Philosophers and scientists agree that an important characteristic of theories is their internal coherence. I propose that there is a particular type of internal coherence, termed “unit coherence,” that has received insufficient attention from psychologists. When psychologists theorize about the mechanisms that bring about human behavior, the units in which the variables are expressed need to be consistent throughout the theory; this is unit coherence. The theory of reasoned action is an example of a unit incoherent theory. I explain why (...) this is so and also show how the theory can be made to be unit coherent. I also include examples of unit coherent theories to demonstrate that they are possible to have in psychology. Although the concept of unit coherence is somewhat subtle, it has surprising and far-reaching consequences. (shrink)