What is the reductionism vs holism debate in psychology?

The debate raises questions regarding the extent to which a theory explains a behaviour. 
A reductionist theory in order to explain a complex phenomena reduces its constituent parts. i.e behaviour is greatly simplified. This is unrepresentative of the true nature of human behaviour. 
A holistic theory takes into account all the constituent parts/factors that may contribute to behaviour and accepts that they all play a role in the specific outcomes. Such theories are more realistic and representative of human behaviour but do not establish any causation.