Income Inequality in Relation to Policies, Taxes, and Gender: A Bibliometric Analysis

The main contribution of this study is to fill a knowledge gap related to the institutional, country, and author concentration of income inequality research and its implication for the development of public policies, taxation plans, and opportunities for males and females. Technological rationality is considered a factor with the potential to lead human reality. This is in terms of facilitating a just advantage of human capital over financial capital and real estate. Advancing human capital is not the role of just one tier or one gender in any society. Individuals from all spheres and gender can be capable managers, stockholders, and policymakers and develop the skill to reduce nepotism. In so doing, economic and societal inequalities would be reduced as well as economic and democratic rationality.^[1–3] Economic and demographic rationality encompasses how an individual gets the most out of a given situation based on the individual’s own needs, wants, and desires.^[4]

Income inequality is a direct result of both economic and demographic rationality since rich countries remain rich and developing ones remain poor. However, when analysing this problem using a gender approach it is observed that more men have greater economic affluence when compared to women worldwide.^[5–7] Income inequality is defined as an indicator of how material resources are distributed across the general society. That is a significant reason for causing conflict, limiting cooperation, or creating psychological and physical health stresses.^[8,9] Studying economic inequality and inequity is essential because such inequalities are embedded in social justice and economic efficiency theories. According to Amarante et al.,^[10] the concerns about inequality must be examined from a social justice point of view, inequality of opportunities based on gender and other metrics, and inequality of income and outcomes. Previous bibliometric studies of economic inequality in the last decade revealed that several economists and sociologists had approached the topic by analyzing the causes of rising wealth inequality.^[11,12]

Furthermore, the role of life-cycle wealth and inherited wealth using theoretical and non-theoretical models has been examined. It was found that a solid correlational and causal relationship exists between the unequal distribution of economic resources and economic growth. These results were found by carrying out a bibliometric review of 146 papers (both theoretical and empirical) that studied the subject using a clusterization method based on keywords such as wealth holders, empirically grounded models, wealth transfers, economic growth, and wealth inequality. It is worth highlighting that an empirical exercise common among these studies is a correlational analysis on per capital GDP level, primary school enrollment ratio, as well as the Gini coefficient of 70 countries that include democracies and non-democracies, including United States, United Kingdom, Australia, Italy, Netherlands, Spain, among others economies.^[13]

In the work of Crespo et al.,^[14] it is discussed that income inequality was due to differences in circumstances and possessions of or lacking opportunities in a given society. They also indicated that the attempt to compensate for societal inequalities is lacking in the literature. However, the work of Crespo et al.,^[14] provides an exciting and relevant view of the subject. More scientific research needs to be conducted-regarding economic inequality and its relationship to public policies, tax structures, and gender.

Concerning the evolution of studies regarding the distribution of income, research that includes empirical income distribution in a community reveals that wealthy families have a certain degree of incentive to isolate themselves from the rest of the economy in order to provide the highest level of education for their children at the lowest cost, causing low-income families to be isolated from the rest of the population, thus inducing persistent or permanent poverty for sectors of the population being unable to generate sufficient human capital investment in their children to escape from low-paying occupations.^[15]

In contrast, the theory of inequality measurement from Pigou– Dalton states that if wealth is transferred from a more affluent person to a poorer person, without reversing their ranks, inequality goes down, and of course, a transfer in the opposite direction makes inequality go up;^[16] thus, in a capitalist democracy, as all individuals are formally equal and all institutions are open to everyone, the highest classes have to enter the proverbial competition for scarce resources and the upward mobility for the lower classes is possible, since, in a formally democratic society, a high social position is based on some achievement legitimized by merit.^[17]

In that sense, Atkinson,as cited in Wernerová,^[18] proposes that the notion of a good society is incompatible with the current level of economic inequality since this is progressive and tries to show ways to help reduce inequality by analyzing distribution problems, appealing to government officials to take action and lead to the regulation of economically negative influences such as unemployment, poverty, globalization;^[18] other authors such as Lenski’s and Wallerstein’s were primarily interested in how major shifts in the mode of production produce changes in the macrostructure, considering the societal or international distribution of re- sources-usually over relatively long periods.^[19]

Given the level of discussion related to the evolution of the subject, the present bibliometric analysis seeks to address this need and is guided by the following four pertinent Research Questions (RQ) and six Research Goals (RG).

Our bibliometric study approach relates to income inequality considering the following RQs and RGs:

–RQ1: How many articles addressing economic inequality were produced between 1961-2021?

RG1: Assess the general productivity of articles/ scholars per country, including a diachronic analysis of papers published since the first mention in the literature (1961–2021)
RG2: Breakdown the geographic analysis of income inequality at the country, institutional, and level of journal

–RQ2: What are the most relevant keywords associated with income inequality?

RG3: Conduct an analysis of publications by subject area to unearth income inequality in the literature between 1961–2021
RG4: Present the research approaches by keywords found in the literature.

–RQ3: Who are the most influential authors in income inequality research found in the literature between 1961 – 2021?

RG5: Determine the influence structure by citation (authors), based on the number of citations.

–RQ4: What are the characteristics of the publications on income inequality of the most productive nations with emphasis on the economic context? (policies, tax structure, and gender).

RG6: Assess the information showing a descriptive analysis of income equality in terms of (policies, tax structure, and gender breakdown) in the most productive countries.

