Reductionism

Reductionism is a really key issue in psychology because to some extent it can be applied to every theory, and every approach or paradigm. 

Reductionism refers to the breaking down of complex phenomenon into smaller components. It's the opposite of holism, where complex phenomena are looked at in their entirety.

Reductionism/holism is a continuum rather than an either/or description of a theory or approach. Some of the more reductionist approaches are behaviourism and the biological approach, whilst the more holistic approaches are humanistic psychology, the individual differences approach and social psychology.

Essentially, the more simplistically a behaviour is explained, the more reductionist the theory.