Despite a strong rights-based emphasis, clearly the HF model has its roots in psychology. The HF model was originally developed and described by a psychologist, Sam Tsemberis,  and the strategic choice to build quantifiable measures into the model, to develop an evidence base for its effectiveness, is characteristic of the ambition of psychology as a disciplne to assert its credentials as a science, in contrast to social work and housing management, which typically make no such claims.

More to the point, it has some very sound social psychology at its heart, especially around questions of empowerment and choice, and the negative effects of institutionalisation. Although the strengths model - itself a branch of positive psychology - and a growing understanding of trauma in trauma-informed care are both in principle options for the services that adopt HF as practice, they are so closely interwoven, both in practice and in policy statements on HF, that they are now almost inseperable.

The 'psychology' in PIEs is potentially far broader, although in practice homelessness services that have adopted this approach, meeting largely the same psychological and emotional neeeds, have likewise found that the strengths model and an awareness of trauma are the source of many insights and many operational responses ( see  'Working with trauma': HERE).

Housing First and PIEs may have developed originally as separate, even quite contrasting approaches,  or at least as 'parallel lines', but they are now coming back together (see 'Housing First and PIEs: where parallel lines met?' : HERE). There is ample scope for debate currently as to how far the HF and PIEs approaches should be seen as compatible, with their contrasting attitudes to principles and pragmatism (HERE) and with the potential currently to deepen the dialogue by focusing some of the Pizazz self assessment process more clearly on the application in HF (HERE).

It would be possible to see accommodation in an ordinary flat - singleton ('scattered site') housing - as being 'psychologically informed', at least in so far as it may meet profound psychological and emotional needs for the individual - including safety and control. The policy of HF is certainly psychologically informed, if it stems for a recognition that rapid housing makes it more possible for services to then address other psychological needs, where other approaches have failed.

More recently we are finding that in the UK, where the PIEs approach is now well established, the self assessment and service development process for PIEs - known as the Pizazz - is proving useful to help to embed HF principles in operational practice; and to help share the learning between services on these more contextualised issues that the HF principles in themselves give no guidance on.

What is at the moment less clear is how far the PIEs approach - and the Pizazz - may be useful for Housing First services outside the UK.   In part this may depend not so much on the applicability of the fundamental principles of HF, but on a closer examination of the kinds of housing stock available - and the terminology to describe them. In the analysis of the historical development of PEs and HF that we have presented as 'parallel lines' now meeting (HERE), we had suggested that the PIE approach may be particularly helpful in congregated HF sites, in permanent supported housing.