The historical roots and seminal research on health equity: a referenced publication year spectroscopy (RPYS) analysis

Health equity was a cornerstone of the Millennium Development Goals (MDGs) and a major goal in the Sustainable Development Goals (SDGs) [1]. It has been endorsed by many international and national agencies as an essential element in health system development [2‐4]. There is consensus in the international community that health is a fundamental human right [5], and health equity reflects social fairness and justice [6, 7]. Inequity in health can bring detrimental effects on economic development, social vibrancy, and national security [8].

The concept of health equity involves multiple dimensions. In the literature, “health disparity”, “health inequality” and “health inequity” are often used interchangeably [8]. They may measure differences in health system structure (e.g. availability of financial and human resources), health care delivery (e.g. accessibility and quality of care), or health care outcomes (e.g. individual and population health) across populations. Increasingly, researchers agree that health equity is a more appropriate term for defining health system development goals. This is because inherent biomedical differences in health exist among populations; but health is also determined by socioeconomic factors and the differences in health outcomes caused by these modifiable factors are unacceptable. Therefore, the allocation of health care resources and services should be prioritized toward the socioeconomically disadvantaged. These people are likely to suffer from more health problems and need more care. In health care, equity indicates “equal access to available care for equal need, equal utilization for equal need, and equal quality of care for all” [9]. In other words, health inequity measures needs-adjusted inequality in health [10, 11]. Clearly, the concept of health equity is primarily based on the value judgements of morality, justice and human right norms. The most concise and accessible definition of health inequity was articulated by Whitehead in the early 1990s and Braveman in 2006 as “the systematic, unnecessary, potentially avoidable differences in health or the major socially determined influences on health between groups of people who have different relative positions in social hierarchies according to wealth, power, or prestige, which can be shaped by policies” [11, 12]. Unfortunately, debates about what differences are unnecessary, avoidable, unfair and unjust will never disappear.

Intellectual history is the foundation for studying an area and is helpful for the researcher to understand the key concepts and comprehensive development process of a research area. It gives researchers an insightful perspective into what their work positions and how their work fits into the overall area. The term “health equality” first appeared in the article “Equality and Health” written by Meltsner in 1966 [13]. This article is commonly accepted as the first published health equity study in several bibliometric studies [14‐16], despite some differing personal opinions [17, 18]. However, as a multidisciplinary concept with a long history, the theoretical roots of the term “health equality” can be traced back far before the year of 1966 and has a far-reaching theoretical and practical history. Health equity-related studies had already emerged before the appearance of the terms. At present, the origins or intellectual roots of the concept of health equity is still unclear. A better understanding of the historical context of health equity will render a refreshing perspective on how it all started, in which direction the research field is moving, and what gaps in research need to be filled.

In this study, we investigated the historical roots of health equity studies through identifying the seminal works of relevant research and analyzing publication and referencing trends using the method of Reference Publication Year Spectroscopy (RPYS) proposed by Marx et al. in 2013 [19]. Such a study is necessary to help researchers develop a better understanding of the origin and evolutionary pathway of health equity studies, which is essential for an adequate interpretation of previous studies, the articulation of new research questions and the adaptation of methodological approaches.

We performed a RPYS [19] analysis on health equity studies. It quantified the impact of studies in the area of health equity by analyzing the cited references before and after the term(s) appeared [20]. RYPS assumes that a new research area usually evolves on the basis of previous discussions among researchers in the scientific community; and the relationship of current research to past literature plays a significant role, which is usually expressed in the form of citations [21]. The premise of citation theory is that the more frequently cited scientific publications contribute more to the advancement of knowledge [19]. A citation analysis provides insights into the historical context of science. It offers the opportunity for researchers to trace the origin of a new research area even before its concept and terms are formalized.

