Professor Murphy investigates aggression based on how an aggressor perceives information in a situation and how they make decisions about being aggressive. Professor Murphy is the most likely a psychologist in the subfield.

Cognitive.

Explanation:

Cognitive psychology is demonstrated as the domain of psychology that primarily deals with the study and understanding of human mind and the mental processes that determine their behavior. It involves the study of thinking, ability to reason, perception, decision-making, ability to make judgments, etc.

As per the question, Professor Murphy would most likely be categorized as the 'cognitive psychologist' as he investigates and attempts to understand that how 'the mental processes like perception of an information and decision making abilities' has lead to the individual's aggressive behavior. Thus, 'cognitive' sub-field is the answer.