To answer the four RQs, we propose the following six RGs. Each research goal is designed to furnish additional answers to the four RQs posed. The model below in Figure 1 shows the conceptual framework for the RQs to the RGs.

Figure 1:
Model of Research Questions and Research Goals to Understand Income Inequalities.

As shown in Figure 1, there are four primary RQs and six RGs. This is because each research question was based on the purpose of the research. Therefore, research questions 1 and 2 have two RGs to answer the research questions fully. In addition, RQs 3 and 4 have one RG each, to dig deeper into the analysis to answer the RQs posed.

This paper aims to provide a general overview of research conducted between 1961-2021 addressing income inequality. Specifically, bibliometric analysis techniques of performance analysis and science mapping are used to analyze the three themes of policy structures, taxation, and gender breakdown that impact income equality. This is in line with the seminal work of Donthu et al.,^[20] These themes will be further evaluated by eximining into the relevant factors: general productivity, research approaches, and influence structure at a country, institution, author, and journal level. This approach is diagrammed in Figure 2.

Figure 2:
Bibliometric Analysis Linked to the Relevant Themes and Factors of Income Inequality.

As shown in Figure 2, to analyze income inequalities, the independent variable, we posed four RQs and six RGs. The independent variables of policy structures, taxation, and gender breakdown, helped to facilitate the analysis. The dependent variables, general productivity, research approaches, and influence structure at a country, institution, author, and related journal level were evaluative themes and factors selected further to provide a broad analysis of income inequality. This is one way this research adds value to the body of research by examining into the themes and factors. The outlined themes and factors are thoroughly discussed in subsequent sections of this paper. The rest of this manuscript is structured as follows: Section 1: Methods— explaining the methodology of the bibliometric study; Section 2: Selection of the databases and keywords; Section 3: Data analysis— using bibliometric techniques showing both relationships and evaluative powers. Section 4: Results section—presenting the outcomes of the bibliometric analysis—descriptive data regarding variables and levels related to the research problem to provide objective and verifiable answers to the research questions. Section 5: Summary of the main findings and conclusions of the paper. Finally, implications for policy and practice and further research are advanced at the end of the manuscript.

This paper aims to provide a detailed overview of research conducted between 1961-2021 addressing income inequality. Specifically, bibliometric analysis techniques of performance analysis and science mapping are used to analyze the three themes of policy structures, taxation, and gender barriers that impact income equality. The bibliometric methodology has been postulated and used successfully by seminal researchers such as Donthu et al.,^[20] Jain et al.,^[21] and Buhren et al.,^[22] In addition, the bibliometric methodology has gained traction over the years to analyze large databases that would be too cumbersome and impractical to be analyzed by hand Donthu et al.,^[20]

This bibliometric analysis of the literature addressing income inequality evaluated the structural relationships between and among research constituents, including the metrics, authors, countries, institutions, and related topics. In addition, performance analysis and science mapping were conducted as we examined income inequality. These are two techniques recommended by Donthu et al.,^[20] for this type of research to be valid, reliable, and relevant.

On the other hand, the World Inequality Database (WID) houses extensive data on the evolution and distribution of income, and wealth, globally, both inside and between countries, combining different data sources available (such as financial data, survey data, and national accounts) in a systematic way.^[23]

Bibliometric analysis is a form of scientific communication that allows researchers to track the progression in the field of science, identify trends in research topics across disciplines, and learn how scientists develop knowledge and disseminate findings. Bibliometric studies can also be used for knowledge evolution (Amate-Fortes et al.,^[24] Donthu et al.,^[20] and to obtain an overview of the state of research in various scientific disciplines that have increased significantly since the evolution of information technologies and the emergence of the Internet, the development of various specialized tools and software, the availability of data/information online, have increasingly facilitated the systematization of information and has boosted the development and publication of these studies.^[25]

Some of the questions that bibliometric studies can answer relate to issues such as mapping vast amounts of available research that describe developmental trends and status in a comprehensive, systematic, and replicable manner. This is based on a set of quantitative methods used to measure and assess scientific research at the journal, country, region, institution, and author levels.^[26] In this sense, the main contribution of the present study is to fill the gap by identifying, synthesizing, and evaluating extant research on income inequality.

The bibliometric review leads to a comprehensive view of the general productivity, research approaches, and influence structure of the research field related to income inequality to answer four RQs. The analysis carried out in the present work took into consideration descriptive quantitative bibliometric indicators (Donthu et al.,^[20] addressing income inequality. The process followed is explained to achieve the RGs and answer the four RQs. (1) how keywords were selected, (2) types of documents considered, (3) the most important indicators, (4) the specific analysis for bibliometric review, and (5) ending with the discussion regarding the findings and research proposal for future research.

The data were collected from Scopus and the World Inequality databases. Scopus is a source-neutral abstract and citation database curated by independent subject matter experts, with tools that generate precise citation search results and automatically update researcher profiles. Currently, the Scopus database includes more than 75 million records, with 68 million post-1970 records. These records include more than 8.5 million Open Access articles, 23,500 peer-reviewed journals, 740 book series, 300 trade publications, articles in press from over 8,000 titles, and many references. Records are obtained from national and international publishers, focusing on the (1) arts and humanities, (2) business, (3) law, (4) medicine, (5) science, (6) social sciences technology, and (7) other subject areas.

