# Trends in Equal-Contribution Authorship: A Large-Scale Bibliometric Analysis of Biomedical Literature

Binbin Xu

► To cite this version:

Binbin Xu. Trends in Equal-Contribution Authorship: A Large-Scale Bibliometric Analysis of Biomedical Literature. 2025. hal-05218321

HAL Id: hal-05218321

https://hal.science/hal-05218321v1

Preprint submitted on 21 Aug 2025

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>
# Trends in Equal-Contribution Authorship: A Large-Scale Bibliometric Analysis of Biomedical Literature

Binbin XU

EuroMov Digital Health in Motion

Univ. Montpellier, IMT Mines Ales, France

binbin.xu@mines-ales.fr

## Abstract

Equal-contribution authorship, in which two or more authors are designated as having contributed equally, is increasingly common in scientific publishing. Using ~480,000 tagged records extracted from over 45 million bibliographic XML files (PubMed and PMC, 2010–2024), we examine temporal trends, journal distributions, and geographic patterns. Results show a sharp rise after 2017, with both high-output mega-journals and smaller, discipline-specific journals contributing to the growth. Journal-level analysis indicates a median increase from ~19% in 2015 to over 30% in 2024. Geographically, China accounts for the largest share (40.8% of tagged cases), followed by the United States (15.2%) and Germany (5.2%). Normalizing to 2015 baselines, China shows a 13.1× increase by 2024, while even the slowest-growing countries more than tripled their rates. Collaboration networks highlight strong China–USA ties and identify mid-volume countries serving bridging roles. These findings document a broad shift toward shared first-author credit across journal sizes and regions within the biomedical literature. While this practice may improve equity in credit attribution, its rapid expansion merits continued monitoring and further analysis of domain-specific, institutional, and policy contexts.

## 1 Introduction

Over the last two decades, scientific publications have increased at an almost exponential rate. A recent study showed that the total number of articles indexed in Scopus and Web of Science was close to 2.9 million in 2022, 47% higher than in 2016 and almost doubled the figure in 2013 [Hanson et al., 2024]. However, this represents only a small portion of the total scientific publication. As of 2025, PubMed alone indexes over 38 million references. Not only has the number of publications grown rapidly, but there has also been a clear shift from solo scholarship toward team-based research. In biomedical fields, this growth in team size has been accompanied by new conventions to highlight contributions and responsibilities, including explicit designation of "equal contribution" (often in co-first authorship) and, more recently, co-corresponding authorship. Early journal-specific studies documented a clear increase in equal-contribution statements in high-impact medical journals between 2000 and 2009 and in specialty venues thereafter [Akhabue and Lautenbach, 2010].

Teams have expanded substantially across science: typical team size roughly doubled over the latter half of the twentieth century and continues to grow [Wuchty et al., 2007, Fortunato et al., 2018]. In biomedicine, authors per article increased from 3.99 (2000) to 6.25 (2020), while single-author papers declined about threefold [Jakab et al., 2024]. Growth includes “hyperauthorship” (very large bylines) with attendant attribution challenges [Cronin, 2001]. Collaboration correlates with higher citation impact
but exhibits diminishing marginal returns at very large team sizes [Larivière et al., 2015]. The article-
level citations also tend to rise with author and reference counts in ecology research, consistent with
incentive effects without implying causation [Fox et al., 2016]. Within biomedicine, the division of labor
has produced an expanding stratum of “middle authors” [Mongeon et al., 2017]. These macro-trends
provide neutral context for the increased use of explicit equal-contribution designations examined here.

These authorship signals have prompted the development of guidance from editorial bodies and stan-
dards organizations. For example, the International Committee of Medical Journal Editors (ICMJE) for-
malized authorship criteria and encouraged contributorship statements to improve transparency around
individual roles [of Medical Journal Editors, 2025, International Committee of Medical Journal Editors,
2025]. The CRediT taxonomy [NISO CRediT Working Group, 2022] introduced a structured vocabulary
of 14 roles for describing individual contributions, which has now been adopted by many publishers.
Major publisher policies now define how equal contribution is recorded. In practice, many publishers
such as Nature, BMJ, PLOS align with ICMJE recommendations on authorship and require contrib-
utorship statements using the CRediT taxonomy so that equal-contribution designations are explicitly
declared rather than inferred [Portfolio, 2025, BMJ, 2025, PLOS, 2025]. Policies typically cap formal
equal-contribution labels (e.g., a single co-first and a single co-senior set) and direct additional equalities
to the contribution statement, though implementation varies by journals.

