Professor Kelly's research involves describing and explaining similarities and differences in people's patterns of thinking, feeling, and behaving. Professor Kelly MOST likely studies a(n) theory.

Personality  

Explanation:

In psychology, the study of personality is one of the main branches of this science.

The study of personality involves the description and explanation of different patterns of thinking and behaving in terms of some traits that people have or don't (personality traits). These traits would usually explain the difference between people's patterns of feeling, thinking and behaving.

In this example, Professor Kelly's research involves describing and explaining similarities and differences in people's patterns of thinking, feeling, and behaving. Therefore, we can say that he most likely studies a Personality theory.

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