Computational models are increasingly being used to assist in developing, implementing and evaluating public policy. This paper reports on the experience of the authors in designing and using computational models of public policy ('policy models', for short). The paper considers the role of computational models in policy making, and some of the challenges that need to be overcome if policy models are to make an e ective contribution. It suggests that policy models can have an important place in the policy process because they could allow policy makers to experiment in a virtual world, and have many advantages compared with randomised control trials and policy pilots. The paper then summarises some general lessons that can be extracted from the authors' experience with policy modelling. These general lessons include the observation that o en the main benefit of designing and using a model is that it provides an understanding of the policy domain, rather than the numbers it generates; that care needs to be taken that models are designed at an appropriate level of abstraction; that although appropriate data for calibration and validation may sometimes be in short supply, modelling is o en still valuable; that modelling collaboratively and involving a range of stakeholders from the outset increases the likelihood that the model will be used and will be fit for purpose; that attention needs to be paid to e ective communication between modellers and stakeholders; and that modelling for public policy involves ethical issues that need careful consideration. The paper concludes that policy modelling will continue to grow in importance as a component of public policy making processes, but if its potential is to be fully realised, there will need to be a melding of the cultures of computational modelling and policy making.

Drawing on theories of generalized exchange and the norm of indirect reciprocity, we conceptualize subordinates’ organizational citizenship behavior directed toward the organization (OCBO) and directed toward peers (OCBI) as antecedents of managerial trustworthy behavior and examine how managers’ affective trust in subordinates mediates this relationship. We also investigate the extent to which this mediation is moderated by the level of collectivism in a society. Data were collected from 741 managers and 2,111 subordinates in 18 countries representing all major cultural regions of the world. We find support for our hypothesized moderated mediation in that managers’ affective trust in subordinates mediates the relationships between both subordinates’ OCBO and managerial trustworthy behavior, and subordinates’ OCBI and managerial trustworthy behavior across the different countries studied. Further, managers’ affective trust in subordinates only mediates the relationships between both types of citizenship behavior and managerial trustworthy behavior when collectivism is low to medium but not when it is high. Implications for research on cross‐cultural psychology, trust, and organizational citizenship behavior are discussed.

Theory of Change diagrams are commonly used within evaluation. Due to their popularity and flexibility, Theories of Change can vary greatly, from the nuanced and nested, through to simplified and linear. We present a methodology for building genuinely holistic, complexity-appropriate, system-based Theory of Change diagrams, using Participatory Systems Mapping as a starting point. Participatory System Maps provide a general-purpose resource that can be used in many ways; however, knowing how to turn their complex view of a system into something actionable for evaluation purposes is difficult. The methodology outlined in this article gives this starting point and plots a path through from systems mapping to a Theory of Change evaluators can use. It allows evaluators to develop practical Theories of Change that take into account feedbacks, wider context and potential negative or unexpected outcomes. We use the example of the energy trilemma map presented elsewhere in this special issue to demonstrate.

Central government guidance seeks to ensure and enhance the quality of practice and decision-making across – and sometimes beyond – government. The Magenta Book, published by HM Treasury, is the key UK Government resource on policy evaluation, setting out central government guidance on how to evaluate policies, projects and programmes. The UK Centre for the Evaluation of Complexity Across the Nexus was invited to contribute its expertise to the UK Government’s 2020 update of the Magenta Book by developing an accompanying guide on policy evaluation and ‘complexity’. A small multidisciplinary team worked together to produce a set of guidance, going through multiple stages of work and drawing on a variety of sources including academic and practitioner literature and experts and stakeholders in the fields of evaluation, policy and complexity. It also drew on Centre for the Evaluation of Complexity Across the Nexus’ own work developing and testing evaluation methods for dealing with complexity in evaluation. The resulting Magenta Book 2020 Supplementary Guide: Handling Complexity in Policy Evaluation explores the implications of complexity for policy and evaluation and how evaluation can help to navigate complexity. This article, designed primarily for practitioners who might be interested in this guidance and how it was developed, describes the processes involved, particularly related to the interdisciplinary dialogue and consultation with other key stakeholders that this involved. It also briefly outlines the content and key messages in the guidance, with reflections on the experiences of the authors in developing the guide – including the challenges and insights that arose during the process, particularly around the challenges of communicating complexity to a broad audience of readers.

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