In this study we treat such labels as descriptive metadata (not evaluative) and harmonize cross-
publisher heterogeneity via JATS/PubMed tags. At the metadata level, the Journal Article Tag Suite
[Beck, 2011] (JATS) and PubMed/PMC pipelines [PubMed Central, 2025] support machine-readable
flag, e.g., the equal-contrib="yes" attribute on <contrib> and explicit corresponding-author elements.
They provide a technical basis for corpus-scale measurement.

Despite these developments, the literature lacks a comprehensive, longitudinal account of equal-
contribution authorship across the biomedical record. Existing studies typically focus on selected journals,
a single discipline, or short time windows, and often rely on manual searches or unstructured PDF text.
For example, Akhabue and Lautenbach [Akhabue and Lautenbach, 2010] quantified equal-credit state-
ments in five general medical journals (2000-2009), and subsequent analyses reported similar growth in
selected specialties. Huang et al. [Huang et al., 2016] examined 53,603 pharmacy and anesthesia papers
(1995-2014) from top journals to quantify the prevalence, distribution, and byline patterns of co-first and
co-corresponding authorship. Related work on co-corresponding authorship [Tian et al., 2024], a different
yet related designation of responsibility, confirms that multi-role attribution is spreading across coun-
tries, disciplines, and journals, but comprehensive coupling analysis using large-scale structured metadata
remains lacking.

This gap has implications for research assessment. Committees, project funders, and hiring panels
often rely on author order as a proxy for leadership and contribution. As shared first authorship becomes
more common, the interpretive value of ordered position may diminish unless explicitly supplemented by
author statements.

Here, we present a corpus-level analysis of equal-contribution authorship from 2010-2024 using machine-
readable metadata from PubMed and PubMed Central. Leveraging JATS/PubMed tags (e.g., values of
equal-contrib on <contrib>), we (i) quantify global trends in the prevalence of papers with ≥ 2 equal-
contribution authors; (ii) map journal- and year-specific patterns; and (iii) examine geographic patterns
via affiliation data. This approach complements prior journal- or domain-focused studies by exploiting
standardized metadata across a large collection of biomedical literature. By establishing a longitudinal
baseline, this work documents the current state and recent evolution of equal-contribution authorship and
provides a reference point for future studies on co-corresponding authorship and other contributor-role
designations.

The remainder of this article is organized as follows: Section 2 describes the data sources and process-
ing steps. Section 3 presents the results, including overall trends, journal-level patterns, and geographic
analysis. Section 4 discusses the implications of these findings and concludes the article.
## 2 Data

In the biomedical field, two major bibliographic databases exist: PubMed and PubMed Central (PMC), which are complementary services of the U.S. National Library of Medicine (NLM). PubMed is the largest free bibliographic database today indexing over 38 million citations and abstracts from MEDLINE, life-science journals, and online books. It also provides links to full text when available (including PMC) and offers annual XML baselines and updates<fnref n="1" />. PMC, on the other hand, contains a free full-text archive of biomedical and life-sciences journal literature, curated under NLM collection policies and distributed via FTP or cloud access (e.g., the Open Access Subset [National Library of Medicine, 2025]). Importantly for this study, PMC and PubMed support machine-readable contributor metadata in JATS/XML (e.g., the `equal-contrib="yes"` attribute on `<contrib>` and `corresp="yes"` for corresponding authors), enabling corpus-level measurement of equal-contribution authorship.

It is important to note that the two platforms are not systematically cross-linked: many PubMed records have no PMC counterpart and vice versa, and identifiers (PMID, PMCID, DOI) are sometimes missing or non-bijective. Accordingly, we match on available identifiers, de-duplicate overlaps, and exclude PMC `<sub-article>` content to curate a single, analysis-ready dataset.