Additionally, the data retrieved from WID included the frequency of nominal variables based on the name of specific documents that are input for the World Inequality Lab Dataset.^[7]

The WID focuses on the evolution and the distribution of income, and wealth, globally, both inside and between countries, combining different data sources (such as financial data, survey data, and national accounts) systematically.^[23]

The sample of literature reviewed on income inequality spanned the years between 1961-2021. From an analysis of the Scopus database, we present the most relevant and influential articles between 1961 – 2021. The Scopus database was selected based on three criteria (1) vast array and comprehensiveness of digital records, (2) the frequency with which the Scopus data is updated in real-time, and (3) robust ability of the database to debug and process data using Boolean terms (Jain et al.,^[21] Mugomeri et al.,). ^[27,28] There were articles excluded based on the key terms selected. This is in aligned with the work of Transfield et al.,^[31] According to Jain et al.,^[21] the first step in running queries in bibliographic databases is the identification of the relevant keywords. In this research, using the Scopus database, the four primary keyword searches are summarized in Table 1.

As seen in Table 1, the first keyword algorithm entered was (“income inequality”) yielding over eleven thousand papers; second, (“income inequality” policies), yielding over three thousand papers; third (“income inequality” taxes) yielding close to one thousand papers; fourth we entered (“income inequality” gender) yielding close to six hundred papers. For each keyword search, the parameter “ALL” was selected as the document type to ensure an objective comparison among all papers available. In addition, using the option “show all abstracts” and the sorting option; “cited by highest” were all in place to get the desired results. With the appropriate key words algorithm, the data analysis would be trustworthy.

According to Kulaki,^[32] “Bibliometric is defined as the use of statistical methods to analyze the bibliometric publications data such as peer-reviewed journal articles, books, conference proceedings, periodicals, reviews, reports, and related documents. It has been widely used to present the relations of research domains with quantitative methods.” This present research employs this definition to deepen readers’ understanding of income inequality. Various studies have used different software packages such as Bibliometrix R, CiteSpace, BibExcel, and VOSviewer. In this research, we utilized BibExcel and VOSviewer due to the access to these programs and their ability to manage and analyze the data uploaded from Scopus. This is consistent with the works of Persson et al.,^[33] and Jain et al.,^[21] The data analysis considered three independent variables: general productivity, research approaches, and influence structures. These independent variables help provide an analytical and descriptive view of the evolution of the research articles addressing “income inequality” and are represented in Table 2.

Predicated on the research design presented in Table 2, the results show an overview of the main aspects relevant to the evolution of the keyword “income inequality” in terms of general productivity, research approaches, and influence structure at the country, institution, journal, author, and keyword level. Additionally, the results of the bibliometric analysis provide a descriptive comparison that considers the combination of the terms “income inequality” with “policies,” “taxes,” and “gender” (see Table 1), ensuring a worthwhile Ck concentration.

To understand better the level of concentration of productivity in terms of documents published at the country, institution, and journal levels, the Ck concentration index was used. This is an index that presents a sum of the shares in publication given a “K” largest institution, journals or countries in this scientific field, calculated as expressed in formula 1.^[34]

Formula 1: Ck concentration index

Source: Adapted from.^[34]

Where K represents the number of documents published in a journal, country, or institution related to the scientific field.

Using the Ck concentration index, a description of the relevant concentration in the production and citation of a specific subject in regions, institutions, and journals is provided.

The way to interpret the index is as follows: Ck < 33% = low concentration; 34% < Ck < 67% = moderate concentration; Ck > 68% = high concentration. In the next section, the results of total research productivity by country, using the Ck concentration index as the method of analysis is presented.

In this section, the RQs along with the RGs will be answered. Then, the results are presented based on each research question.

General Productivity of Articles Addressing “Income Inequality”

–RQ1: How many articles addressing economic inequality were produced between 1961-2021?

RG1: Assess the general productivity of articles/scholars per country, including a diachronic analysis of papers published since the first mention in the literature (1961 – 2021).
RG2: Break down the geographic analysis of income inequality at the country, institutional, and journal levels.

To answer RQ1, the descriptive analysis of “income inequality” was used as the base topic that leads to a more holistic understanding of scientific production and general contribution to scientific research. The results indicate a diachronic perspective of the number of documents published since the concept was mentioned in the available literature.

Regarding papers published since the first mention in the literature (1961 – 2021), it is relevant to highlight the positive tendency represented in the slope shown in Figure 3.

Figure 3:
Number of Documents Published that Include “Income Inequality”.

As seen in Figure 3, the production of scientific research increased steadily over time. The first article recorded from the Scopus Database was in 1961. By 2021, the number of articles grew exponentially by over 1000%. The analysis yielded 1,071 related articles addressing income inequality. Noteworthy is that in the year 2014, the production of scientific research on “income inequality” surpassed 500 publications, reaching 532 related articles. In the next section, an examination of the productivity of articles by country will be presented.

Concerning article productivity per country (see Table 3), it is evident from the analysis that the most productive countries in terms of scientific contribution related to “income inequality” are grouped by subject reflecting concentration index at 3, 7, and 10 degrees. Table 3 presents the 20 most productive nations where research was conducted on income inequality.

As seen in Table 3, it is evident that 80% of the productivity is concentrated in 10 prolific countries. These countries are the United States (3,883), United Kingdom (1,251), China (691), Germany (645), Canada (564), Australia (492), Italy (432), Spain (414), Netherlands (344), and France (338). It is worth highlighting that 46% of “income inequality” research is siloed in two wealthy developed North American countries, (1) The United States and (2) Canada. Why is there such a concentration of articles addressing income inequality in these two countries? The geographic analysis will provide the answer to this question.

