RESEARCH ARTICLE - Am MIT spezialisierte KI-Forschung

RESEARCH ARTICLE

Research coauthorship 1900–2020: Continuous,
universal, and ongoing expansion

Mike Thelwall

and Nabeil Maflahi

University of Wolverhampton, Vereinigtes Königreich

Keine offenen Zugänge

Tagebuch

Schlüsselwörter: coauthorship, collaboration, scientometrics, Scopus, team science

Zitat: Thelwall, M., & Maflahi, N.
(2022). Research coauthorship
1900–2020: Continuous, universal, Und
ongoing expansion. Quantitative
Science Studies, 3(2), 331–344. https://
doi.org/10.1162/qss_a_00188

DOI:
https://doi.org/10.1162/qss_a_00188

Peer Review:
https://publons.com/publon/10.1162
/qss_a_00188

Erhalten: 2 Dezember 2021
Akzeptiert: 27 Februar 2022

Korrespondierender Autor:
Mike Thelwall
m.thelwall@wlv.ac.uk

Handling-Editor:
Ludo Waltman

Urheberrechte ©: © 2022 Mike Thelwall and
Nabeil Maflahi. Published under a
Creative Commons Attribution 4.0
International (CC BY 4.0) Lizenz.

Die MIT-Presse

ABSTRAKT

Research coauthorship is useful to combine different skill sets, especially for applied
problems. While it has increased over the last century, it is unclear whether this increase is
universal across academic fields and which fields coauthor the most and least. In Beantwortung,
we assess changes in the rate of journal article coauthorship 1900–2020 for all
27 Scopus broad fields and all 332 Scopus narrow fields. Although all broad fields have
experienced reasonably continuous growth in coauthorship, In 2020, there were substantial
disciplinary differences, from Arts and Humanities (1.3 authors) to Immunology and
Microbiology (6 authors). Alle 332 Scopus narrow fields also experienced an increase in the
average number of authors. Immunology and Classics are extreme Scopus narrow fields, als
exemplified by 9.6 authors per Journal for ImmunoTherapy of Cancer article, wohingegen 93%
of Trends in Classics articles were solo in 2020. The reason for this large difference seems to
be the need for multiple complementary methods in Immunology, making it fundamentally
a team science. Endlich, the reasonably steady and universal increases in academic
coauthorship over 121 years show no sign of slowing, suggesting that ever-expanding teams
are a central part of current professional science.

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EINFÜHRUNG

Research collaboration may be essential to address complex societal challenges (Hall, Vogel
et al., 2018), as exemplified by the need for cooperation to reduce the impact of the COVID-
19 pandemic (Cai, Fry, & Wagner, 2021; Chakraborty, Sharma et al., 2020). International col-
laboration is also widely believed to be beneficial (Matthews, Yang et al., 2020), and huge
authorship teams involving large numbers of countries (Adams & Gurney, 2018) or hundreds
of people (Thelwall, 2020) are also essential for some problems. This article focuses on col-
laboration as expressed in journal article coauthorship for the practical reasons that this is the
type for which the largest scale evidence is available (z.B., 62% of articles found by a review of
team science used bibliometric data; Hall et al., 2018), and this type of collaboration is impor-
tant for research evaluation.

There is some empirical evidence for the value of coauthorship. Journal articles with more
authors tend to be more cited in many different contexts (Abramo & D’Angelo, 2015; Didegah
& Thelwall, 2013; Fuchs, Paine, & Sauterey, 2016; Larivière, Gingras et al., 2015; Thelwall,
2020). Trotzdem, the extent to which coauthorship is a citation advantage varies between
fields and countries (Avkiran, 1997; Chi & Glänzel, 2017; Sooryamoorthy, 2009; Thelwall &
Maflahi, 2020; Thelwall & Sud, 2016), and the reasons are poorly understood (Hall et al.,

Research coauthorship 1900–2020

2018; Melin, 2000; Wagner, Whetsell, & Mukherjee, 2019). A larger number of authors is also
associated with greater social media interest (Haustein, Costas, & Larivière, 2015), and coau-
thoring may help researchers to write more articles (Abramo, D’Angelo, & Di Costa, 2009),
although perhaps not if fractional counting is used (Lee & Bozeman, 2005).