<figure><img src="image_2.png" /><figcaption>**Figure 1.** Data-flow diagram showing the filtering, matching, and date-restriction steps applied to PubMed and PubMed Central (PMC) records to produce the final analysis set for 2010-2024.</figcaption></figure>

A schematic of this workflow, including counts at each step and the identifier matching strategy, is shown in Figure 1. We processed PubMed and PubMed Central (PMC) in four stages. We began with the full corpora (PMC ≈ 7 million references, 146 GB compressed `.tar.gz` XML files; PubMed ≈ 38 million references, 46 GB compressed `.tar.gz` XML files), accessed in July 2025. The downloaded XML files containing contributor tags (e.g., equal-contribution indicators) were then extracted. Basic cleaning was applied, including discarding malformed XML, excluding `<sub-article>` translations/supplements, and removing obvious duplicates. This step yielded 437,561 PMC-tagged and 325,416 PubMed-tagged records. In the third step, we reconciled overlaps into three subsets: PMC-only (212,394), PMC-PubMed matched (221,918), and PubMed-only (102,635) using DOI, PMCID, and PMID. Considering that earlier-stage records may not be well indexed with corresponding tags, we restricted the analysis window to 2010-2024. The final dataset contains 479,274 records in total, comprising 157,131 PMC-only, 220,760 PMC-PubMed matched, and 101,383 PubMed-only records for the longitudinal analyses.

### Annual Publication Counts per Journal

To normalize the yearly proportion of equal-contribution articles by journal, we computed the annual total number of publications for each journal in the combined dataset. This step required matching and merging bibliographic metadata from the raw PubMed and PMC collections while removing duplicates

<footnote n="1">¹accessible on https://ftp.ncbi.nlm.nih.gov/pubmed</footnote>
across sources. The PubMed dataset includes, for each article, fields such as journal abbreviation, publication date, and keys such as PMID, DOI, and PMC ID (if available). The PMC dataset contains similar bibliographic information, with multiple publication date fields (`pub_date_ppub`, `pub_date_epub`, `pub_date_pmc-release`). To avoid double counting, we applied a hierarchical deduplication strategy using DOI as the primary key, followed by PMID, and then PMC ID when DOI was unavailable. In cases of key collisions, PubMed entries were given precedence as the baseline source. For PMC records, the article date was defined as the earliest of the three available publication dates. For each record, we retained the journal identifiers (PubMed's *journal_abbr* and PMC's *journal.nlm ta*), the standardized article date, and the key bibliographic identifiers. This merged and deduplicated table was then used to generate annual publication counts per journal, which formed the denominator for calculating normalized yearly ratios of equal-contribution authorship.

## Publication Country or Region Extraction

We noticed that publications from certain regions tend to involve more team-based research, prompting investigation into whether geographic patterns exist in the prevalence of equal-contribution authorship. To examine this, we mapped the country or region of each author's affiliation. Affiliation data were extracted separately for PubMed and PMC using custom parsing scripts tailored to their respective XML structures. For PubMed, the script iterated through `<AffiliationInfo>` elements, retaining the full affiliation string and isolating the last segment (typically the country or region). For PMC, affiliations were mapped to authors via `<xref ref-type="aff">` links, with a fallback to inline `<aff>` tags when no cross-reference was present. Text cleaning steps included removing label and email elements and standardizing whitespace. Country information was first taken directly from `<country>` tags when available; otherwise, it was heuristically inferred from the last segment of the affiliation string.

In a post-processing step, extracted country names were matched against the ISO 3166-1 standard. Strict matches were assigned the corresponding ISO 3166-1 alpha-3 code. Remaining unmapped entries were manually reviewed, applying location-based heuristics such as city name lists and associated postal code patterns (notably for the USA, UK, and China). Affiliations that could not be resolved to a country, about 0.74% of the total were left unassigned. Original affiliation strings were retained in all cases for traceability.

# 3 Results

Please note that, all statistics reported here are descriptive; we do not infer policy intent, quality, or appropriateness.