This section presents a geographic analysis at the country level, providing a more in-depth analysis of the productivity in the top 10 most productive countries over the last 60 years (1961 – 2021). A country-level analysis during this period will provide a better understanding of the most prolific countries in this research regarding income inequality, as shown in Table 4.

As shown in Table 4, there was a total of 3,885 papers addressing income inequality. Each decade saw a progressive increase of articles globally. For example, between 1961-1971, there were a total of six articles; by 1981, this number was 55; 1991 saw this number more than doubling to 141; In 2001, there were 392 articles; in 2011, this number more than tripled to 1,056. In 2021, the USA produced 2,235 articles. To better understand the data presented in Figure 4 and further substantiate the findings, a line graph that reflects the differences in productivity in the most productive countries from (1961 – 2021) is presented.

Figure 4:
Performance of Scientific Research Productivity per Country 1961-2021.

Figure 4 shows a marked difference in the production of the articles from the United States in 2021 compared to the other countries, considering both the times series and the last record (296 publications compared with 133 for China, the second best). To enrich this information, it is essential to augment the presented data by including information disaggregated at the institutional level, country of origin, and overall productivity with the corresponding Ck level.

From the application of the concentration index to the data, it is evident that in the most productive institutions, the results show that top-tier academic or not-for-profit institutions generated the majority of “income inequality” research. Table 5 shows the concentration of “income inequality” research segregated by the institution’s name and the documents by affiliation based on the articles published in income inequality in the Scopus database.^[35]

In Table 5, it is evident that more than 50% of the total publications available were concentrated in 8 institutions presenting a Ck (8) with 53%, which is an important indicator of concentration in income inequality research. The analysis of journal level will be discussed in the section that follows.

In terms of the leading journals, the list of the ten most productive journals is represented by scientific publications specialized in the related subject areas such as social sciences, income and wealth, development, applied economies, and modeling. Table 6 shows the total production of documents and the accumulated percentage (%A*).

From Table 6, it is observed that an index of Ck (4) of around 50% of productivity (Social Science and Medicine, Review of Income and Wealth, Social Indicators Research and World Development journals) is clustered in three top-tier journals. (1) Social Science and Medicine, 20%; (2) Review of Income and Wealth, 14%; (3) Social Indicators Research, 13%. This is a revealing, relevant concentration level of article production related to income inequality.

In summary, the information included in this section contributes to understanding the general productivity relating to RQ1. Additionally, from the analysis of data, the evolution of the research field “income inequality” is characterized and evidenced by a steady increase over time, growing exponentially, with a principal level of concentration in some wealthy developed North American countries (the United States and Canada); this remarkable level of concentration is also present at the institutional and journal level. In answering RQ1, 16,376 articles addressing income equality were analyzed in this research. The data analysis to answer research question two follows in the next section.

• –RQ2: What are the most relevant keywords associated with income inequality?
  • RG3: Present the research approaches by keywords found in the literature
  • RG4: Conduct an analysis of publications by subject area to unearth income inequality in the literature between 1961 – 2021.

This research analyzed the most relevant keywords associated with income inequality. In addition, the repeatedly used keywords/ phrases in the title/abstract of the papers in this bibliometric analysis was identified.

Regarding a related keywords analysis to the “income inequality,” the top 20 main keywords related to the field are presented in Table 7.

As evidenced in Table 7, the most frequently used keywords (top 3) regarding income inequality were “income distribution” mentioned in 2,213 documents, “inequality” mentioned in 1,375 documents, and “poverty” mentioned in 1,019 documents. These terminologies keep with the theme because income distribution in a country, especially along gender lines, favors males more than females. Therefore, the imbalance of income and inequalities leads to poverty, especially among women.,^[36] Kawachi et al.^[37]

To better understand income inequality, we used the VOSviewer software to analyze the database visually, constructing a bibliometric networks based on bibliographic data from the source Scopus file, that included fields related to author, keywords, affiliations, and references fields, focunsing on a unit of analysis per keywords and co-ocurrence, selecting the options full counting and total link strength, with a network visualization, framed shapes, and curved lines, using predetermined cluster colors. Therefore, this specialized software helps construct networks of scientific publications, journals, researchers, organizations, countries, keywords, or terms^[38–40] van Eck et al.,^[38] Waltman et al.,^[41] considering a threshold with a minimum occurrence of a term of 71 and a binary counting method. Figure 5 shows the keyword correlated graphic in a network visualization figure.

Figure 5:
Bibliometric Network by Keywords.
Source: VOSviewer.^[42]

Figure 5 shows the visual structure formed by the diversity of keywords found in the literature, a strategy used by.^[42] The terms are represented and mapped based on the frequency of their occurrences. The network of relationships is identified by five colors and their respective links, based on the following data regarding occurrences and relevance scores for the overall keyword analysis.

The following occurrences are further represented based on the relevance of scores described in Table 8, that contains data obtained directly from VOSViewer, where the column “occurrences” indicates the number of documents in which any given keyword occurs, and “relevance scores” refers to a number that the software calculates for each term to represent specific topics covered by the text data (the higher the score, the more representative of the papers analyzed), allowing to excluding terms with a low relevance score, which are general and unspecific terms.

