Co-first authorship (also called joint first authorship, shared first authorship, or equal contribution) means two or more authors are formally marked as having contributed comparably to a paper, so that traditional byline position — where “first author” signals “did the most hands-on work” — does not misrepresent the group’s actual division of labor.
It is layered on top of ordinary co-authorship, not a substitute for it: every co-first author still has to independently meet the applicable authorship criteria (most commonly ICMJE’s four-part test) before the equal-contribution question is even relevant. For the underlying concept, worked examples, and the CRediT “degree of contribution” distinction, see CASRAI’s Co-First Authorship dictionary entry. This guide goes one level deeper into the part researchers usually get stuck on: what the mechanism actually looks like at specific publishers, and what happens to that designation after the paper is published — in reference managers, CVs, and citation databases.
How the major publishers actually handle it
ICMJE deliberately leaves author order and equal-contribution notation to the author group and the journal — there is no single cross-publisher standard. In practice, that means the exact footnote wording, the symbol used, and how many authors can share the designation varies by publisher, and sometimes by journal within the same publisher. The three most commonly asked-about are covered below, verified against each publisher’s own current guidance where a live, public policy statement exists.
Nature / Nature Portfolio
Nature’s author guidance states that if authors regard it as essential to indicate that two or more co-authors are equal in status, they may be identified by an asterisk symbol with the caption “These authors contributed equally to this work,” placed immediately under the address list. If more than three co-authors are equal in status, Nature’s guidance says that should be indicated in the author contributions statement instead of the symbol/footnote mechanism. Nature Portfolio journals separately allow one set of co-authors to be marked as having contributed equally (joint first authorship) and, independently, a second set to be marked as having jointly supervised the work (joint senior/last authorship) — using a distinct symbol for each group. Individual Nature Portfolio journals (for example, Scientific Data) implement the same underlying mechanism with journal-specific instructions for the footnote text, so always check the specific journal’s own formatting guide before submission rather than assuming every Nature-family title behaves identically. Source: Nature’s author formatting guidance and Nature Portfolio authorship editorial policy (nature.com), current as of this writing.
Elsevier
Elsevier’s standard Guide for Authors template (used, with journal-level variation, across ScienceDirect titles) permits a numbered footnote against the relevant author names reading “These authors contributed equally to this work,” using the same superscript-footnote mechanism the guide also uses for present/permanent-address notes and designating a senior author. Because Elsevier requires a CRediT contributor statement from the corresponding author on submission, its guidance also directs authors to state equal contribution in the CRediT/author-contributions section of the manuscript as well as (not instead of) any footnote — the two mechanisms are meant to reinforce each other, not replace one another. As with Nature, the exact wording and formatting requirements can vary by individual Elsevier journal, so the specific journal’s “Guide for authors” page is the authoritative source at submission time.
PLOS
PLOS’s currently published authorship and submission-guidelines pages (journals.plos.org) describe authorship requirements built around the CRediT taxonomy — every author must be assigned at least one CRediT contributor role at submission, and PLOS supports CRediT’s optional “degree of contribution” qualifier (lead / equal / supporting) on a given role, which is the mechanism-of-record PLOS points authors toward for signalling equal contribution on a specific task. As of this review, PLOS’s live public guidance pages do not spell out a separate, named footnote-symbol convention or a stated numeric cap on co-first authors the way Nature’s does explicitly. In practice, published PLOS papers commonly still carry a plain-language “These authors contributed equally to this work” note near the author list, following the same broad convention used across the field — but authors should not assume a specific symbol, wording, or author-count limit without checking the target PLOS journal’s current submission guidelines directly, since this guide could not verify a codified PLOS-wide policy on that specific point at the time of writing.
Other publishers
Wiley, Springer, SAGE, and most other major publishers support some version of the same “contributed equally” footnote, generally documented in each journal’s own “Instructions for Authors” rather than a single company-wide policy page. The safest working assumption for any target journal: the mechanism (a symbol next to the relevant names, resolving to a footnote) is close to universal, but the exact wording, the number of authors it can cover, and whether it needs to be echoed in a separate contributions statement are journal-specific decisions you should confirm before submission, not something to infer from a different publisher’s policy.
How co-first authorship is cited and tracked after publication
The footnote a journal prints on the article PDF is not the end of the story — it has to survive the trip into reference managers, CVs, and citation databases, and in practice it often doesn’t cleanly.
- Reference-manager exports (Zotero, EndNote, Mendeley, BibTeX). These tools import the author list and order from the source metadata (typically Crossref or the publisher’s own record), but there is no standard citation-metadata field for “equal contribution” that most reference managers surface or preserve. The exported citation will show the same author order as the source record, with no visual indicator that two or more of those names are formally co-first — the equal-contribution marker is a presentation-layer footnote on the published PDF/HTML, not something reliably carried in the structured citation metadata most tools consume.