1.1. The Evolution of Research Coauthorship

Historically, collaboration appeared in science when it transformed from an amateur to a pro-
fessional occupation (Beaver & Rosen, 1978), and this presumably expressed itself increas-
ingly often in coauthored papers. The size of research teams and consequently the number
of authors per paper increased after the Second World War in richer nations, driven by the
cost of research, as part of the development of “big science” (Price, 1963). Coauthorship
has become more common and easier with faster, cheaper travel, the internet (Melin,
2000), and cloud computing (Langmead & Nellore, 2018), but is still more likely between
partners that are geographically close (Hoekman, Frenken, & Tijssen, 2010). It has been partly
driven by funding programs that mandate or encourage collaboration (Melin, 2000), with large
grants associating with greater and more diverse collaborations (Bozeman & Corley, 2004),
aligning with the big science idea (Price, 1963).

Many empirical studies have found evidence of increasing coauthorship in individual fields
(z.B., economics: Jones, 2021; translation studies: Rovira-Esteva, Aixelá et al., 2020), aber die
focus here is on science-wide trends. International coauthorship increased substantially in the
Web of Science ( WoS), aus 10% In 1990 Zu 25% In 2011 (Wagner, Park, & Leydesdorff,
2015). An investigation of 20 million WoS research articles from 1955 (science and engi-
neering), 1956 (social sciences), oder 1975 (arts and humanities) Zu 2000 found increases in
average (arithmetic mean) team sizes and the proportion of coauthored articles in all cases,
with solo research dominating the arts and humanities, solo articles being about half of all
social science articles in 2000, and team publications being the norm in science and engi-
neering (Wuchty, Jones, & Uzzi, 2007). An updated and larger scale investigation with the
Science Citation Index Expanded, Social Sciences Citation Index, and Arts and Humanities
Citation Index 1900–2011 found that an increasing proportion of articles had more authors
when split into two groups: natural and medical sciences and social sciences and humanities
(Larivière et al., 2015). The percentage of solo papers also decreased substantially in both
groups during this period. According to various field-specific catalogues, coauthorship
increased between 1800 Und 1999 in mathematics, Physik, and logic, although the main
expansions occurred in different half centuries in each case (Wagner-Döbler, 2001). Despite
these studies, no prior published academic journal article seems to have analyzed long-term
trends in research coauthorship across all broad or narrow fields of science.

1.2. Types of Coauthorship

Coauthorship occurs for at least two reasons that seem to apply to all areas of scholarship:
between PhD students and supervisors, and for social reasons, such as to work with friends
(Jha & Welch, 2010; Melin, 2000). Researcher characteristics also affect the likelihood that
they collaborate (not equated with coauthorship) (Van Rijnsoever & Hessels, 2011). Other rea-
sons for coauthorship vary between fields and probably change more over time, such as the
need for large-scale studies or diverse sets of skills to tackle specific problems. At the individ-
ual level, researchers may coauthor an article because they have complementary areas of
expertise for common interests, and so can produce more complex studies, but they may also

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Research coauthorship 1900–2020

join to access scientific instruments or other resources (Thorsteinsdottir, 2000; Tomáška,
Cesare et al., 2020).

There are many ways of contributing to research projects, only some of which might usually
be classed as collaboration or recognized with a coauthorship attribution (Katz & Martin,
1997; Laudel, 2002). Zum Beispiel, a minor contribution to a project might not be thought
of as a collaboration and some teams might only allocate a coauthorship to major contribu-
tionen. Other contributions might generate an acknowledgment instead, or no formal recogni-
tion (Cronin, 2001; Laudel, 2002). Im Gegensatz, noncollaborators might receive gift authorships
(Chawla, 2020) and team leaders might be listed as last authors for overall management of a
project or laboratory without a specific contribution to a study. Daher, the criteria for authorship
are not fixed. There are some attempts to standardize the requirements for authorship, Wie-
immer. Zum Beispiel, the CRediT (Contributor Roles Taxonomy) system lists the following 14
authorship roles (CASRAI, 2022): Konzeptualisierung; Datenkuration; Formale Analyse; Funding
Erwerb; Untersuchung; Methodik; Projektverwaltung; Ressourcen; Software; Supervi-
sion; Validierung; Visualisierung; Writing—original draft; Writing—review & Bearbeitung. From this
list, it may be assumed that any of these types of contribution, if substantial enough, might
qualify a person to be a coauthor. Trotzdem, other guidelines make more strict specifica-
tionen, such as a requirement that all contribute to writing the article and are accountable for
“all aspects of the work” (ICMJE, 2021).

