“Legal epidemiology” is the scientific study and deployment of law as a factor in the cause, distribution, and prevention of disease and injury in a population.1 Its emergence as a distinct field reflects the indispensability of law to modern public health practice.2–4 Proponents of the field aim to remove 2 persistent barriers to the effective use of legal action for public health: the limited extent of rigorous and timely evaluation of the impact of law and legal practices on health5,6; and the inattention in training and practice to the important legal functions played by nonlawyers in the health system.7,8 In the authors' assessment, the research necessary to identify and spread best legal practices is too often never carried out. Legal interventions affecting millions of Americans are often not evaluated for years, if at all. Innovations that show promise in research or practice are sometimes not scaled, so they either do not spread or spread too slowly. The unintended (or incidental) effects of laws on population health often remain unidentified and unexplored.2,9 Limited professional training in law, disciplinary boundaries, and, arguably, a cultural tension between law and other health disciplines continue to limit the full integration of law into public health.5–8,10 The publication of this special supplement of JPHMP is an opportune time to take stock. The purpose of this commentary is to describe the emergence of legal epidemiology, its key methods and tools, and the challenges it faces going forward.
The Emergence of Legal Epidemiology
Rigorous research on the health effects of legal interventions goes back at least 50 years.3 The field of public health law—as a practice of lawyers specializing in how law authorizes, requires, and limits public health work—goes back even further, to the latter part of the 19th century.11 Despite their shared focus on law, these 2 strands of public health law work proceeded largely independently of one another. Public health lawyers were usually not trained or supported to participate in empirical research, and scientists who studied the health effects of laws empirically often lacked legal expertise or a sense that they were part of a “field” of public health law research.6
What we are now calling the field of legal epidemiology is the product of decades of independent work of many researchers and practitioners.3 Based on their many years of involvement and experience in legal epidemiology, the authors consider 2 institutional developments to have been particularly helpful to the ultimate emergence of this distinct field. The first was the founding of the Public Health Law Program (PHLP) at the Centers for Disease Control and Prevention (CDC) in 2000.12 PHLP's early work included several functions that would prove to be important models for the development of a more comprehensive understanding of public health law: providing grant funding for empirical research on the effects of law13 and conducting such research intramurally14; supporting an external collaborating center—the Center for Law and the Public's Health at Georgetown and Johns Hopkins universities—to provide legal expertise on matters such as emergency powers15; and providing legal technical assistance to support state, tribal, and local health departments in understanding the opportunities, limitations, and barriers to using law to address health.12 Today, PHLP works to institutionalize legal epidemiology within CDC and to foster an ethic under which legal research is as rigorous and transparent as CDC's other research on public health issues.16 In recent years, PHLP has administered a new cooperative agreement with ChangeLab Solutions focused on public health law, developed competency models for public health law practice17 and legal epidemiology,18 and supported the Public Health Law Academy19 and the STLT Legal Epidemiology Project, all of which aim to build the internal legal epidemiology capacity of state, tribal, local, and territorial health departments.
The second important institutional development for legal epidemiology was the 2009 founding of the Public Health Law Research (PHLR) program by the Robert Wood Johnson Foundation (RWJF).6,20 PHLR's mission was to support more frequent and rigorous empirical study of law as a factor in public health through research funding and the development of helpful methods and tools. Over the next 6 years, PHLR funded more than 80 research grants, produced a textbook on public health law research methods,21 and advanced the emerging practice of policy surveillance.22,23
Both PHLR and PHLP were part of an informal network of organizations working in various facets of public health law, including the Network for Public Health Law, ChangeLab Solutions, and the Public Health Law Center. Like PHLR, these organizations were funded in whole or in part by RWJF, which also supported a staffer to coordinate with the Association of State and Territorial Health Officials, the National Association of County and City Health Officials, and the American Public Health Association.7,24,25 Participants from these organizations shared the vision of integrating the empirical and legal strands of public health law and highlighting the law-related work of nonlawyer public health practitioners and researchers in the field. The idea that public health law should be modeled as a “transdisciplinary” field integrating the professional work and tools of lawyers and the research, policy development, implementation, and diffusion work routinely carried out by nonlawyers was informed by a decade of collaborative work within this network. The former was labeled “public health law practice,” and the latter came to be called “legal epidemiology.”10 The concept of transdisciplinarity was chosen for its emphasis on the true integration of disciplines at the level of theory, methods, and conceptual tools.26
The transdisciplinary model depicted public health law practice and legal epidemiology as equally important facets of a single field.10 In subsequent work, the collaborators specified 5 “essential public health law services” (5EPHLS) that help define what the public health law field does.7 The 5EPHLS took the transdisciplinary model and, in the mode of the 10 Essential Public Health Services,27 put it into the form of a set of observable, measurable, and improvable services that are “necessary components of a strategic effort that can enhance the chances that lifesaving legal reforms can be devised, tested, and spread as quickly as possible.”7(p751) We are hopeful that the transdisciplinary model and the 5EPHLS will bring greater clarity about the field to those who have already placed themselves within it and also help expand awareness of legal epidemiology among the many researchers, practitioners, educators, and funders who have been tacitly engaged in important public health law work without necessarily thinking of it in terms of law or legal epidemiology.2,28,29