As shown in Table 8, the published income inequality research with a high relevance score and relatively low occurrences were distribution, labor, market, wage, welfare, social, and state. The data show that high occurrences do not equate to relevance to the subject of income inequality, as is reflected in mortality having a comparatively low relevance score of 0.1592 and 163 occurrences in the text data compared to labor with an occurrence score of 82 and a very high relevance score of 2.3899. Therefore, the data show that the high occurrence of related key terms to the subject of investigation, income inequality, does not equate to relevance.

In answering RQ2, the second RG analyzed addresses publications by subject area. The distribution by subject area by focusing on articles about subjects related to income inequality is analyzed. Table 9 presents the relevance of essential concepts such as distribution, labor market, wage, and welfare to scientific production regarding income/economic inequality. Table 9 shows the results of the analysis.

As Table 9 shows, the vast concentration of research is in the subject areas of Social Sciences (5,787), Economics, Econometrics, and Finance (5,066), followed by Business Management and Accounting (1,532), Medicine (1,254), and Environmental Science (952).

In sum, to answer RQ2, the top 20 research papers addressing income inequality yielded a total of 18,186 research papers between 1961-2021. This large number of documents based on the keyword search is significant in this research. The analysis of publications by subject areas yielded 13, 659 occurrences in the literature. The relevance score generated by the positive or negative comments based on income inequality shows that the highest relevance score of 3.4449 was in reference to the keyword income having the highest number of occurrences of 2,854. The bibliometric analysis also revealed that 80% of the scientific scholarly production is mainly from the Social Sciences, Economics, and Environmental Science.

1. –RQ3: Who are the most influential authors in income inequality research found in the literature between 1961 – 2021?
   • RG5: Determine the influence structure by citation (authors), based on the number of citations

To study the citation structure of economic/income inequality research, methodology used by Laengle, et al.,^[43] was followed. In Table 10, the total production of papers published by year, the total citation of those articles, an indicator named “Average Citation per Publication per Year” (ACPY) was incorporated to analyze the growth of the field. This analysis took into consideration that if a year has P publications and C citations in 44 publication years, within the evaluation period 1968 to 2021, then ACPY of 1966 will be C/44P; 1974 will be C/43P; 1978 will be C/42P . . . respectively). This helped to identify the most influential authors on the topic of income inequality shown in Table 10.

The results presented in Table 10 show that the Average Citation Per Year (ACPY) that is higher than 100 as an indicator of influence includes 25 periods, meanwhile in 5 periods, there was an ACPY over 50 (but less than 100); in 2 periods it presented an ACPY over 25 (but less than 50), and in 18 periods presented an ACPY over 5 (but less than 25).

For the analysis related to the most productive and influential authors, from the bibliometric analysis, data in terms of TP = Total Papers; TC = Total Cites, CPP = citations per paper, and H index for the authors regarding the papers published related to income inequality is presented. In Table 11, the top 20 authors’ average Citations Per Paper (CPP) is included.

Table 11 shows a highly relevant data analysis considering the influence of the most influential authors on the topic of general productivity on income inequality based on the analysis performed using the Scopus database. The higher production of total papers is not necessarily an equivalency of higher influence, as evidenced in Kennedy’s total of 5,907 citations and a calculated index of Citations Per Paper (CPP) of 369.2. In contrast, Kawachi et al.,^[37] has a total of 9,219 cites but a lesser calculated index for CPP of 119.7); the critical aspect is looking at the CPP weighting for relevance.

In answer to RG5, the most cited papers addressing income inequality are chronicled below. The following Table 12 includes information related to the respective papers in terms of a Journal (J), Total Citations (TC), title, authors, year of publication, and the average Citation per Year (C/Y), the results obtained in the quantitative analysis are presented in Table 12.

Table 12 presents interesting data regarding indicators about total citation and citation per year, since the first publication entitled “Health inequalities among British civil servants: the Whitehall II study”. The “Whitehall II” study is the most cited work on the list, with a total of 2,407 citations and a citation per year proportion of 80. Conversely, the second most cited article corresponds to the work “Practitioners’ corner: A note on the theme of too many instruments,” with a citation per year of 179 and an overall citation score of 2,148. Furthermore, four of the top 10 journals are domiciled in the United Kingdom and four in the United States. The remaining two journals in the top 10 publications (see Table 12) are from France, another developed European country. These journal ranking results are in line with the overall findings that most of the articles addressing income inequality have been produced in developed countries. To better understand the relationship between the total number of citations and the proportion year, the following Figure 6 shows a comparative contrast between those two variables.

Figure 6:
Descriptive Comparison by Contrast in Total Cites and Citation per Year Indicator.

In Figure 6, it is apparent that some highly cited papers are not necessarily the most influential works over the last 60 years (1921 – 2021). Interestingly, a significant work with noticeable citation per year may not have a high total citation, reflecting a contrast among indicators. For example, “The association between income and life expectancy in the United States, 2001-2014,” – published by JAMA – Journal of the American Medical Association, see Table 12, accounts for only 891 Total Citations (TC) and Citations per Year (C/Y) of 178.2, compared to “The Lancet” that had 2,407 TC and a C/Y of 80.2.

To better understand those relevant works, further information about the ten most influential articles is described in Table 13.

As shown in Table 13, the most cited and influential papers present various research works related to healthy behaviors, social environments, job design, and the consequences of income inequality. Furthermore, these articles highlight themes related to financial sector development, per capita density, shares of income and wages, data on Gini coefficients, wage gaps in OECD countries, household wealth, consumption patterns, in-kind public services, and population health. These articles were assessed as most influential from a huge pool of papers published between 1961 and 2021. Finally, Table 14 lists the number of published papers on income inequality related to policies, taxes, and gender.

