Co-First Authorship

Co-first authorship (also called joint first authorship or shared first authorship) is the practice of designating two or more authors as having contributed equally to a work, so that no single person occupies the traditional “first author” slot alone. It is a labelling convention layered onto co-authorship, not a separate authorship test: every co-first author still has to independently satisfy the applicable authorship criteria (most commonly the ICMJE’s four-part test described in CASRAI’s Authorship entry) before the equal-contribution question is even relevant. What co-first authorship adds is a way to signal that byline order, on its own, would misrepresent how the work was actually done.

Why it exists

In fields that use a contribution-ranked byline (biomedicine and the life sciences most consistently), the first-author position is conventionally read as “the person who did the most hands-on work.” That convention breaks down when two or more people genuinely did comparable amounts of that work — a single first-author slot cannot represent two people at once, and simply listing one of them first would imply a ranking that isn’t true. Co-first authorship resolves that mismatch: the group keeps a byline order (someone still has to be printed first, for citation and indexing purposes) but attaches an explicit note stating that the listed order does not reflect unequal contribution.

How it’s marked

The standard mechanism is a shared symbol — typically a dagger (†) or asterisk (*) — placed next to each co-first author’s name, resolving to a footnote such as “These authors contributed equally to this work.” Some journals extend the same convention to the last-author position (“co-senior” or “co-last” authorship) using a second, distinct symbol. Exactly how many authors a journal will allow to share the first-author designation, and the precise footnote wording it expects, varies by publisher — CASRAI’s Co-Authorship entry covers this signalling convention, and the byline-order conventions it sits within, in more depth; this entry focuses specifically on the first-author case and what happens to that signal after publication.

Co-first authorship vs. CRediT’s degree-of-contribution qualifier

Co-first authorship is a byline-level signal, resolved between the authors themselves and not governed by any single external standard — ICMJE explicitly leaves author order to the author group. It is related to, but distinct from, the CRediT taxonomy’s optional “degree of contribution” qualifier (lead / equal / supporting), introduced in the NISO CRediT standard (ANSI/NISO Z39.104-2022). CRediT’s “equal” qualifier can mark two contributors as equal on a specific, named role (for example, both marked “equal” on Formal analysis) without that implying anything about first-author position on the byline — a CRediT statement and a co-first-authorship footnote are two independent mechanisms that are commonly published side by side, not substitutes for each other.

Why the credit routinely gets lost after publication

Because co-first authorship is expressed as a footnote and a typographic symbol rather than as structured, machine-readable metadata in most publishing workflows, the designation is prone to disappearing between the published PDF and the downstream systems that CVs, tenure dossiers, and bibliometric tools actually draw on. Two concrete, documented points illustrate the gap:

• PubMed’s own equal-contribution field is display-only. Per the National Library of Medicine’s technical bulletin, PubMed added an <EqualContrib> XML element in September 2017 so publishers could flag equally-contributing authors, and the flag does appear in the citation display. It is not, however, a searchable field — a search for “first author” in PubMed still returns only the person printed first, with no way to query for co-first status directly.
• A 2026 PLOS Biology piece argues the omission is systemic, not a one-off. Wenying Shou’s “We need to correct the wide-spread omission of equal contribution in article indexing” (PLOS Biology 24(4): e3003746, April 2026) documents that major indexing and discovery systems — PubMed, ORCID, Google Scholar among them — routinely strip the equal-contribution designation even when a journal’s own PDF states it clearly, and argues this distorts downstream credit: seminar invitations, citation-based metrics, and evaluation tools built on author position tend to default to treating only the printed-first name as “the” first author. The piece calls for standardizing machine-readable equal-contribution metadata (in XML formats such as JATS) and for major indexers to actually enforce it, rather than leaving it to inconsistent publisher-by-publisher implementation.

The practical downstream effect for research administrators and researchers alike: a CV, ORCID record, or institutional faculty-activity system that lists publications as a flat reference list — pulled from a citation manager or an automated ORCID/PubMed import — will often show a co-first author’s name in whatever position the source database happened to preserve, with no field to indicate “equal contribution” at all. The author has to manually annotate the entry (e.g., appending “(co-first author)” or “*equal contribution” next to the citation) for the distinction to survive into a CV or tenure/promotion dossier, and that manual step is easy to omit, inconsistently applied across a career’s worth of publications, and invisible to any automated CV-building or import tool. A preprint bibliometric analysis of roughly 480,000 PubMed/PMC records (Xu, “Trends in Equal-Contribution Authorship,” 2026, not yet peer-reviewed) reports that equal-contribution designations on biomedical papers have risen sharply since 2017 — meaning more researchers are affected by this recognition gap each year, not fewer.

Worked examples

Example 1. Two graduate researchers in the same lab independently designed and ran complementary sets of experiments for a single manuscript, contributing comparable intellectual and hands-on effort. The published paper lists them first and second, both marked with a dagger footnote reading “These authors contributed equally to this work,” followed by the supervising investigator as corresponding author. All three meet the underlying authorship criteria independently; the co-first designation only clarifies that the first two names should not be read as ranked relative to each other.

Example 2. A journal permits up to three co-first authors and asks that the equal-contribution footnote name every qualifying author explicitly rather than using a generic “the first N authors” phrasing, so a reader who only sees the citation (not the full footnote) can still identify exactly who shares the designation.

Counter-example. A paper lists a fourth-year PhD student first and a research assistant second, with no equal-contribution footnote and no stated shared symbol, purely because the PhD student happened to draft the manuscript last among the group. This is ordinary contribution-ranked authorship, not co-first authorship — the absence of an explicit equal-contribution marker means the byline order should be read as the group’s real ranking (or at least not stated to be unranked), regardless of how close the two contributions actually were in practice.

References

• ICMJE, “Defining the Role of Authors and Contributors” (Recommendations, current edition)
• ANSI/NISO Z39.104-2022, Contributor Roles Taxonomy (CRediT)
• National Library of Medicine, “Equal Contribution for Authors in PubMed,” NLM Technical Bulletin, September–October 2017
• Shou, W. “We need to correct the wide-spread omission of equal contribution in article indexing.” PLOS Biology 24(4): e3003746 (2026). doi:10.1371/journal.pbio.3003746
• Xu, B. “Trends in Equal-Contribution Authorship: A Large-Scale Bibliometric Analysis of Biomedical Literature.” arXiv preprint, 2026 (not peer-reviewed).