The task which the Scottish Centre for Crime and Justice Research ( SCCJR) was required to conduct was to systematically review the existing evidence on the impacts of the criminalisation of the purchase of sex. This remit meant that other legislative approaches and models such as decriminalisation or regulation were not included in the review  ; the absence of a comparative dimension is an obvious limitation in that the lack of broader comparison with other models (i) could potentially present issues as pertinent to the criminalisation model alone, rather than as issues associated with prostitution more broadly (ii) presents challenges in assessing any impact of the criminalisation of the purchase of sex in comparison to other models (such as decriminalisation, regulationist).
The review was tasked with assessing existing 'evidence' relating to the impact of the criminalisation of the purchase of sex. This involved consideration of what constitutes 'evidence' and how it is presented: a challenging task given the wide-ranging literature surrounding prostitution more generally. We have attempted to retain a focus on empirical evidence however, the underpinning discourses surrounding prostitution and the debates which contextualise legislative developments provide an important context for the review and merit ongoing consideration  .
The review draws upon a range of evidence, constituting a variety of different methodological approaches which are underpinned by distinct and significant ideological and theoretical positions. This problematizes any attempt to read the evidence as 'value free' or 'neutral' and evidence assessments, including this review, require caveats and contextualisation. The different positions that underpin data collection and indeed, how an issue is defined and re-presented mean that this review is limited in its ability to present 'neutral evidence' which can support policy and decision-making processes. Definitions of the 'problem' and the 'solutions' that underpin the available evidence need to be considered from the political perspectives that determine them.
A systematic review of evidence presents a number of challenges in this area, given the limited number of papers which are informed by empirical data collection and analysis, and due to the limitations of existing evidence on the impacts of criminalisation. This has meant that a small number of studies have been cited extensively throughout recent literature and used consistently to argue specific points. These limitations emerge partly from the challenging nature of prostitution research where the associated stigma and marginalisation make potential participants 'hard to reach' and thus results in a partial overview of impacts. Additionally, the introduction of legislation to criminalise the purchase of sex has been introduced to various countries at different points in time and in many countries the legislation is recent and potential impact is preliminary. The review draws upon existing evidence, highlighting findings, methodological approaches (i.e. methods used in primary research) (see Annex) and the broader contexts within which studies included in the review are located. Where possible, gaps in existing knowledge are identified and evidence deficits highlighted.
Relevant databases and bibliographies were searched to broaden the scope of the study. A Research Advisory Group ( RAG) was established to support the Scottish Government composed of key academics working in this area from Scotland and England, and representing different positions within current debates. RAG members distributed requests for relevant material around their networks and a number of published and unpublished papers, conference presentations and evaluation studies were received by the research team following such requests. Colleagues and practitioners from Scotland and abroad passed on relevant papers and reports.
The review is limited to papers in English. The search was initially conducted between October and December 2015 with a second scan of available evidence in February and March 2016. Quantitative (largely attitudinal surveys) and qualitative studies (interviews and observations) (see Annex One) have been included. The range and scope of various evidence and publications in this area presented a number of challenges for the research team and attempts were made to scope and sift the wide-ranging material to ensure that the most robust evidence was included in the review (discussed further below).
The Annex lists many of the empirical studies which form the basis of this review, however it has proved difficult to meet the expected standards required for an assessment of evidence since many of the studies included are qualitative in nature and do not fit with most systematic evidence assessment tools designed more generally, for quantitative outcome studies. Some of the key studies referred to in documents are not written in English and while English summaries were included, this does not allow us to access the more detailed evidence contained in the full document/report. Many papers and reviews cite the work of other researchers where original studies have been unobtainable or, in some cases, not available in English. We have therefore drawn on a broader range of studies which makes the study less 'scientific' than a systematic review, or even a rapid evidence assessment would aspire to be. While we recognise this is a limitation it does, however, allow for consideration of a more comprehensive range of evidence. Methodological difficulties are intrinsic to this field of study meaning that policy makers will need to take into consideration issues other than the existing evidence base.
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