As indicated in Table 14, the descriptive analysis regarding the total number of papers published from 1961 to 2021 related to income inequality in terms of policies, taxes, and gender, reflects a positive trend over the last 60 years. Furthermore, the data indicate that research related to income inequality as a general topic had the highest level of publications, as follows: income inequality (11,453) by policies (3,325), taxes (990), and gender (594). To obtain a more precise sense of the prevalence of income equality vis-a-vis policies, taxes, and gender, the following graphic, Figure 7, presents a comparative analysis of the total productivity considering general and specific subjects.

Figure 7:
Comparative Graphic in Scientific Productivity in the Study of Income Inequality and Related Subjects in Terms of Policies, Taxes, and Gender.

As shown in Figure 7, both the trends and the total productivity of the subjects of interest, income inequality, policies, taxes, and gender indicate that comparatively, income equality reflects the highest number of publications per year11, 453, when measured against other pertinent factors of policies, taxes, and gender, yielding 4,909 articles combined. Regarding the contribution per country, Table 15 comprises the total productivity per country for income inequality and related subjects: policies, taxes, and gender.

Concerning the data in Table 15, the two most productive countries are the United States and the United Kingdom (two developed countries), yielding 5,134 for all the subjects examined in the scope of the study. China, a quasi-developing country, has the third place for publications (691) addressing inequality and policies. Germany is fourth for articles addressing taxes (85), and Canada is third (44) for related topics that address gender. Figure 8 reflects the compelling contrast in each related subject based on the total production of papers focusing on income inequality.

Figure 8:
Comparative Analysis by Country and Related Subjects.

As shown in Figure 8, it is clear that the highest productivity numbers (3,883) correspond to the United States in all the subjects of interest for the present research, a difference that is also noticeable with the second most productive country being the United Kingdom (1,251). Based on the evidence from our research, it is clear which nation is leading the field on the topic of income inequality globally. Regarding the overall influence of the publications in Scopus, Table 16 shows the total citation per year of the subjects of interest.

Table 16 presents data in terms of total citation descriptive analysis, where it is possible to observe that the higher quantity is related to policies as a related subject (26% of the total citation), followed by taxes (9%) and gender (6%) as the third more cited among all the considered categories. To better understand this relationship between income inequality, policies, taxes, and gender, Figure 9 shows a graphic with the comparative tendencies with the subjects of interest.

Figure 9:
Comparative Tendencies for Income Inequality as a General Subject with Related Topics of Interest in Policies, Taxes, and Gender.

As shown in Figure 9, the corresponding trends for the citations related to income inequality have the highest productivity overall, but with a negative trend in recent years. These numbers concerning citations for each topic of interest have policies in second place, followed by taxes and gender. Therefore, less has been written about gender versus policies

In short, the information included in this section is helpful in the understanding of the general productivity relating to RQ3, where the paper “Health inequalities among British civil servants: the Whitehall II study” is the most cited work. Noteworthy, the better proportion of publications related to income inequality corresponds to the work “Practitioners’ Corner: A note on the theme of too many instruments.” Also, the most cited and influential papers are focused on subjects such as healthy behaviors, social environments, job design, the consequences of income inequality, shares of income and wages, data on Gini coefficients, wage gaps in OECD countries, household wealth, consumption patterns, in-kind public services, and population health. Together, the results produced by this bibliometric analysis answered RQ3. Now, the focus is on answering RQ4.

1. –RQ4: What are the characteristics of the publications on income inequality of the most productive nations with emphasis on the economic context? (policies, tax structure, and gender).
   • • RG6: Assess the information showing a descriptive analysis of income equality in terms of (policies, tax structure, and gender breakdown) in the most productive countries.

To answer RQ4, the frequency of nominal variables based on the name of specific documents addressing income inequality, policies, tax structure, and gender was analyzed, using the data obtained from the World Inequality Lab Dataset.^[49] The number of occurrences of the nominal variable “related document as input” and their corresponding relevance score (treating the qualitative nominal variable as a joint keyword to determine the number of keywords included in the mapping process) is presented in Table 17.

Table 17 shows the documents that are relevant input for the world inequality lab datasets. These documents exhibit a high relevance score that represents specific topics covered by the text data and thus portray the income inequality analysis. Hence, the most representative topics about income inequality are Pre-tax labor income, pre-tax labor income ranking, number of tax-units married couples, single adults, fiscal capital, total fiscal income ranking, capital component, corporate debt, equity liability, the market value of the corporation, non-equity liability, government financial, excluding cash, dwelling, national land, private land, government land, life insurance, private pension, personal pension, corporate pension, government pension, national business, corporate business, government business, personal business, profit business, and financial assets.

To better understand the contrast among relevant documents and their relevance to income inequality in most productive countries in terms of scientific research, Figure 10 shows in terms of density representation, the current type of information available in countries such as the United States, United Kingdom, China, Germany, Canada, Australia, Italy, Spain, Netherlands, and France.

Figure 10:
Graphic Representation of the Importance by Density in the Type of Document Available from the World Inequality Datasets.

