What Are Preprints? Understanding Pre-Peer-Review Scientific Papers

How Preprints Work

The traditional publication process works like this: researchers complete a study, write a manuscript, submit it to a journal, wait for peer reviewers to evaluate it, revise the manuscript based on feedback, and eventually see it published. This process typically takes 6 to 18 months from submission to publication, and sometimes longer. During that entire period, the findings are invisible to the rest of the scientific community.

Preprints bypass the waiting period. Researchers upload their manuscript to a preprint server, where it is screened for basic standards (not spam, not plagiarism, contains actual scientific content) and then made publicly available, usually within 24 to 48 hours. The manuscript receives a permanent digital object identifier (DOI), making it citable. The authors can then simultaneously submit the paper to a journal for traditional peer review while the preprint is already available for anyone to read.

Most preprint servers allow authors to post updated versions of their manuscript as it evolves through peer review. When the paper is eventually published in a journal, the preprint page typically links to the published version, and many journals link back to the preprint. The preprint remains available even after publication, preserving the record of the original version.

Major Preprint Servers

arXiv (pronounced "archive") was founded in 1991 and is the oldest and largest preprint server. It primarily serves physics, mathematics, computer science, quantitative biology, quantitative finance, statistics, and electrical engineering. In these fields, preprints are so deeply embedded in the culture that many researchers read arXiv daily and consider the preprint the primary version of a paper, with the journal publication being a formality.

bioRxiv (pronounced "bio-archive") launched in 2013 and serves the biological sciences. It has grown rapidly and now receives thousands of preprints per month. BioRxiv played a significant role during the COVID-19 pandemic, when the urgency of sharing research results made the traditional publication timeline unacceptable.

medRxiv (pronounced "med-archive") launched in 2019 specifically for clinical and health sciences research. Because medical preprints can directly influence patient care and public health decisions, medRxiv applies slightly more rigorous screening than bioRxiv, including checks for patient consent and ethical approval.

Other notable preprint servers include SSRN (social sciences, economics, law), ChemRxiv (chemistry), EarthArXiv (earth sciences), and PsyArXiv (psychology). All are free to read and free for authors to use.

Advantages of Preprints

Speed. Preprints are available within days of completion, compared to months or years for traditional publication. During fast-moving situations like disease outbreaks, natural disasters, or rapidly evolving technology, this speed difference can be critical. The COVID-19 pandemic demonstrated this clearly: preprints enabled researchers worldwide to build on each other's work in real time, accelerating vaccine development and treatment research.

Open access. All preprints are freely available to anyone with an internet connection, regardless of institutional affiliation or ability to pay subscription fees. This democratizes access to scientific knowledge, allowing researchers in developing countries, independent scholars, and the general public to read the latest findings.

Priority establishment. In competitive research fields, posting a preprint establishes a public, timestamped record of a discovery. This can protect against being scooped by another research group working on the same question. The preprint's DOI serves as a permanent record of when the work was first made public.

Community feedback. Preprints can receive feedback from the broader scientific community before formal peer review. Researchers can comment on preprints, identify errors, suggest improvements, and offer alternative interpretations. This crowd-sourced feedback can improve the paper before it reaches the journal review stage.

Limitations and Risks of Preprints

No peer review. The most fundamental limitation is that preprints have not been evaluated by expert reviewers. Peer review is imperfect, but it does catch errors, identify methodological weaknesses, and improve the clarity and accuracy of papers. A preprint may contain mistakes that peer review would have caught, ranging from minor statistical errors to fundamental design flaws.

Premature media coverage. Journalists sometimes report on preprints as if they were peer-reviewed publications, especially when the findings are newsworthy. This can spread preliminary or incorrect results to the public before they have been properly vetted. During the COVID-19 pandemic, several widely reported preprints were later found to have serious flaws and were not published in peer-reviewed journals.

Variable quality. Because preprint servers perform only basic screening, the quality of preprints varies enormously. Some are polished manuscripts from leading research groups that will be published with minimal changes. Others are preliminary results, poorly designed studies, or even work containing significant errors. Readers must evaluate each preprint individually rather than relying on the imprimatur of a reputable journal.

Citation complications. Citing preprints in academic work requires noting that the paper has not been peer-reviewed. Some journals and grant agencies have specific policies about citing preprints. The norms around preprint citation are still evolving and vary across disciplines.

How Preprints Fit into the Publishing Ecosystem

Preprints and peer-reviewed journals are not competing alternatives. They serve complementary roles in the research communication system. Preprints provide immediate visibility and enable rapid feedback, while journals provide structured peer review, editorial curation, and long-term archiving. Most preprints eventually appear as peer-reviewed journal articles, sometimes with substantial changes based on reviewer feedback. Studies tracking the fate of preprints in biology have found that roughly 70% of bioRxiv preprints are eventually published in peer-reviewed journals, typically within 6 to 12 months of the initial preprint posting.

Different fields have very different attitudes toward preprints. In physics and mathematics, posting to arXiv before journal submission has been standard practice for decades, and the preprint is often considered the definitive version of the work. In biology and medicine, preprints are a more recent phenomenon and some researchers remain skeptical, particularly in clinical fields where unvetted findings could influence patient care decisions. In the social sciences, adoption varies widely by discipline. Understanding the norms of the field you are reading helps you calibrate how to interpret preprints in that area.

Funding agencies have increasingly recognized preprints as legitimate research outputs. The National Institutes of Health, the Wellcome Trust, and many European funding bodies now allow researchers to cite preprints in grant applications and progress reports. This institutional endorsement has accelerated preprint adoption across fields that were previously resistant to the practice.

How to Evaluate a Preprint

When reading a preprint, apply the same critical evaluation skills you would use for any paper, but with additional scrutiny. Check the methods section carefully, because this is where peer reviewers most commonly identify problems. Look at the authors' credentials and institutional affiliations. Check whether the preprint has received any public comments or critiques on platforms like PubPeer or in the preprint server's comment section. Search for whether a peer-reviewed version has been published since the preprint was posted, and if so, read the published version instead, as it may contain important corrections.

Be especially cautious about preprints making dramatic or counterintuitive claims. Extraordinary claims require extraordinary evidence, and without peer review, the bar for skepticism should be higher. If a preprint claims to have cured a major disease, overturned a well-established theory, or found a revolutionary new technology, wait for peer review and replication before accepting the claim.

Track the preprint's trajectory over time. If a preprint posted a year ago has still not been published in a peer-reviewed journal, that may indicate that the work had problems that prevented it from passing peer review. Conversely, if the peer-reviewed version contains significant changes from the preprint, those changes often reflect issues identified during review that are worth understanding. Preprint servers typically link to the published version when one exists, making it straightforward to check.