1.3. Disciplinary Differences in Coauthorship

There are disciplinary differences in the likelihood of coauthoring papers. Informal collabora-
tion without coauthorship is thought helpful in the social sciences and humanities, wohingegen
formal collaborations are believed to be more important in the natural sciences (Lewis, Ross, &
Holden, 2012). Many other factors (discussed below) seem less applicable to the social sci-
zen, arts, and humanities. Trotzdem, social sciences and humanities scholars may also
experiment with collaboration because it is promoted as an effective research strategy, sogar
though it is rare in their home fields (Graham Bertolini, Weber et al., 2019).

Fields that need to combine to create pooled resources, such as global biodiversity data
sets or specialized nuclear reactors, engender new coauthorships of different types based
on these resources or their production (z.B., Arita, Karsch-Mizrachi, & Cochrane, 2021;
D’Ippolito & Rüling, 2019). Ähnlich, large health-related cohort studies may need multiple
centers to collect all the data and researchers in later stages may need to collaborate and
coauthor with earlier researchers to understand or process all the data collected (Eblen,
Fabsitz et al., 2012). In some areas of science, researchers operate largely as part of teams
rather than individuals, and so collaboration and coauthorship are core to their work
(Ziman, 1994).

There is some empirical evidence about disciplinary differences in the extent of
coauthorship.
International coauthorship has been more common in basic fields (Frame
& Carpenter, 1979). Trotzdem, evidence from National Science Foundation funded
studies in the United States suggests that it grew more rapidly during 1997–2012 in applied
fields and particularly in the medical sciences and emerging applied hybrid fields (Coccia
& Bozeman, 2016). The fields in which single authorship is most common are mathematics
and the arts and humanities (Farber, 2005) and there are more authors per paper in the
natural and medical sciences compared to the social sciences and humanities (Larivière
et al., 2015).

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1.4. Research Questions

Although the above brief review suggests that coauthorship is broadly increasing, the extent of
the historical trends and fine-grained disciplinary variations in coauthorship have not been
systematically evaluated. This is an important omission for future attempts to identify the
causes of the apparent coauthorship advantage. This article assesses the historical evolution
of the number of coauthors of journal articles at the finest grained cross-science level yet. Es
also contrasts with a previous WoS study (Larivière et al., 2015) by using a different data
source: Scopus. It focuses on coauthorship as an easily available metadata type, wenngleich
there are a range of types of contribution to research (including none) that can translate into an
authorship attribution (Larivière et al., 2016; Mongeon, Smith et al., 2017; Rahman, Mac
Regenstein et al., 2017). It also focuses on global trends in coauthorship team sizes rather than
changes in the nature of coauthorships because there is little data from which to deduce the
nature of a collaboration on a science-wide or long-term scale. The following research ques-
tions drive this study.

(cid:129) Has the average number of authors per publication for journal articles increased during
1900–2020 in all Scopus broad and narrow fields, and is it still increasing in 2020?
(cid:129) Which Scopus broad and narrow fields had the highest and lowest average number of

authors per publication in 2020?

2. METHODEN

The raw data for this study is the metadata records of all Scopus documents of type journal
article published between 1900 Und 2020, as downloaded by the Scopus API up to September
2021. Scopus was chosen for wider coverage than WoS (Martín-Martín, Thelwall et al., 2021)
and a greater number of narrow fields (332 vs. 252), supporting a finer grained analysis. Sehen
the Discussion for coverage limitation issues.

Scopus broad fields (n = 27) and narrow fields (n = 332, excluding empty fields) war
used for the classification process (for a list of narrow fields and journals, see Elsevier
(2021)). The fields are predominantly journal-based and inferior to article-based classifica-
tionen (Klavans & Boyack, 2017), but were used for transparency and the availability of
expert-validated field names. Articles classified by Scopus into multiple broad fields were
included within each one. Including these duplicates, 88 million articles were analyzed.
Numbers for each broad field and year are in the online supplement (https://doi.org/10
.6084/m9.figshare.17064419).