According to Figure 10, it is clear that the access to information related to financial assets, income, fiscal income, life insurance, dwelling, post-tax national income, net personal wealth, the book value of the corporation, consumption of fixed capital of national economy, and corporate non-financial asset represent valuable information to study income inequality. Therefore, the text mining technique using VOSviewer was utilized in obtaining the graphic recommended by Van Eck et al.^[38,40]

In brief, the information included in this section helps understand the relevance of each topic for the research related to income inequality and the importance of density in the type of document available from the world inequality datasets to address RQ4, where subjects such as pre-tax labor income ranking, number of tax-units per married couple, single adult, fiscal capital, total fiscal income ranking, capital component, corporate debt, and equity liability are noticeably relevant in the field.

Economic inequality at the individual, country, and institutional levels continues to have adverse impacts on already marginalized populations across the globe.^[5–7] From the bibliometric analysis of scholarly publications utilizing the key terms income inequality between 1961 and 2021 (see Table 1), only six of the top 20 countries contributing to the scientific literature on income equality were from developing nations. Additionally, of the top ten countries that published research on income inequality, only one country is categorized as developing, China, due to income disparity, and the other nine are categorized as developed. From the data analysis (see Table 3), and in answer to RQ1 and RG1, more than 80% of the scholarly publications are concentrated in 50% of the top 20 of the most productive countries (1961 – 2021). The concentration of publications regarding economic inequality in developed nations reflects the global disparity in income and economic equality, where except for Brazil and South Africa, which rank 11^th and 16^th in our bibliometric analysis (see Table 3), there are no other developing countries from the 2022 Wealth Inequality by Country^[50] ranked in findings.

Furthermore, it is essential to note that the United States and China, arguably the two largest economies in the world, represent approximately 50% of all publications dealing with income inequality from (1961 – to 2021). This revelation begs the question of why is research relating to income inequality concentrated in the USA and China when according to the 2022 World Inequality Report and the World Economic Forum Global Risks Reports, economic inequality continues to be a significant issue in other non-researched prolific developing countries and regions across the globe. It is also interesting to note that there are no data reflecting research on economic inequality from regions and countries such as Africa, Asia (excluding China), Mexico, India, Central, and South America, or the Caribbean, where there continues to be a high prevalence of income and gender inequality (Altuzarra et al.,).^{[5–7,51,52]}

The United States had the highest number of “income inequality” publications during 1961 – 1921, representing approximately 34% of total publications with the search term. Interestingly, the most productive institutions during the period of investigation are domiciled in the United States and represent 45% of all institutions. We found it to be revealing that most of the institutions in the United States researching “income inequality” are Ivy League or Carnegie tier 1 classified schools. This finding leads to the question as to why more non-Ivy League institutions in the United States or, for that matter, globally, are not investing time and resources in understanding and explicating “income inequality” research. Another question that requires further research is whether “economic inequality” research would significantly impact policy changes impacting individuals, countries, and regional levels if the researchers doing “income inequality” research were domiciled in the countries and regions that reflect the phenomena. The data analysis also shows that more than 50% of “income inequality” research is published in four top-tier journals.

To understand the most relevant concepts related to the key term “income inequality,” a combination of Donthu et al.,^[20] (2021) performance analysis and science mapping with a particular focus on the co-word citation, bibliographic coupling, network metrics, clustering, and visualization analyses was used. From the bibliometric performance analysis, the top five or 80% of key research areas are social and environmental science, medicine, business, management, accounting, economics, and finance. These areas of concentration are in line with the key terms of poverty, inequality, income distribution, class, and gender that are the primary concentration of “economic inequality” research (see Figure 3). From Table 9, it is clear that there is greater interest in publications that focus on income inequality, distribution, and labor than other related areas, such as the impact “of income inequality” on gender and mortality rates.

Interestingly, the most productive authors in income inequality literature do not have the highest number of citations per paper. For example, Kennedy has more than triple the citation per paper (369.2 vs. 119.7, see Table 11) compared to Kawachi et al.,^[37] (1997). However, Kawachi has an H-index that is four times as high. Hence, the influence of these authors is not just based on the citations per paper but also on the total citations. The data analysis showed that the most cited papers might not represent the most influential authors (see Figure 4). From the analysis of the Scopus dataset of publications, it is evident that publications relating to economic/income inequality, gender, taxes, and related policies have grown exponentially over the last 60 years. The most cited publications are income inequality with more than 192, 000 compared to policies, taxes, and gender over 50,000, 17,000, and 12,000, respectively (see Table 15).

Income and economics, along with gender and tax inequality, continue to be areas of significant concern for individuals, countries, regions, and globally.^{[1,3,5,6,23,53,54]} The COVID-19 pandemic continues to increase the income gap between the rich and poor in developing and developed countries.^[55] With the rising income and economic inequality globally,^[9] there must be a diversity of researchers representing a broad cross-section of countries, cultures, and experiences so that the impact and reach of the research are not housed within a limited group of researchers and countries, with whom the economic and income status is counter to the countries and cultures on which their research is based. From the data analyzed, it is apparent that economic/income inequality research is concentrated in a limited group of authors, regions, and countries. This concentration of economic/inequality research in a few top-tier institutions and developed countries is concerning since income and economic inequality is less prevalent, comparatively, in these developed countries where the top-tier authors and scholarly works are domiciled.

The data from our research indicate that there needs to be increased focus by authors and institutions on factors such as gender, taxation, and income distribution policies, all of which are symptomatic of income and economic inequalities globally. For example, an analysis of the data revealed that in 2021 there were seventeen times more articles produced by scholars (1071 vs. 62) on income inequality than there were on gender. Nevertheless, according to extant research-(Amate-Fortes et al.,^[5,6,24] income inequality continues as a significant factor negatively impacting gender equity.