- Indexing databases (PubMed, Scopus, Web of Science, Google Scholar). Coverage of equal-contribution status is inconsistent across these systems. PubMed added a dedicated XML element for equal-contribution flags in 2017, and that flag can appear in PubMed’s citation display, but it is not a searchable field — you cannot query PubMed specifically for “co-first authored” papers. Scopus, Web of Science, and Google Scholar generally treat the byline as a flat, ordered author list for indexing and metric purposes (h-index, citation counts, author-position-based scoring) without a parallel mechanism to flag or search on equal contribution at all.
- CVs, biosketches, and faculty-activity systems. Because the upstream databases mostly don’t preserve the designation, it is the author’s own responsibility to manually annotate a co-first-authored publication on a CV, NIH biosketch, or tenure/promotion dossier — commonly by appending a note such as “(co-first author)” or “*equal contribution” directly after the citation entry. This is a manual step, not something an automated ORCID sync, citation-manager export, or faculty-activity-system import will do for you, and it is easy to omit or apply inconsistently across a publication list built up over years.
- What researchers commonly do to preserve the distinction. Beyond manually annotating CV entries, common practice includes: keeping a copy of the published PDF/footnote as documentation for tenure and grant committees who may not otherwise see it; explicitly noting equal-contribution status in cover letters or personal statements for funding and promotion applications where author position might otherwise be read literally; and, where the target venue supports it, using CRediT’s “equal” degree-of-contribution qualifier on the relevant role(s) in addition to the footnote, since a CRediT statement is more likely to be captured as structured metadata than a PDF-only footnote.
The result is a real, documented gap between what a journal prints and what downstream systems preserve. CASRAI’s Co-First Authorship dictionary entry documents a 2026 PLOS Biology commentary on this exact problem, which argues that major indexers routinely strip the equal-contribution designation even when a journal’s own record states it clearly, and calls for standardized machine-readable equal-contribution metadata (in formats such as JATS XML) rather than leaving preservation to inconsistent, publisher-by-publisher implementation. Until that changes, the practical takeaway for researchers is: don’t assume the footnote travels with the citation — plan to document co-first status yourself wherever it matters for evaluation.
Why the practice exists
Contribution-ranked byline conventions, most consistently used in biomedicine and the life sciences, read the first-author slot as “the person who did the most hands-on work.” That convention breaks down whenever two or more people genuinely did comparable amounts of that work: a single slot can’t represent two people at once, and simply picking one name to print first would imply a ranking that isn’t accurate. Co-first authorship resolves the mismatch without abandoning byline order altogether — someone is still printed first for citation and indexing purposes — by attaching an explicit, visible note that the order does not reflect unequal contribution. It is worth keeping separate from CRediT’s “equal” degree-of-contribution qualifier, which can mark two contributors equal on one specific named role (for example, both “equal” on Formal analysis) without saying anything about byline position; a co-first footnote and a CRediT statement are commonly published side by side as two independent, complementary signals rather than substitutes for each other.
Frequently asked questions
Is there a universal limit on how many authors can be co-first?
No single cross-publisher rule exists. Nature’s guidance specifically routes cases of more than three equally-contributing authors to the author-contributions statement rather than the symbol/footnote. Other publishers set their own practical limits, if any, in journal-specific instructions — check the target journal directly rather than assuming a number.
Does co-first authorship change who counts as “the” first author for citation purposes?
No. Whatever name is printed first in the byline is still what most citation styles, indexes, and reference managers will display and sort by as “the first author” — the equal-contribution footnote is a disclosure layered on top of that printed order, not a change to it. This is exactly the gap described above: the citation format itself usually can’t represent “these two names are tied.”
Is a co-first authorship footnote the same thing as a CRediT statement?
No. A co-first (or co-senior) footnote is a byline-level statement about author order; a CRediT contributor statement is a structured, role-by-role account of who did what, defined by ANSI/NISO Z39.104-2022. They commonly appear on the same paper and are not substitutes for each other — CRediT’s own “equal” qualifier can mark contributors as equal on a specific role without implying anything about byline position.
How is co-first authorship different from co-corresponding authorship?
They mark different things. Co-first authorship signals comparable contribution among authors typically listed early in the byline; the corresponding-author role is an operational designation (who handles submission, peer-review correspondence, and post-publication queries) that publishers increasingly allow to be shared between two or more people. See CASRAI’s guide to how journals handle a shared corresponding-author role for how that convention works and where it overlaps with, but doesn’t duplicate, co-first authorship.
Related CASRAI resources
- Co-First Authorship — the operational definition, worked examples, and counter-example
- Co-Authorship — the broader byline-order and signalling conventions co-first authorship sits within
- Co-Corresponding Authors: How Journals Handle a Shared Corresponding-Author Role
- Pros and Cons of First Authorship
- CRediT Author Statement Samples
- CRediT’s Degree-of-Contribution Qualifier (Lead / Equal / Supporting)
- First Author vs. Second Author