The average number of authors per article was calculated using the geometric mean (based
on averaging the logged numbers of authors, then applying the exponential function) eher
than the arithmetic mean. The geometric mean is a more appropriate measure of central ten-
dency for highly skewed data (Smothers, Sun, & Dayton, 1999) and this is important because
of the presence of some huge teams. Zum Beispiel, a few articles by the 2,862-person ATLAS
collaboration (z.B., https://doi.org/10.1007/JHEP09(2016)074) would skew the arithmetic
means for their fields to unrepresentative numbers. The median is also appropriate for skewed
data but is not fine-grained enough for this study because the average coauthorship values (für
the geometric mean) are all between 1 Und 7, as can be seen in the figures. The harmonic
mean could also have been used to reduce the impact of highly coauthored outliers, Aber
the geometric mean seems more intuitive because it is based on averaging logarithms rather
than reciprocals.

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Articles with no authors were excluded from all calculations because an article must be writ-
ten by someone (excluding fake computer-generated texts). Unauthored articles seemed to be
primarily entries that had been misleadingly classed as journal articles (z.B., special issue intro-
duktionen, errata, corrections, announcements [z.B., “The Topics of the Month of IASR”)], Und
editorials) but might also sometimes be journal articles with indexing errors. In at least one case,
the article authors were not explicitly included in the original article (“Germ-cell migration.
Finding the way to the gonad in Drosophila”, Aktuelle Biologie, 1994, 4(1), 47), either by a pub-
lishing accident or because it is not a full journal article (an annotated picture in the above case).

3. ERGEBNISSE

Full broad and narrow field trends are on FigShare (https://doi.org/10.6084/m9.figshare
.17064419), with the main results reported below.

3.1. Broad Scopus Fields

In all 27 broad Scopus fields, the average number of authors per article has increased over the
Vergangenheit 121 Jahre (Figures 1–4). In nearly all broad fields, single authorship was overwhelmingly
most common in 1900. The two exceptions were Materials Science and Chemistry, with aver-
ages close to 1.5 (Figur 1). Von 2020, most broad fields had at least four authors per paper on
average (geometric mean), including all multidisciplinary and natural science broad fields
(Figur 1), the Energy and Chemical Engineering broad field (Figur 2), and all life science
and health broad Scopus fields (Figur 3). Im Gegensatz, all social science and humanities

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Figur 1. The average (geometric mean) number of authors per journal article for multidisciplinary and natural science Scopus broad fields
(see the raw data for this and all figures at https://doi.org/10.6084/m9.figshare.17064419).

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Figur 2. The average (geometric mean) number of authors per journal article for formal science and engineering Scopus broad fields.

broad fields had less than 3.5 authors per paper in 2020. The largest difference is between
the average of 6 authors per Immunology and Microbiology journal article in 2020 (Figur 3)
Und 1.6 for Arts and Humanities (Figur 4).

3.2. Narrow Scopus Fields

In almost all cases (329 out of 332), the average (geometric mean) number of authors per arti-
cle in the most recent publishing year of a Scopus narrow field (almost always 2020) War
higher than in the first publishing year (usually after 1900). Two of the three exceptions, Nurse
Assisting and Agricultural and Biological Sciences (miscellaneous), were due to a single article
published in their first year. Ignoring this year, there was an increase in average authorship for
these two fields. The third exception, Drug Guides, began publishing in 1973 with four articles
Und, ignoring years with fewer than five articles, has increased to 2020. It also has an overall
increasing trend, albeit with a decrease since 2013 (see online supplement for figure: https://
doi.org/10.6084/m9.figshare.17064419), so is not an exception.

Alternativ, trends could be assessed by comparing the average number of authors for the
Erste 100 articles (or more, counting whole years) against the most recent 100 articles. A mini-
mum of 100 ensures that the average is not dominated by a few articles. With this calculation, alle
332 Scopus narrow fields had an increase in the average number of authors from their inception
Zu 2020 (or their termination in Scopus). Somit, on the basis of both of these types of test, es ist
reasonable to claim that coauthorship has increased in all 332 Scopus narrow fields.

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Figur 3. The average (geometric mean) number of authors per journal article for life science and health-related Scopus broad fields.

3.3.

Immunology and Microbiology: Highest Average Coauthorship

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The Scopus broad field with the highest coauthorship rate, Immunology and Microbiology,
was further investigated for coauthorship rates in its constituent subfields. All seven narrow
subfields have high rates of coauthorship, with the highest being Immunology in most years
(Figur 5). Perhaps confusingly, the “Immunology and Microbiology (alle)” narrow field
includes all types of Immunology and Microbiology, but it does not contain all Immunology
and Microbiology articles.