The Methods of Legal Epidemiology
The development of legal epidemiology as a field has included work on research methods and tools. In particular, methodologists and practitioners in legal epidemiology have sought to enable and highlight the potential for strong causal inference in well-designed observational studies, the importance of using explicit theory in legal epidemiology research, and the use of scientific methods for measuring the features of law as they vary over time and space.21,30–32
Strengthening (and appreciating) causal inference in observational research
Although law is sometimes amenable to pure experimental testing,33 most studies of laws' health effects are observational simply because researchers cannot control the exposure. However, observational researchers in legal epidemiology have been able to take advantage of the way law typically unfolds as a natural experiment.34 The United States is a federal system with considerable legal authority vested not just in states but also in tribal, local, and territorial governments. Laws are enacted and go into effect at specific—and differing—times, in different places, and often with requirements that vary in important ways. Studies that combine multiple design elements can produce evaluations of real-world legal interventions with overall levels of validity and strength of causal inference that are comparable with or even superior to randomized trials, which are typically undertaken within artificial environments.34 Comparing multiple affected subgroups (such as people covered by a new law and people exempted from it), repeating measures over time, testing multiple outcomes, and comparing jurisdictions with different laws can all strengthen causal inference.34 General advances in methods that strengthen causal inference, through such tools as difference-in-differences analysis and synthetic controls, have spread widely in the field and have added to the potential of quasi-experimental studies.35
Explicit use of theory
Observational methods that support credible causal inference are central to legal epidemiology, but the field's roots in sociolegal studies on one hand and epidemiology on the other also encourage a commitment to theory. Causal inference in any observational study can and should be informed by a theory as to the mechanism of effect.36 In legal epidemiology, the mechanism is the process through which a law is enforced, influences behavior or environments, or is internalized as a norm. Theory is also essential to strong design in the first place. In studies of legal effects, the researcher's choice of theory influences what laws to study and what specific features of laws to measure for analysis. The theorized mechanism of effect can point to measures of implementation, the likely timing or patterning of effects, and the kinds of effects to look for in the behavior of targeted individuals or changes in the targeted environments.21
As a transdisciplinary field, legal epidemiology does not rely solely on either sociolegal or epidemiological theories but rather encourages the use of any well-established behavioral or structural theory that is suited to generating hypotheses as to a law's mechanisms and effects.37 For example, a study of safety belt law could use economic or deterrence theory to hypothesize that compliance would rise as enforcement increased and decline as enforcement is relaxed. But the study could instead (or also) draw on the theory of planned behavior from psychology to investigate how enforcement and changes in social behavior could lead to an enduring and self-enforcing social norm that produces stable safety belt use even after enforcement declines. These theories not only generate hypotheses as to how the law works but also suggest measures (such as changing perceptions of safety belt norms) and the expected temporal distribution of effects that can strengthen study design.
Scientific legal mapping
The most distinctive contribution of legal epidemiology has been the formalization of methods, and the development of software, for measuring law itself. Since at least the 1990s, a small cadre of researchers had been developing and practicing rigorous, transparent, and reproducible methods for capturing key features of laws in a form that could be used for multijurisdictional comparative and longitudinal evaluation.38,39 Guided by Alexander Wagenaar, one of the creators of the Alcohol Policy Information System (APIS), and taking up a term proffered by other public health law research innovators such as Jamie Chriqui, PHLR commissioned Charles Tremper and colleagues to set down explicit methods for “policy surveillance,” defined as the ongoing, systematic collection, analysis, and dissemination of policies across jurisdictions and over time.22,23,40 The PHLR team then developed software purpose-built for efficiently capturing and publishing the observable features of law as numeric data.41,42 The software has been used to develop rigorous and freely available legal data sets that enable and reduce the cost of research on legal implementation and effects. Policy surveillance data from APIS, LawAtlas, and the Prescription Drug Abuse Policy System (PDAPS) have been used in more than 400 research articles.42–44 While methodologists still point to problems with legal measurement in research,30,45 we have every reason to hope for steady improvement as the training and practice of legal epidemiology spread.46 In addition to easing public and professional access to public health law, well-designed legal information campaigns using policy surveillance data can expedite the diffusion of healthy public policies, exemplified in the CityHealth project, which uses granular policy legal data to rank 40 cities by the adoption and strength of 9 health policies.47
At the age of 10 years, legal epidemiology has a distinct place within transdisciplinary public health law in which practitioners in the field have explicit roles in developing, implementing, and evaluating law and its impact on public health. With its roots supported by tools and educational materials, and a rising profile in public health globally, we now turn to the future of its adolescence.