From the total 479,274 records, as shown in Table 1, we first observed that the majority of equal-contribution designations involve two authors (73.4%), with smaller fractions for three (17.0%) and four authors (6.1%). More extreme cases with five or more authors sharing first or equal-contribution authorship are rare but present, accounting for about 3.5% of cases. The pattern for corresponding authors is similarly skewed: about two-thirds of articles list a single corresponding author, about 26% list two, approximately 7% list three or more. This skew toward smaller numbers suggests that while collaborative contributions are often recognized at the *first author* level, the role of corresponding author remains more centralized, potentially reflecting editorial practices or the need for a single primary contact.

## 3.1 Trends in Equal-Contribution Authorship

Figure 2 shows a clear and accelerating increase in the absolute number of publications tagged with **equal-contribution** authorship between 2010 and 2024. In the left panel, the yearly totals rise gradually through the early 2010s, followed by a sharper increase after 2017. In the period after 2020, yearly totals
<table><caption>**Table 1.** Distribution of the number of equal-contribution authors and corresponding authors across the combined PubMed-PMC dataset, 2010-2024.</caption><thead><tr><th colspan="3">Equal Contribution</th><th colspan="3">Corresponding</th></tr><tr><th>Number</th><th>Count</th><th>Percent</th><th>Number</th><th>Count</th><th>Percent</th></tr></thead><tbody><tr><td>2</td><td>351955</td><td>73.44%</td><td>1</td><td>316182</td><td>65.97%</td></tr><tr><td>3</td><td>81403</td><td>16.98%</td><td>2</td><td>128414</td><td>26.79%</td></tr><tr><td>4</td><td>29057</td><td>6.06%</td><td>3</td><td>29703</td><td>6.20%</td></tr><tr><td>5</td><td>8510</td><td>1.78%</td><td>4</td><td>4109</td><td>0.86%</td></tr><tr><td>6</td><td>3838</td><td>0.80%</td><td>5</td><td>620</td><td>0.13%</td></tr><tr><td>≥ 7</td><td>4511</td><td>0.94%</td><td>≥ 6</td><td>246</td><td>0.05%</td></tr></tbody></table>

peaked at 94,690 tagged articles in 2024. This inflection would coincide with a broader uptake of explicit equal-contribution statements in journal metadata, which may reflect genuine growth in multi-author collaborative research, stronger reporting norms for shared first authorship, evolving journal policies, or changes in authorship arrangements.

<figure><img src="image_3.png" /><figcaption>**Figure 2.** Trends in the use of equal-contribution authorship in scientific publications.</figcaption></figure>

*Left:* Annual count of publications explicitly labeling at least two authors as having contributed equally, showing a strong upward trend from 2010 to 2024. *Right:* Distribution of the top 100 journals ranked by proportion of equal-contribution articles (2010–2024).

The right panel of Figure 2 focuses on journals in which equal-contribution authorship accounts for at least 15% of all publications from 2010–2024. By normalizing for overall publishing volume during this period, this view highlights smaller but highly specialized journals alongside multidisciplinary mega-journals. While broad-scope journals typically fall in the 15-30% range, several lower-volume journals reach much higher normalized ratios, with some exceeding 40-50%. These high ratios suggest that in some domains, such as oncology, molecular biology, and genomics, equal-contribution authorship has almost become a common convention, potentially reflecting the multi-expertise, multi-center study designs and other collaborative research structures. Many of these high-ratio journals also contribute disproportionately to the steep post-2018 rise shown in the left panel, indicating that the overall trend is driven by both growth in mega-journals and increases within specialized journals.

Taken together, these two views highlight complementary aspects of the phenomenon: mega-journals contribute most to the total volume of equal-contribution articles, whereas smaller or more specialized journals can have a higher proportion of such designations. This dual pattern would suggest a broad
evolution in authorship practices influenced by both journal scope and by research culture. Increasingly, modern studies are technically and logistically more complex, often involving multi-omics datasets, large-scale cohorts, interdisciplinary approaches and compressed project timelines. All these can promote broader collaboration and increase the prevalence of shared first-author recognition.