High coauthorship was the norm for 19 of the 20 largest Immunology narrow field journals
In 2020 (Figur 6), with the exception being European Journal of Molecular and Clinical Med-
icine. The highest average coauthorship for any journal with at least 100 articles was 9.6
authors: Journal for ImmunoTherapy of Cancer, apparently due to its reporting of complex
multiexperiment studies (see online supplement for a detailed analysis of this journal:
https://doi.org/10.6084/m9.figshare.17064419).

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3.4. Arts and Humanities: Lowest Average Coauthorship

The Scopus broad field with the lowest coauthorship rate, Arts and Humanities, beinhaltet a
wide variety of Scopus narrow fields in terms of average authorship (Figur 6). Solo authorship
is still the norm in many of the fields, as evidenced by many average author counts being
below 1.4 (this is the maximum geometric mean that guarantees a mode of 1). Presumably,
the fields overlapping with social science or technical specialties are more likely to have

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Figur 4. The average (geometric mean) number of authors per journal article for social science, arts, and humanities Scopus broad fields.

coauthors. The Arts and Humanities Scopus narrow field with the fewest average authors in
2020 is Classics, which is only marginally ahead of Literature and Literary Theory.

Within Classics, only one journal published more than 100 articles in 2020: Trends in Clas-
sics (see online supplement for figure: https://doi.org/10.6084/m9.figshare.17064419). Diese
106 articles included 99 with solo authors, six with two authors and one with four.

4. DISKUSSION

Two important data limitations of this study are that it relies on articles accurately reflecting
coauthors and Scopus accurately indexing them. Both assumptions are not true in some cases.
Insbesondere, hugely coauthored articles may list the consortium as the author, and Scopus
seems to limit the number of coauthors to 100, at least for some papers. This mainly affects
highly coauthored articles, Jedoch, and its influence is reduced by using geometric mean
averaging in this paper. “Authors” that are consortia include “The European Polycystic Kidney
Disease Consortium” listed in Scopus as the author of “The polycystic kidney disease 1 gene
encodes a 14 kb transcript and lies within a duplicated region on chromosome 16.” This
seems to be rare, Jedoch, with only 14 Scopus first-author names including “consortium”
und nur 268 including “collaboration” (z.B., “Event Horizon Telescope Collaboration”),
although other consortium names are almost certainly also indexed.

Another important limitation is that the extent to which Scopus’s coverage of academic
journals has changed over time is unclear, although it is known that its total coverage has

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Figur 5. The average (geometric mean) number of authors per journal article for Immunology and Microbiology Scopus narrow fields.

expanded greatly (Thelwall & Sud, 2022). Some journals may have been not produced or lost
during the Second World War as universities and libraries were looted, damaged, or destroyed
(Van der Hoeven & Van Albada, 1996), Zum Beispiel. Journals from fields that died out may also
be more difficult to obtain, as might early journals that were published by scholarly societies
and distributed relatively informally before the commercialization of much academic publish-
ing (Fyfe, Coate et al., 2017). Scopus may also have been more (or less) inclusive in its selec-
tion policy for older years, given that it does not attempt to index everything (Mabe & Amin,
2001). More recently, because the coverage of Scopus changes every year, the inclusion of
single large journals or groups of journals from a country or publisher can also impact the
results (Thelwall & Sud, 2022). It is impossible to know how reliable the results are from dif-
ferent decades because of changes over time in the coverage of Scopus, but it seems likely that
older results are increasingly less reliable, as some of the data may have been lost when ret-
rospectively added after Scopus was created. The results should therefore be interpreted as
accurate reflections of Scopus but not necessarily uniformly representative of published aca-
demic research since 1900.

The disciplinary differences in coauthorship rates shown above overlap with and echo
some previous studies. This is true for the low rate of coauthorship for mathematics (Farber,
2005) and the social sciences and humanities (Larivière et al., 2015). The results extend prior
findings with updated, finer grained, and longer term information. Insbesondere, while the arts
and humanities are known to have the lowest coauthorship rates, Classics and Literature and

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Figur 6. The average (geometric mean) number of authors per journal article for arts and humanities Scopus narrow fields.