The Future Development of an Evolving Field
The principal challenge facing legal epidemiology in the next 10 years is to become more firmly embedded within public health education, practice, and research. Our experience suggests that several developments could help: more specific instruction in public health law in public health education; more legal epidemiology training and technical assistance for public health practitioners; and greater and more systematic support for research on the implementation and health effects of law. Continued efforts to strengthen public health law education and research in law schools would also be useful.
Exposure to legal epidemiology starts with primary public health and legal education.48 More than 15 years ago, an Institute of Medicine committee looking at public health education noted the lack of strong content in law and policy.8 It is not clear that this problem has been solved, and the emergence of legal epidemiology has added some complexity. It has been, in our opinion, conventional to treat law and policy either as a single concept or as fungible.30 Legal epidemiology has questioned this, pointing out that law is not just one means of specifying a desirable practice but also the primary social tool for generalizing or scaling up practices judged collectively beneficial, forestalling negative behavior, and setting powers, duties, and limitations on public and private entities.11,30 As captured in the 5EPHLS, effective work with law in public health is built on skills that future health professionals can be taught.7,48 Current standards issued by the Council on Education for Public Health (CEPH) require education in policy development, advocacy, and evaluation, but the word “law” is entirely absent from the document's training requirements.49 Proponents of legal epidemiology would benefit from working with CEPH to help build out legal studies within public health education. Success in meeting this challenge will be measured in the number of schools that make not just health policy but also public health law standard courses for all undergraduate and master of public health (MPH) students.
We would also like to see legal epidemiology built out and institutionalized within the current ranks of public health professionals. General awareness of law's importance in health is always useful2 but does not go far enough. Legal epidemiology emphasizes the development of greater professional “ownership” of legal functions among the nonlawyers who are in the position to carry out the essential public health law services such as policy development, implementation, and evaluation.7,50 Enunciating legal competencies18 and offering continuing education such as the Public Health Law Academy, can help at the individual level. The Public Health Accreditation Board (PHAB), which has convened a working group to consider better incorporating the 5EPHLS into accreditation standards, is offering a great example for systems-level change.51
Any discussion of strengthening research in legal epidemiology starts with recognizing the important work already being done. In areas as diverse as gun control,52 obesity,53 tobacco regulation,54 and vaccination,55 researchers have produced important evidence to guide policy making and implementation. Studies have not only explored the impact of particular legal interventions56,57 but also investigated the effects of legal infrastructure on public health practice58 and advanced understanding of how laws not intended to influence health can have powerful incidental effects.59,60 A scan of publications by CDC authors identified 185 legal epidemiology articles published in 83 different journals between January 1, 2011, and May 31, 2015.16 The addition in fall 2019 of a National Library of Medicine MeSH term for “legal epidemiology” is a validation of the field and will make it easier to track its growth in the future.
The glass of legal research studies is thus far from empty, but even an optimist would be hard-pressed to say it was half-full. Many important domains where laws impact health—housing, for example61—either lack research or have research that is limited in scope or design.20,30,62 A study of National Institutes of Health funding between 1985 and 2015 identified only 510 awards that focused on health law-making or the health effects of laws and/or its enforcement.5 Funding levels both reflect and influence researcher interest in studying law, appreciation of the importance of legal studies, access to necessary data, and other practical and cultural factors that all need to be better understood and tackled for the field to progress.63
The field would benefit from determining why legal epidemiology research is not more frequently funded, and taking action as appropriate to address those causes.7,64 It is also important to make the research more strategic and systematic. Sometimes policy makers require and even fund evaluations of the laws they pass, but this is rare. If we recognize that laws are “treatments” to which thousands or millions of people are exposed, conducting research on the impacts and side effects is important. We have advance notice that new laws will be coming into effect, which makes it possible to plan and implement timely studies of implementation and impact. People who believe that legal epidemiology is important will have to continue to make the case for integrating early evaluation of impact in the policy development and diffusion cycle.
Law schools, which train lawyers for public health law practice, also have a role to play in legal epidemiology. Although it remains a niche legal specialty, public health law has grown as a legal field over the past 3 decades.65 Law schools train future public health lawyers, and they can also help deepen the professional talent pool through joint JD-MPH programs and by offering public health law and legal epidemiology courses that bring together law and public health students in one course.48 In a transdisciplinary field, it is not expected that lawyers become scientists, any more than that scientists become lawyers. Rather, the ideal is for lawyers to have a sufficient grasp of legal epidemiology to effectively cooperate across the 5EPHLS.
The emergence of legal epidemiology is steadily enhancing understanding and deployment of law as a core element of public health. The field encompasses theories, conceptual frameworks, methods, and software tools that make legal work in public health more feasible, efficient, and accessible. The field speaks to—and makes demands on—the full range of public health professionals, researchers, and teachers. It points toward the opportunity to more rigorously and systematically evaluate law's health effects—and the costs of failing to do so.66 To thrive over the next decade, legal epidemiology will need to reach more public health professionals in training and broaden its base of institutional support. And, of course, more individuals such as those who have contributed to this issue can advance the field by adding new ideas and impetus to its methods and models.
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