Based on the findings presented here, it is suggested that further research should be targeted at developing research centers across institutional and author classifications, other than top-tier institutions and high-impact factor authors. Thereby assuring that a broader range of research is reflected in the literature that encompasses the perspective and possible directions from developing countries where there is a higher prevalence of income inequality and the related issues of gender, taxation, and policy disparities, as shown in Figure 11.

Figure 11:
Directions for further research.

Figure 11 shows a summary of the areas that researchers may explore as they seek to add value to the income and inequality research body of knowledge. It is also recommended that a mentorship consortium for institutions and authors be developed to assist with incubating and distributing a broader base of researchers and institutions that focus on income inequality research and the related factors of gender, taxes, and policies.

Of the top twenty most productive (scholarly publications) countries in the world (see Table 3), fifteen are ranked developed and five developing by the United Nations Human Development Index (HDI). This bibliometric research reviewed scholarly publications over the last 60 years, and of these, the overwhelming majority of publications have emanated from institutions and authors that are domiciled in developed countries. Based on the^[50] most countries that reflect income inequality are designated developing countries. Therefore, it would make sense that income inequality research would be a natural byproduct of authors and institutions domiciled in these developing countries. However, this is not the case based on this bibliometric research.

Hence, further research into the factors that determine the domicile of income inequality research needs to be carried out. Answering the questions outlined in Figure 11 will help clarify why most income inequality research resides in four economically and traditionally patriarchal societies, the United States, United Kingdom, China, and Germany. It would be interesting to see the perspectives and empirical research of scholars domiciled in large developing economies such as Africa, Mexico, and Asia (excluding China, Japan, and South Korea, which are included in the top 20 productive nations – See Table 3) as well as South and Central America.

Another area for further research would be to analyze scholarly research using other bibliometric databases than Scopus, which was used for this research—for example, conducting a cross-sectional analysis using Web of Science and Microsoft Academic Search, Crossref API (DOI focused), JSTOR and Grobid (source for PDF or scanned documents). Finally, it would be worthwhile to see if the findings from this research that utilized Scopus would result in similar findings regarding the concentration of income inequality research in top-tier, often Ivy League institutions and acclaimed authors in developed nations.

The top 50 most cited and influential papers (see Tables 12 and 13) regarding income inequality over the last 60 years reveal that the top five papers with the most citations are dated, with the most recent publication date being 2009 with the Oxford Bul¬letin of Economics and Statistics. Hence, there is great potential for future income inequality research that reflects current and di¬verse paths to address issues of taxes, gender, and policies on in¬come disparity from a diverse cultural, economic, country, or re¬gional perspective. Our bibliometric research elucidates the need for more current scholarly work on antecedent and latent factors impacting income inequality, especially in the light of massive income and economic disparities exposed by the COVID-19 pandemic. Another area for further research would be gender and taxes relating to income inequality, as these areas are the least researched based on the bibliometric data analyzed.

As a matter of conclusion for the present work, this bibliometric analysis of scholarly publications relating to income/economic inequality over the last 60 years (1961 – 2021) adds to income/ economic literature by providing a multidimensional perspective of authorship, diffusion of work (citation), and domicile of scholarly works. The results presented here help to elucidate gaps in research and shed light on the need to expand authorship of works to include scholars from developing countries and a greater diversity of scholars and institutions. The research and dissemination of scholarly works through a wider authorship group and regional and country participation will add greater validity and relevance to income and economic inequality research, as the drivers of the research would have a more significant stake in sharing publications that will help to highlight and ultimately eliminate income/economic inequality, within their own countries and globally.

This bibliometric research reveals that the literature on income inequality has grown exponentially over the past 60 years. However, this growth is concentrated in only a few countries, with the United States, United Kingdom, and China housing the majority of publications and most prolific authors. These top three research prolific countries, through their research institutions and authors, have a responsibility to provide mentorship and resource support to other institutions and authors in developing countries so that a wider body of research that is less skewed to the perspectives of developed countries is reflected in the literature.

The findings suggest that the research available in terms of income inequality is highly concentrated in a few countries that are developed economies, such as the s United States, United Kingdom, China, Germany, and Canada, and most of the most productive institutions that are most productive are also located in developed countries; this data is relevant since income inequality represents a global situation for the vast majority of economies, as well as being a transversal topic for subjects related to the impact of income on poverty, distribution of resources, healthy behaviors, social environments, job design, financial sector development, wages, and income rates are of great interest and are essential and needed areas for continued research.

In that context, it is essential to recognize that these findings indicate the study’s contribution to income inequality. Moreover, since published research provides the raw material for policies and law initiatives and concerning the quantity and effectiveness in the transition of those initiatives to existing law, the contribution of those countries, institutions, and authors has a noticeable representation in wealthy and developing nations.

Within this interpretation, a study of this nature suggests that there is a need for developing countries to increase the level of support and resources allocated to understand income inequality, mainly to increase the knowledge on the topic and enhance the quality and suitableness considered in the process of creation of policies and laws.

Finally, other lines of research should include comparative analysis related to the differences among nations relating to productivity and trends since 1961, including grouping techniques of comparison showing the possible changes in the proportion of the income inequality of nations and the productivity in research, so that it is possible to understand the actual or potential impact of the research field in creating better conditions for the corresponding population.