered normal science is a field that is closer to the truth than a pre-paradigmatic field. I think that psychologists working in other fields of psychology would be wise to build on intelligence research instead of trying to work with theories that lack consensus. This won’t be possible with every topic because modern intelligence theory does not span all of human behavior. But where intelligence is relevant, it provides a firmer foundation that any pre-paradigmatic theory.


Arden, R., Luciano, M., Deary, I. J., Reynolds, C. A., Pedersen, N. L., Plassman, B. L., . . . Visscher, P. M. (2016). The association between intelligence and lifespan is mostly genetic. International Journal of Epidemiology, 45, 178-185. doi:10.1093/ije/dyv112

Carroll, J. B. (1993). Human cognitive abilities: A survey of factor-analytic studies. New York, NY: Cambridge University Press.

Gottfredson, L. S., & Deary, I. J. (2004). Intelligence predicts health and longevity, but why? Current Directions in Psychological Science, 13, 1-4. doi:10.1111/j.0963-7214.2004.01301001.x

Kuhn, T. S. (1996). The structure of scientific revolutions (3rd ed.). Chicago, IL: University of Chicago Press.

Jensen, A. R. (1998). The g factor: The science of mental ability. Westport, CT: Praeger.

Jung, R. E., & Haier, R. J. (2007). The Parieto-Frontal Integration Theory (P-FIT) of intelligence: Converging neuroimaging evidence. Behavioral and Brain Sciences, 30, 135-154. doi:10.1017/S0140525X07001185