Note: I wrote the following discussion (last year) to be a Nature Comment but it was not to be!
Nature articles on evidence-based policymaking often present what scientists would like to see: rules to minimise bias caused by the cognitive limits of policymakers, and a simple policy process in which we know how and when to present the best evidence. What if neither requirement is ever met? Scientists will despair of policymaking while their competitors engage pragmatically and more effectively.
Alternatively, if scientists learned from successful interest groups, or by using insights from policy studies, they could develop three ‘take home messages’: understand and engage with policymaking in the real world; learn how and when evidence ‘wins the day’; and, decide how far you should go to maximise the use of scientific evidence. Political science helps explain this process, and new systematic and thematic reviews add new insights.   
Understand and engage with policymaking in the real world
Scientists are drawn to the ‘policy cycle’, because it offers a simple – but misleading – model for engagement with policymaking. It identifies a core group of policymakers at the ‘centre’ of government, perhaps giving the impression that scientists should identify the correct ‘stages’ in which to engage (such as ‘agenda setting’ and ‘policy formulation’) to ensure the best use of evidence at the point of authoritative choice. This is certainly the image generated most frequently by health and environmental scientists when they seek insights from policy studies.
Yet, this model does not describe reality. Many policymakers, in many levels and types of government, adopt and implement many measures at different times. For simplicity, we call the result ‘policy’ but almost no modern policy theory retains the linear policy cycle concept. In fact, it is more common to describe counterintuitive processes in which, for example, by the time policymaker attention rises to a policy problem at the ‘agenda setting’ stage, it is too late to formulate a solution. Instead, ‘policy entrepreneurs’ develop technically and politically feasible solutions then wait for attention to rise and for policymakers to have the motive and opportunity to act.
Experienced government science advisors recognise this inability of the policy cycle image to describe real world policymaking. For example, Sir Peter Gluckman presents an amended version of this model, in which there are many interacting cycles in a kaleidoscope of activity, defying attempts to produce simple flow charts or decision trees. He describes the ‘art and craft’ of policy engagement, using simple heuristics to deal with a complex and ‘messy’ policy system.
Policy studies help us identify two such heuristics or simple strategies.
First, respond to policymaker psychology by adapting to the short cuts they use to gather enough information quickly: ‘rational’, via trusted sources of oral and written evidence, and ‘irrational’, via their beliefs, emotions, and habits. Policy theories describe many interest group or ‘advocacy coalition’ strategies, including a tendency to combine evidence with emotional appeals, romanticise their own cause and demonise their opponents, or tell simple emotional stories with a hero and moral to exploit the biases of their audience.
Second, adapt to complex ‘policy environments’ including: many policymakers at many levels and types of government, each with their own rules of evidence gathering, network formation, and ways of understanding policy problems and relevant socioeconomic conditions. For example, advocates of international treaties often find that the evidence-based arguments their international audience takes for granted become hotly contested at national or subnational levels (even if the national government is a signatory), while the same interest groups presenting the same evidence of a problem can be key insiders in one government department but ignored in another.
Learn the conditions under which evidence ‘wins the day’ in policymaking
Consequently, the availability and supply of scientific evidence, on the nature of problems and effectiveness of solutions, is a necessary but insufficient condition for evidence-informed policy. Three others must be met: actors use scientific evidence to persuade policymakers to pay attention to, and shift their understanding of, policy problems; the policy environment becomes broadly conducive to policy change; and, actors exploit attention to a problem, the availability of a feasible solution, and the motivation of policymakers, during a ‘window of opportunity’ to adopt specific policy instruments.10
Tobacco control represents a ‘best case’ example (box 1) from which we can draw key lessons for ecological and environmental policies, giving us a sense of perspective by highlighting the long term potential for major evidence-informed policy change. However, unlike their colleagues in public health, environmental scientists have not developed a clear sense of how to produce policy instruments that are technically and politically feasible, so the delivery of comparable policy change is not inevitable.
Box 1: Tobacco policy as a best case and cautionary tale of evidence-based policymaking
Tobacco policy is a key example – and useful comparator for ecological and environmental policies – since it represents a best case scenario and cautionary tale. On the one hand, the scientific evidence on the links between smoking, mortality, and preventable death forms the basis for modern tobacco control policy. Leading countries – and the World Health Organisation, which oversees the Framework Convention on Tobacco Control (FCTC) – frame tobacco use as a public health ‘epidemic’ and allow their health departments to take the policy lead. Health departments foster networks with public health and medical groups at the expense of the tobacco industry, and emphasise the socioeconomic conditions – reductions in (a) smoking prevalence, (b) opposition to tobacco control, and (c) economic benefits to tobacco – most supportive of tobacco control. This framing, and conducive policymaking environment, helps give policymakers the motive and opportunity to choose policy instruments, such as bans on smoking in public places, which would otherwise seem politically infeasible.
On the other hand, even in a small handful of leading countries such as the UK, it took twenty to thirty years to go from the supply of the evidence to a proportionate government response: from the early evidence on smoking in the 1950s prompting major changes from the 1980s, to the evidence on passive smoking in the 1980s prompting public bans from the 2000s onwards. In most countries, the production of a ‘comprehensive’ set of policy measures is not yet complete, even though most signed the FCTC.
Decide how far you’ll go to maximise the use of scientific evidence in policymaking
These insights help challenge the naïve position that, if policymaking can change to become less dysfunctional, scientists can be ‘honest brokers’ and expect policymakers to use their evidence quickly, routinely, and sincerely. Even in the best case scenario, evidence-informed change takes hard work, persistence, and decades to achieve.
Since policymaking will always appear ‘irrational’ and complex’, scientists need to think harder about their role, then choose to engage more effectively or accept their lack of influence.
To deal with ‘irrational’ policymakers, they should combine evidence with persuasion, simple stories, and emotional appeals, and frame their evidence to make the implications consistent with policymakers’ beliefs.
To deal with complex environments, they should engage for the long term to work out how to form alliances with influencers who share their beliefs, understand in which ‘venues’ authoritative decisions are made and carried out, the rules of information processing in those venues, and the ‘currency’ used by policymakers when they describe policy problems and feasible solutions. In other words, develop skills that do not come with scientific training, avoid waiting for others to share your scientific mindset or respect for scientific evidence, and plan for the likely eventuality that policymaking will never become ‘evidence based’.
This approach may be taken for granted in policy studies, but it raises uncomfortable dilemmas regarding how far scientists should go, to maximise the use of scientific evidence in policy, using persuasion and coalition-building.
These dilemmas are too frequently overshadowed by claims – more comforting to scientists – that politicians are to blame because they do not understand how to generate, analyse, and use the best evidence. Scientists may only become effective in politics if they apply the same critical analysis to themselves.