RPYS has been successfully used to detect the historical roots, milestone works and evolutionary pathways of a number of research areas, ranging from philosophy to medicine, computer science and climate change [19, 21‐32]. For example, RPYS analyses revealed that Garfield is one of the pioneers and most influential researchers in bibliometric studies, and his work was influenced by that of Shepard [32]. To the best of our knowledge, no RPYS studies have ever been conducted in the area of health equity research.

Meltsner published the first article using the term “health and equality” in 1966. The growth of health equity studies was slow initially, with far less than 100 relevant publications each year. Since 1990, studies on health equity started to accelerate. The annual volume of publications on health equity eventually exceeded 1000 in 2003 and continued to grow until reaching the current level of 7132 in 2017 (Fig. 1). From 1900 to 2018, the majority were published in public health (e.g. 38.56% on public environmental occupational health) and health services (e.g. 13.56% on health care services and 8.73% on general internal medicine) journals, followed by social science journals (e.g. 6.06% in biomedical social sciences and 2.99% on social sciences other topics).

The cited references of health equity studies can be traced back to 1801. The distribution of cited references by RPYs indicated a three-stage development in health equity studies (Fig. 2): origins (pre-1965), formative (1966–1990), and development and expansion (1991–2018). There were many small peaks in DoNCR in the origins stage, compared with a few major peaks occurring in the formative and development and expansion stages. In total, we identified 17 significant peaks with seminal works: 12 in the origins stage (Fig. 3), 2 in the formative stage (Fig. 4), and 3 in the development and expansion stage (Fig. 5).

Approximately 31 seminal works contributed to the 17 peaks (Table 1). Most of the seminal works were conducted by researchers in the US (19, 61.3%), the UK (7, 22.6%) and the Netherlands (3, 9.7%). The first seminal work conducted by Chadwick from Great Britain appeared in 1942, which revealed variations in life expectancy associated with social class. It signals the emergence of health equity studies. The ideology of health equity was eventually endorsed by the international society through the World Health Organization (WHO, 1946) declaration based on the foundational works of Chadwick (1842), Engels (1945), Durkheim (1897) and Du Bois (1899). Since then, the scope of health equity studies has been extended from a focus on mortality and morbidity to a view on health and wellbeing, from a wealth-related perspective to perspectives of a variety of social classifications (e.g. social stigma and discriminations), from quantitative descriptions to qualitative understanding of root causes, and from a national approach to an international approach. The WHO Commission on Social Determinants of Health (2005) provided a systematic framework for health equity studies.

This study detected the historical roots and seminal works of health equity using RPYS. A total of 17 pronounced peaks in RPYs were identified over the years from 1800 to 2018. These citation peaks are indicative of milestone works in research on health equity: 31 seminal studies. The historical evolutionary path in health equity studies is characterized by three distinctive stages: origins (1800–1965), formative (1966–1991), development and expansion (1991–2018).

The origins stage (1800–1965) is in line with the development of modern public health. Poor sanitation and living conditions of the poor and working class were proved to be associated with high mortality and poor population health, triggering governmental interventions through public health acts. These populations were deemed highly susceptible to infectious diseases in epidemiology, and if not managed properly, could impose serious health risks to the entire society. At this stage, there was not much discussion about unequal access to health care services. Instead, environmental and occupational health and safety for the entire society was the major focus. However, the “Constitution of the World Health Organization” (1946) and the preamble to the United Nations “Universal Declaration of Human Rights” (1948) signals a development in conceptualizing health equity [18]. Since the 1950s, research instruments for studying health equity have been developed, providing theoretical and methodological foundations for research into health equity.

In the formative stage (1966–1990), the terms “health disparity”, “health equality” and “health equity” were formally defined. Sociological studies made a significant contribution to the development of health equity research. This was driven, at least partly, by the grounded theory proposed by Glaser et al. (1967). This stage also features the wide acceptance of social class theories and their use in epidemiological studies. However, the lack of a specified sub-discipline and relevant research methods restricted the growth of health equity studies.