Literary Theory do not seem to have been identified as the least collaborative or with the low-
est level of coauthorship. Ähnlich, Immunology and Microbiology does not seem to have
been previously singled out as the highest coauthorship area, at least with the Scopus classi-
fication scheme. Other fields seem to be better known for large authorship teams, wie zum Beispiel
astronomy (many large telescopes) (Kahn, 2018), genetics (Human Genome Project) (Dinh
& Cheng, 2018), and particle physics (Large Hadron Collider) (Kahn, 2018), but Immunology
and Microbiology seems to have consistently large coauthorship teams for its studies because
of the typical need to combine multiple methods for a single article. Microbiology has been
important for human health since germ theory displaced the miasma theory of disease in the
late 19th century and was used to develop treatments and cures. This critical role, together
with the complexity of its topic, may be part of the reason why studies are unusually elaborate
and comprehensive, needing large teams of scientists. These teams are sometimes within a
single medical center or hospital-university connection, rather than being higher profile inter-
national collaborations.

The apparently continuing increase in authorship raises the question of whether it is reach-
ing a natural peak in any field. Plotting the authorship data on a graph with a logarithmic y-axis
can help check this. On such a graph, exponential growth (d.h., the rate of growth is constantly

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Research coauthorship 1900–2020

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Figur 7. The average (geometric mean) number of authors per journal article for all 27 Scopus broad fields with a y-axis log scale. Das (Und
all graphs in the paper) can be inspected in detail in the source spreadsheet available on FigShare (https://doi.org/10.6084/m9.figshare
.17064419).

increasing) would become a straight line and constant or decreasing growth rates would trans-
late into a line bending downwards. With this interpretation, the logarithmic graphs for all
fields for the authorship data show no sign of the rate of increase of authorship slowing down
(Figur 7). Tatsächlich, the rate of increase seems to be accelerating in many broad Scopus fields.
Daher, the data suggests that the level of authorship is not close to reaching a natural maximum,
and raises the possibility that authorship team sizes will continue to grow in all broad fields for
the foreseeable future.

5. CONCLUSIONS

Coauthorship has increased universally since 1900 in terms of broad and narrow fields, Aber
with substantial differences between broad and narrow fields. The graphs in this paper can
serve as benchmarks for future studies to check against the coauthorship rate of any field,
to see how it compares to the overall average for the corresponding Scopus broad field (Wenn
geometric means are used).

The fact that the average number of authors per paper has been universally and reasonably
steadily increasing for 121 years with no sign of reaching a natural maximum, even in its most
prevalent fields, demonstrates a fundamental change in academic research and the possibility
of a continued acceleration in article author numbers for the foreseeable future. This suggests
that team size growth may be a fundamental part of modern academic research, Zum Beispiel

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Research coauthorship 1900–2020

due to the increasingly complex nature of studies needed to investigate beyond the expanding
research frontier, or extrascientific pressure from funders to collaborate, leading to larger
Autorenteams.

The increases in the average numbers of authors per paper suggest, but do not prove
(because of differing authorship attribution cultures), that collaboration has also increased
steadily. Although all academic specialties are different, with their own trends and require-
gen, the results suggest that research group leaders and research managers should plan
on the basis that coauthorship teams will continue to get larger and that this increase will need
to be supported. This support might take the form of investing in infrastructure for coau-
thorship, encouraging communication between different sets of researchers that may coauthor,
and encouraging the formation of larger research groupings or intergroup coauthorship. Diese
types of support may help researchers to form and operate within the increasingly larger coau-
thorship teams that are appearing in all types of research. Managers should be sensitive to
disciplinary differences and the importance of variety in research, Jedoch.

At the individual researcher level, the increases in average numbers of authors per article
suggest that it may be beneficial to seek opportunities to enhance cooperation with other indi-
viduals or teams, perhaps leading to future coauthorships. It may also be useful for a researcher
to look out for the advantages that coauthorships have brought to published research in their
field so that beneficial types of coauthorship can be sought.

BEITRÄGE DES AUTORS
Mike Thelwall: Methodik, Writing—original draft, Writing—review & Bearbeitung. Nabeil
Maflahi: Writing—review & Bearbeitung.

COMPETING INTERESTS

The authors have no competing interests.

FUNDING INFORMATION

This research was not funded.

DATA AVAILABILITY

Full broad and narrow field trends are in the supplementary material (https://doi.org/10.6084
/m9.figshare.17064419).

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