How necessary or valuable it is that all those involved in a service or community for those more vulnerable should work to or abide by one particular "model" of human psychology, for the service to be most effective? There has been some debate over this question, since the first formulations of a PIE, through the 'classic' model that gradually emerged, until eventually that idea was discarded, for the development of PIEs 2.0

The original paper that first coined the term PIE - based simply on observation of what was actually happening in the more innovative services - at the time at least (2010) had said:
"There is no one set of beliefs that the staff of a PIE need to sign up to, no overall view of the nature of human nature, or even of the underlying problems of the ‘membership’. So it might be any form of psychological theory that might inform the work of the staff, from psychodynamics to behaviourism, from Gestalt to evolutionary psychology, Transactional Analysis, Dialectical Behavioural Therapy, Neuro-Linguistic Programming to existential humanism, and all points between and beyond."

from: Social Psychiatry and Social Policy for the 21st Century (Part One) : The Psychologically Informed Environment

This quite modest and pragmatic approach is echoed in the notes from the (UK) North East Region 2012 Homelessness Strategy workshop on PIEs:
"Adopting "a psychological model" need not mean adopting a particular psychological theory or approach – though that may help – but is more a commitment to recognising psychological and emotional needs. This helps to develop a coherent way of working within each organisation, balancing (for example) consistency with personalisation etc."

from: North East Region PIEs workshop summary notes

On the other hand, the 'Psychologically Informed Services' guidance of 2012 document suggests:
"The development of an explicit service philosophy and practice that is discussed with and adopted by all staff will help promote ownership of behaviour by individuals and recognition of the impact of anti social behaviour. If this is developed with the clients, then it will have even greater ownership and will help shift the power balance. This approach also results in a more predictable set of outcomes for specific behaviours, enabling individuals to make more informed choices about their actions.

from: Psychologically informed services

One thing, however, is crucial here: whatever psychological model is to be adopted, if there IS to be a particular, single model, then it should not be simply imposed on all staff, irrespective of their own thoughts and experiences. Staff - and this includes volunteers and service users - must always be free to question, to doubt, to challenge and to think for themselves.

Elsewhere it is argued that it is reflective practice that mobilises the thinking and discussions that are central to being a PIE. The Pizazz self assessment handbook, "Useful questions", is explicit that reflective practice can only be effective if all workers, volunteers, users etc are free to explore their own way of thinking, and to question any model that is proposed to be applied.

Further reading

Psychologically informed services by Helen Keats, Peter Cockersell, Robin Johnson and Nick Maguire

Social Psychiatry and Social Policy for the 21st Century (Part One): The Psychologically Informed Environment by Robin Johnson & Rex Haigh

North East Region PIEs workshop summary notes collated by Sheila Spencer

Creating a Psychologically Informed Environment; assessment and implementation by Claire Ritchie

PIEs, SPIEs and Homo SAPIENs by Tom Harrison

Webinars/viewing

Psychologically Informed Environments and Trauma Informed Care (HomelessLink webinar), Claire Ritchie & Jo Prestidge 

Introduction to Psychologically Informed Environments (Fullfilling Lives training programme,) Ray Middleton & Robin Johnson

A 'Handy' guide to being a PIE (2013) by Robin Johnson (video)

Does it take a psychologist to be a PIE? by Robin Johnson