As shown in Table 2 and Figure 1, pensions are a topic of interest to researchers.

Since the first article was published, in 1936 up to the present day, they have experienced a very significant growth. Especially, at the end of the 20th century, there was an increase in interest for researchers. It can also be observed that interest has increased even more in the recent years, as the number of articles published has grown exponentially. It is worth noting that more articles have been published on pensions in the last ten years than in the previous 75 years, which shows the growing interest in this topic today.

Figure 1 also exhibits the significant evolution of the number of citations received by the articles published in each period, demonstrating the interest in the future sustainability of pensions. The decline in the number of citations in the last period should be interpreted with caution, as not enough time has yet passed for these articles to generate citations, especially those published in the previous few years.

The articles on pensions have been classified into several thematic areas by the database. Figure 2 depicts these subject areas and indicates the percentage of published articles. The results show that the 3 most relevant areas are economics, econometrics and finance (32.77%), social sciences (28,45%) and business, and management and accounting (12.12%). These 3 areas account for 73.34% of the articles published. The remaining areas, including medicine, environmental sciences, and mathematics, account for the remaining 26.66%.

The age-related pension problem of the social security system is an economic problem of financing: the shortage of funds in relation to the pensions to be paid. The solution involves econometric analyses that anticipate the future situation, that foresee possible scenarios based on the current situation. In other words, it is mainly a problem of economics, finance, and econometrics.

The journal that has issued the most articles is the International Social Security Review, which is specialised in the matter, as its name suggests. Specifically, it has published 122 papers (9.4% of the total number of articles published) since 1970, the year in which it published its first article on pensions. It can also be seen that this journal has received a total of 1092 citations (10.24% of the total number of citations received by articles on pensions). As for the nationality of these journals, it is worth noting that eight are published in Europe and two in the United States. The UK and the Netherlands share four of the European journals. It is also significant that 2 of the 10 journals are in quartile 1 of the SCImago Journal Rank (S.J.R.); the rest (except for 1 discontinued journal) are in quartile 2, which indicates a good interest in the subject of pensions in high-impact journals.

A total of 112 countries have published articles on pensions. These countries are represented in Figure 3.

The most prolific country is the U.S.A., with 18.95% of publications (244 articles), followed by the U.K. with 12.43% (160 articles), and Germany with 7.38% (95 articles). China and Spain are in fourth and fifth place with 76 and 72 articles, respectively.

Concerning the h-index, the top 3 positions coincide with the most prolific countries; the U.S.A. with 29, the U.K. with 22, and Germany with 21. However, the fourth position goes to the Netherlands with 14 and France and Canada share the fifth position with 13.

Figure 4 depicts networks showing cooperation among the 47 major countries that have published pension research. The colour of each of the spheres corresponds to a grouping of countries, while the size represents the number of articles published per country.

Eleven different groups have been observed. The red group includes the Czech Republic, Hungary, Lithuania, Poland, Slovakia, and Slovenia. The countries associated with the green colour are Belgium, Colombia, France, Ireland, Luxembourg, and Tunisia. The blue cluster, headed by Spain, also includes Brazil, Israel, Portugal, and Austria. The yellow group is led by China, with Canada, Japan, Taiwan, and Turkey as collaborators. The purple group, led by the U.K., comprises Russian, Switzerland, and Malta. The light blue includes the Netherlands, Italy, Denmark, and Finland. Sweden, Norway, Argentina, and Chile make up the orange cluster. The brown set, headed by Australia, includes Hong Kong, Malaysia, and Singapore. The pink group is led by the U.S.A., with India, Mexico, and New Zealand as collaborators. The salmon group comprises Germany, Romania, and South Korea. Finally, Ghana and South Africa make up the light green cluster.

The author with the highest number of publications is Pestieau, P. with 12 articles, followed by Williamson, J.B. with 11. However, Meijdam, L. has ten articles, the author who has received the most citations (182). The second author regarding the number of citations received is Pestieau, P. In terms of the results for the average number of citations per article, Claussen, B. with 18.67 is in the first position, followed by Meijdam, L. with 18.20 and Holzmann, R. with 16.71. It is curious that Verbon, H.A.A. published 6 of his 7 articles with Meijdam, L.

Different collaborative networks have been created from the co-authorships of the principal authors. These networks are shown in Figure 5.