In the third stage (1991–2018), social epidemiology was established with a specific mission to study health equity issues. The concept of health equity was further clarified in the early 1990s [15, 16]. Researchers started to examine health equity issues from national and international perspectives based on the theory of health systems. Driven by the WHO and the World Bank, a consensus on the measurements of the social determinants of health and the methods for decomposing the contributions of socioeconomic factors on health equity was reached. This contributed to a rapid growth in health equity studies. The WHO proposed a global goal for achieving health equity through universal health coverage.

Overall, health equity studies have made a significant contribution in driving the global agenda of “health for all and health in all” [61, 62]. A systems approach beyond the health sector is recognized as essential for achieving health equity [63]. Researchers from the UK, the US, and the Netherlands led the development and growth of research into health equity. This finding is consistent with previous studies [15, 64]. The WHO was the leading agency driving the agenda of health system development with a strong focus on health equity. The efforts of the WHO were supported by its member countries, the World Bank, regional organizations such as the OECD and the European Union, and many non-governmental organizations (NGOs). However, scant studies have been conducted from a political perspective [65, 66], although actions on the social determinants of health are often intensely political [67]. Some researchers have called for increasing attention on socio-political analyses of health equity studies, which will not only offer novel theoretical insights but also methodological innovations [68, 69].

Significant progress has been achieved in measuring health equity [55, 56, 66, 70‐75] and decomposing contributions of various determinants of health inequity [76‐82]. But there is a lack of understanding of the pathways and mechanisms of how the social determinants of health lead to health inequalities. The existing studies are criticized for a bias towards socioeconomic factors (e.g. income level, social class, educational achievement, and occupational status), midstream factors (e.g. biology, health-related behaviors, environment, and health benefits) and downstream factors (e.g. healthcare services access and use factors) [67]. This has limited the usefulness of health equity studies in political and policy decision making [83‐85]. Recently, several theoretical frameworks have been proposed to facilitate research into the causal pathways between upstream social determinants of health (e.g. material, social, political, and cultural circumstances) and health inequity [86‐88], such as social disadvantage and life course approaches [89, 90]. The focus of health equity studies has been extended to concerns of a much broader range of socioeconomic and upstream factors, such as poor health services and health outcomes experienced by the sexual minorities [91‐93] and those with disabilities and mental health problems [66, 94] as a results of victimization and discrimination compounded by stigma [95, 96]. However, there is a lack of understanding about how these factors interact with other socioeconomic factors [96]. Dover and Belon developed a framework integrating the existing social determinants of health framework and the health services utilization framework [97]. It is expected that more studies will emerge addressing health equity concerns from an economic perspective [98‐100]. Despite the progress, health equity studies remain a challenge due to the long and complex causal pathways linking individual and social factors to health and our limited ability to study this topic using randomized control trials [89].

The historical origins of health equity research can be traced back to the early 1800s and is embedded in the development of public health. It has become a major focus of the sub-discipline of social epidemiology, involving a set of measurements and methodological approaches. Sociological studies made a significant contribution to the development of social epidemiology. The concept and principles of heath equity have now been widely accepted in the international community thanks to the efforts of the WHO. Health equity studies are moving from evidence gathering to the identification of cost-effective policies and governmental interventions. This may lead to the emergence of another sub-discipline “Political Epidemiology” as suggested by Pega and Kawachi from Harvard University [101]. New frameworks and analytic approaches are needed to investigate the causal pathways between the social determinants of health and health inequalities [88, 97].

There are several limitations in this study. Firstly, this RPYS study relied on the WoS collection, similar to previous RPYS studies. The WoS database covers high quality journals. But it is biased toward publications in the English language. However, the RPYS method has a focus on references, which expands the coverage of publications and may also reduce the language bias. Secondly, the RPYS does not analyze citation relationships and networks. Future studies should consider the use of co-citation analyses, bibliographic coupling, and direct citations in consideration of research fronts [102].