## 3.2 Journal-level Prevalence Patterns

In the first results section, we examined which journals publish the most equal-contribution articles in absolute numbers. This provides an indication of scale but does not show how individual journals have changed over time. Some journals have used this designation for many years, while others appear to have adopted it more recently, with rapid increases thereafter.

<figure><img src="image_4.png" /><figcaption>**Figure 3.** Changes in equal-contribution authorship over time at the journal level. Heatmap of 143 journals with a median proportion of equal-contribution articles ≥ 20% in the last five years (2020–2024); colors indicate yearly percentages. These values are descriptive only and should not be interpreted as evaluative of editorial policies or journal quality.</figcaption></figure>

Figure 3 shows a heatmap for 143 journals in which the median share of equal-contribution articles over the last five years is at least 20%. Two main patterns can be observed. Some journals have maintained high levels throughout the period, while others began at relatively low levels and then rose sharply, mostly after 2018. These increases may reflect changes in journal policy, shifts in editorial practices, or the influence of large-scale collaborative articles.

Over the last five years, 33 journals exhibited median equal-contribution prevalence of ≥ 40%. Among these, the highest individual journal reached 83.2%, with several others consistently above 60% and a substantial group in the 40-55% range. Many of these outlets are concentrated in oncology, cell biology, molecular medicine, and translational research, areas characterized by multi-center studies, diverse expertise, and team-based workflows.

The presence of journals with ≥ 50% prevalence indicates that, in certain research communities, equal-contribution authorship has shifted from an occasional designation to a routine convention. In contrast, large multidisciplinary journals show elevated ratios around 40%, but with broader variation reflecting diverse disciplinary norms. These findings suggest that both domain-specific practices and editorial conventions influence the adoption of equal-contribution credit.

These patterns are consistent with broader trends toward larger, more interdisciplinary research teams, as discussed in Section 4.
<figure><img src="image_5.png" /><figcaption>**Figure 4.** Relationship between annual journal output and the proportion of equal-contribution authorship (Left), and yearly distribution of equal-contribution ratios across all journals (Right). The scatter plot shows individual journal-year points, color-coded by year; the boxplots summarize the spread and median ratio for 2015-2024.</figcaption></figure>

Across all journals, the yearly boxplots (Figure 4) showed an upward shift in the proportion of papers with equal-contribution authorship. The median ratio was increased from about 19.2% in 2015 to 30.1% in 2024. The widening interquartile range indicates that not only is the typical adopting journal more likely to use this practice, but the variability in adoption rates between journals is also growing. The scatter plot on the left shows no simple relationship between a journal's annual output and its equal-contribution ratio: high ratios occur in both low-volume and high-volume journals, although extreme values tend to appear in lower-output titles.

Overall, these results suggest that the rise in equal-contribution authorship is a broad trend spanning journals of different sizes, with certain fields and editorial policies driving ratios well above the global median.

## 3.3 Geographic Pattern

### Computation of country ratios

For each article with at least one equal-contribution author, all unique countries present in the author affiliation list were extracted (using the ISO 3166-1 mapping procedure described in the Data section). To avoid over-weighting articles with many authors from the same country, we applied a fractional counting approach. Each country was assigned a weight $w_{i,c}$ for article i:

$$w_{i,c} = \frac{n_{i,c}}{\sum_{c'} n_{i,c'}}$$

where $n_{i,c}$ is the number of occurrences of country c in article i's affiliation list (after parsing and ISO 3166-1 mapping). By construction, $w_{i,c} \ge 0$, $w_{i,c} = 0$ if $n_{i,c} = 0$, and $\sum_{c'} w_{i,c} = 1$ for each article i.

We then aggregated these per-article weights over all articles to obtain country totals and proportions at the corpus level.

