Reductionism in psychology is traditionally regarded as an exercise in drawing data from a physiological field in order to explain psychological or behavioural phenomena. The data taken from the physiological level are seen in this view as more basic and more real in terms of a hierarchy of sciences, and come closer to providing "hard facts" due to physiology lying lower on a hierarchy of sciences than psychology.

However, reductionism is also considered here in a number of different ways, including:

1. the breaking down of wholes into parts in science; 2) reduction to disciplines such as mechanics or computing through modelling; 3) reduction "upwards" to higher levels than psychology; 4) single concepts in psychology; and 5) individualism as a reductive outlook.

The hierarchy of sciences is critiqued in terms of indeterminacies that prevent the lower-level sciences from fully constituting the higher-level sciences. A "downwardly determinative" outlook is endorsed which sees the contents of lower-level sciences as being distributed or guided by the systems that the higher-level sciences represent. Non-specific precipitants at higher levels are seen to control the more widely distributed but more specialised phenomena at lower levels, and a twin-aspect theoretical outlook is endorsed which sees a given unit of analysis as being a whole in itself and part of a larger system.

The implication of this for reductionism in psychology is that whilst an elimination of psychological terms in favour of physiological terms is rejected, explanations that include physiology in a complementary role may have some value, although such contributions may appear insignificant in the light of the higher-level concepts that flow down to psychology from higher level disciplines.