Each of the colours corresponds to a cluster grouping its main collaborators. Nine distinct clusters have been found. The red cluster comprises eight authors: Fang I., Gong J., Li Y., Liu W., Liu X., Ning M., Pan W., and Zhang Y. The green cluster groups seven authors: Ding Z., Guo Y., Li X., Si X., Wei H., Zhang W., and Zheng S. The blue cluster is made up of five authors: Huang J., Liu Q., Shen I., Sun S., and Wang H. The yellow group is made up of six authors: Brosing S., Luo R., Rozelle S., Yue A., Zhang I., and Zhao Q. The purple cluster consists of five authors: Deng D., Han J., Meng Y., Zhang X., and Zhang Z. The light blue set also consists of five authors: Chen X., Jia L., Qi T., Sun A., and Zhong S. The orange cluster is made up of four authors: Chen Z., Li Z., Sun J., and Zeng Y. The same is true for the brown cluster, formed by: Wang Y., Xin I., Yang Q., and Zhang J. Finally, the three authors composing the pink cluster are: Liu L., Rettenmaier A.J., and Saving T.R. The brown cluster has the highest number of citations with 62, followed by light blue with 33, orange with 25, blue with 20, and purple with 19 citations. The remaining clusters have fewer than 10 citations.

As established by previous research in other areas of study, in order to carry out a correct analysis of the strategic diagrams, the study has to be divided into two stages [60,73]. For this reason, researchers have divided the period analysed into two periods: the first period from 1936 to 2010 and the second period from 2010 to 2021. The first period is the longest one, but the scientific production is smaller than the second one. The first period includes a total of 625 articles, while the second period includes 662 articles.

For each period, researchers will make a strategic diagram representing two dimensions on a Cartesian coordinate axis: centrality (relationship of one node of the graph to another) and density (strength of the relationship between the entities within each node). Thus, the upper right quadrant shows motor themes, i.e., closely related to each other and others. The lower right quadrant shows basic and cross-cutting themes. The upper left quadrant shows isolated themes or highly developed themes internally but not linked to the others, as is the case with motor themes. Finally, the lower left quadrant shows emerging or declining themes.

Figure 6 represents the strategic diagram of the first period from 1936 to 2010.

As can be seen, it presents three driving themes (upper right quadrant): early retirement, old age security, and pensions. These are the well-developed and essential themes of pensions research.

It can be seen that there are two basic themes (lower right quadrant): ageing population and privatisation. Studies on private pension systems have increased, especially in the last years. Figure 6 also presents two emerging or disappearing themes (lower left quadrant): old age pensions and pension plans. When researchers look at the next period, researchers will see whether these are emerging or disappearing themes. Finally, it can be seen and two highly developed themes: dentist and refugees. These themes are highly developed but unrelated to each other. The size of each of the circles reflects the h-index and, as can be seen, pensions and the ageing population have the highest h-index.

As can be observed in Figure 7, the second period from 2011 to 2021 shows four terms in each of the quadrants, except for the driving themes, which are five. The driver themes are humans, investment, social security system, pension-fund, and coronavirus infection. The appearance of a COVID-19 related issue is striking, although it is logical. As for the basic themes: pension reform, social security scheme, pension insurance, and old age pension, it can be seen that old age pension was, in the previous period, an emerging rather than a declining theme. However, the pension plan was a declining theme. In this section, the most important theme is covering studies on pension reforms. These reforms aiming at the sustainability of the system have become increasingly necessary. The emerging or disappearing themes are child allowance, personal pension, interest rate, and social pension in this period. Finally, highly developed themes, but unrelated to each other, are farmers, optimisation, labour force, and policy modelling.

Human is the main driving theme for this second period. This theme has been covered in 168 articles and has received 1152 citations with an h-index of 16. The second most important topic is the one dealing with the reform of pension schemes (pension reform) with a total of 51 articles, 327 citations, and an h-index of 10. In third place, although with much less importance, are the pension funds, with a total of 11 articles, 20 citations, and an h-index of 3.

Indeed, the ageing population in developed countries and the decline in the number of employed people relative to the number of retired people has led to a rethinking of the reform of traditional pension systems. The use of private pension plans and alternative life insurance to supplement state pensions is increasingly recommended, which is reflected in the scientific literature or recent years. The economic support of human beings at retirement age is a central problem in the most developed economies.

The study of trends in pension research ends with the analysis of the 3389 keywords. It has been performed through a network visualisation map based on the co-occurrence of keywords shown in Figure 8.

Figure 8 shows how the topics of interest in pension research have evolved. Since 1995, aspects, such as organisation, country development, and care for the elderly, were the most important topics. As time went by, the social security system and pensions gained more and more ground in the articles published, ending up in the days with an increasing concern about the much needed reform of the pension system to ensure sustainability. Furthermore, the results show that from the beginning of the period, 2010 or 2012, there was a concern about the sustainability of the social security system and the necessary pension reform. It also shows how society has become increasingly aware of the need for private coverage to complement social security pensions and how new alternative products have emerged, even to private pension plans, such as unit-linked plans, which combine insurance and investment operations in a single financial product.