### General geographic pattern

Figure 5 shows the geographic distribution of equal-contribution authorship, the World Map is obtained from [Natural Earth, 2025]. Results indicate a high concentration, with China accounting for 40.8% of all fractionalized contributions, followed by the United States (15.2%) and Germany (5.2%). South Korea (4.1%), the United Kingdom (3.7%), and Italy (3.1%) also feature prominently. The remaining top-15
countries or regions such as Japan, France, Spain, the Netherlands, Canada, Australia, Taiwan, India, and Switzerland—each contribute between approximately 1.3% and 2.3%. Collectively, the “Others” category, which includes all remaining countries, accounts for 12.3% of the total.

<figure><img src="image_6.png" /><figcaption>**Figure 5.** Geographic distribution of equal-contribution authorship.</figcaption></figure>

*Left:* Pie chart showing the fractional share of the top 15 contributing countries or regions plus “Others”.

*Right:* World map with log-scaled color mapping of article counts per country or region.

This distribution could reflect both the global research landscape and specific disciplinary trends. The particularly high proportion from China may be associated with the scale of biomedical research output in the country, the frequent use of multi-institution collaborations, and cultural or institutional norms favoring shared first authorship. The United States' high share mirrors its overall research volume but is proportionally smaller than China's, suggesting differences in authorship practices. The comparatively high shares for South Korea and Germany may indicate active participation in multi-center studies in high-ratio domains like oncology and molecular biology. The concentration in a small set of countries also suggests uneven adoption of equal-contribution designations globally, potentially reflecting disparities in funding, collaborative infrastructure, and editorial policies.

<figure><img src="image_7.png" /><figcaption>**Figure 6.** Relative increase in publications with equal-contribution authorship across the top 15 contributing countries or regions. All values are normalized to each region's 2015 level.</figcaption></figure>

## Growth pattern over time

When we normalize each country or region's yearly equal-contribution counts to its own 2015 baseline, the growth patterns become much clearer. Although Figure 6 displays data from 2010-2024, we use 2015 as the normalization point to focus on the most recent decade, where data coverage is more complete and
consistent across countries. Values from 2010-2014 are retained in the plot for context, but these earlier years generally have lower coverage and should be interpreted with caution.

China shows the steepest increase, with a 13.1× rise by 2024, nearly double that of the next fastest-growing countries, such as South Korea (7.9×), India (7.6×), and Italy and Switzerland (both 6.8×). Many European countries, the United States, and Japan cluster in the 4–5× range, while Taiwan (3.4×) and the Netherlands (3.9×) are at the lower end, yet still exhibit substantial growth.

These results indicate that the rise in shared first-authorship attribution is not confined to a few regions; even the slowest-growing group has more than tripled its baseline levels over the last decade, suggesting widespread and accelerating adoption of equal-contribution practices across the global research landscape.

## Cross-country collaboration pattern

International collaboration is a key driver of scientific visibility and innovation. In the context of equal-contribution authorship, cross-country partnerships may increase the likelihood of shared first authorship through multi-center trials, distributed datasets, and the pooling of specialized expertise.

To construct the collaboration network, we reused the previously curated contributing-country data. For every unique pair of countries or regions appearing in the same article, one count was added to their shared edge in an undirected adjacency matrix. This process was repeated for all articles, resulting in a symmetric matrix. Edges representing fewer than 0.1% of total co-authorship counts were removed to reduce noise. Node sizes in the graph are proportional to the total weighted degree of each country, and edge thickness is proportional to the co-authorship count between the two connected nodes. Total weights were calculated from the lower triangle of the matrix to avoid double counting.

<figure><img src="image_8.png" /><figcaption>**Figure 7.** Collaboration network among top countries or regions in equal-contribution publications. Node sizes are proportional to weighted degree (total co-authorship counts); edges represent collaboration frequency, and only edges with ≥ 0.1% of total weight are shown.</figcaption></figure>

The collaboration graph (Figure 7) reveals a densely connected core of major research nations, with China (CHN) and the United States (USA) occupying dominant positions in both node size and connectivity. The CHN-USA edge is the thickest (24,193), reflecting a sustained, high-volume partnership. Other strong bilateral links include USA-GBR (13,238), DEU-GBR (7,430), and USA-DEU (11,182), indicating that European and North American hubs remain central. Notably, countries like KOR, ITA, and IND show fewer total edges but maintain strong ties with leading collaborators, often through specific regional or thematic research programs. The “Other” node aggregates smaller contributors and maintains substantial links with CHN (18,996) and USA (49,468), underscoring broad global participation.
# 4 Discussion and Conclusion

This analysis of approximately 480,000 publications tagged with equal-contribution authorship provides a multi-angle view of how this authorship convention has evolved, particularly over the last 10 years.

