Final answer:
The theories proposed to explain paranoid personality disorder, including psychological and sociological ones, lack substantial systematic research partly because of the challenges in empirically verifying the religious experiences often rejected by theorists such as Freud and Durkheim.
Step-by-step explanation:
Most theories that have been proposed to explain paranoid personality disorder have received little to no systematic research. Psychological and sociological explanations from figures like Sigmund Freud and Emil Durkheim share a skepticism towards accepting religious experiences as indicative of any supernatural reality, primarily because these experiences lack empirical verification. This skepticism extends to the field of psychology at large, where concepts and disorders often require empirical research to be validated and widely accepted in the scientific community.