From a general trend perspective, we find a notable and accelerating rise since 2010-2015, with a particularly steep increase after 2017. This growth is evident in both absolute counts and normalized ratios, indicating that the phenomenon is not solely a byproduct of rising publication volume. The adoption appears to be present both in high-output mega-journals and in smaller, more specialized journals in disciplines such as oncology, molecular biology, and genomics.

At the journal level, some journals maintain consistently high ratios of equal-contribution authorship over time, while others show rapid adoption in only the last few years. Trends across all journals indicate a steady median increase from about 19% in 2015 to over 30% in 2024. This pattern suggests that shared first authorship is becoming increasingly commonplace across a substantial portion of the scientific publications.

From a geographic perspective, China accounts for the largest share (40.8%), followed by the United States (15.2%) and Germany (5.2%), with other countries forming a long tail. Normalizing to 2015 levels reveals especially sharp relative growth in China (13.1×), South Korea (7.9×), and India (7.6×). Even the slowest-growing countries or regions more than tripled their 2015 baseline. Collaboration network analysis further shows that while major producers (USA, GBR, DEU, CHN) dominate in volume, certain mid-volume countries (e.g., Japan, India) play important bridging roles that connect different regional or global research clusters.

Equal-contribution authorship is significant because it allows co-first authors to share credit for leading the intellectual and practical aspects of a study, which can influence career progression in competitive academic environments. In principle, it better reflects the collaborative reality of modern science, especially in large, complex, interdisciplinary projects where leadership is genuinely distributed.

However, discussion has emerged around the possible overuse or broader adoption of equal-contribution designations. While most instances are likely legitimate, anecdotal reports suggest that, in some cases, credit may be applied inconsistently, which could affect the clarity of its meaning. Assessing such dynamics is inherently challenging without detailed contextual information, and our intention is not to evaluate individual practices, but simply to note that the rapid expansion merits continued monitoring by the scientific community.

This study also has limitations. Although it draws on over 45 million bibliographic XML records from PubMed and PMC combined, the source metadata are imperfect. Some equal-contribution notes may be missing due to inconsistent tagging or parsing errors. Conversely, wrongly tagged cases are likely rare, as authors and journals tend to be deliberate with such designations. Despite these limitations, our dataset of approximately 480,000 records provides a robust basis for identifying large-scale patterns.

Future work should focus on refining the dataset through improved curation, expanding the analysis to specific research domains, and examining disciplinary and institutional practices in greater detail. Incorporating topic modeling, funding source tracking, or citation impact analysis could help clarify the contexts in which equal-contribution authorship is most prevalent and most meaningful.

# References

[Akhabue and Lautenbach, 2010] Akhabue, E. and Lautenbach, E. (2010). “equal” contributions and credit: An emerging trend in the characterization of authorship. *Annals of Epidemiology*, 20(11):868–871.

[Beck, 2011] Beck, J. (2011). NISO Z39. 96The Journal Article Tag Suite (JATS): What Happened to the NLM DTDs? *The journal of electronic publishing: JEP*, 14(1):106.
[BMJ, 2025] BMJ (2025). Authorship & Contributorship. Online. Available from: https://www.bmj.com/about-bmj/resources-authors/article-submission/authorship-contributorship.

[Cronin, 2001] Cronin, B. (2001). Hyperauthorship: A postmodern perversion or evidence of a structural shift in scholarly communication practices? *Journal of the American Society for Information Science and Technology*, 52(7):558–569.

[Fortunato et al., 2018] Fortunato, S., Bergstrom, C. T., Börner, K., Evans, J. A., Helbing, D., Milojević, S., Petersen, A. M., Radicchi, F., Sinatra, R., Uzzi, B., Vespignani, A., Waltman, L., Wang, D., and Barabási, A.-L. (2018). Science of science. *Science*, 359(6379):eaao0185.

[Fox et al., 2016] Fox, C. W., Paine, C. E. T., and Sauterey, B. (2016). Citations increase with manuscript length, author number, and references cited in ecology journals. *Ecology and Evolution*, 6(21):7717–7726.

[Hanson et al., 2024] Hanson, M. A., Barreiro, P. G., Crosetto, P., and Brockington, D. (2024). The strain on scientific publishing. *Quantitative Science Studies*, 5(4):823–843.

[Huang et al., 2016] Huang, M.-h., Hsieh, H.-T., and Lin, C.-S. (2016). The co-first and co-corresponding author phenomenon in the pharmacy and anesthesia journals. *Proceedings of the Association for Information Science and Technology*, 53(1):1–4.

[International Committee of Medical Journal Editors, 2025] International Committee of Medical Journal Editors (2025). Recommendations for the Conduct, Reporting, Editing, and Publication of Scholarly Work in Medical Journals. Online. Updated April 2025. Available from: https://www.icmje.org/recommendations/.

[Jakab et al., 2024] Jakab, M., Kittl, E., and Kiesslich, T. (2024). How many authors are (too) many? a retrospective, descriptive analysis of authorship in biomedical publications. *Scientometrics*, 129(3):1299–1328.

[Larivière et al., 2015] Larivière, V., Gingras, Y., Sugimoto, C. R., and Tsou, A. (2015). Team size matters: Collaboration and scientific impact since 1900. *Journal of the Association for Information Science and Technology*, 66(7):1323–1332.

[Mongeon et al., 2017] Mongeon, P., Smith, E., Joyal, B., and Larivière, V. (2017). The rise of the middle author: Investigating collaboration and division of labor in biomedical research using partial alphabetical authorship. *PLOS ONE*, 12(9):1–14.

[National Library of Medicine, 2025] National Library of Medicine (2025). PMC Open Access Subset. Online. Bethesda (MD): National Library of Medicine; Available from: https://PMC.NCBI.NLM.NIH.GOV/TOOLS/OPENFTLIST/.

[Natural Earth, 2025] Natural Earth (2025). 1:50m Cultural Vectors, v5.1.1. Online. Available from: https://WWW.NATURALEARTHDATA.COM/DOWNLOADS/50M-CULTURAL-VECTORS/.

[NISO CRediT Working Group, 2022] NISO CRediT Working Group (2022). ANSI/NISO z39.104-2022, CRediT, contributor roles taxonomy. Technical report, NISO, Baltimore, MD.

[of Medical Journal Editors, 2025] of Medical Journal Editors, I. C. (2025). Defining the role of authors and contributors. Online. Available from: https://WWW.ICMJE.ORG/RECOMMENDATIONS/BROWSE/ROLES-AND-RESPONSIBILITIES/DEFINING-THE-ROLE-OF-AUTHORS-AND-CONTRIBUTORS.html.

[PLOS, 2025] PLOS (2025). Authorship policy. Online. Available from: https://JOURNALS.PLOS.ORG/PLOSONE/S/AUTHORSHIP.
[Portfolio, 2025] Portfolio, N. (2025). Authorship Policy. Online. Available from: https://www.nature.com/nature-portfolio/editorial-policies/authorship.

[PubMed Central, 2025] PubMed Central (2025). PubMed Central Tagging Guidelines. Online. Available from: https://www.ncbi.nlm.nih.gov/pmc/pmcdoc/tagging-guidelines/article/tags.html.

[Tian et al., 2024] Tian, W., Cai, R., Fang, Z., Geng, Y., Wang, X., and Hu, Z. (2024). Understanding co-corresponding authorship: A bibliometric analysis and detailed overview. *Journal of the Association for Information Science and Technology*, 75(1):3–23.

[Wuchty et al., 2007] Wuchty, S., Jones, B. F., and Uzzi, B. (2007). The increasing dominance of teams in production of knowledge. *Science*, 316(5827):1036–1039.