The Solo-Authorship Rebound

Matsui (arXiv 2607.10780): across 300M+ OpenAlex works and 26 fields, the decades-long decline in solo-authored papers halts or reverses at ChatGPT's November 2022 release — positive trend break in 23 of 26 fields, largest in Engineering (+2.5 pp/yr) and Business (+1.9), absent in Chemistry and Physics and negative in Arts and Humanities. It survives conditioning on author history and is strongest among authors who had *never* published alone; solo papers stay near their authors' coauthored content while narrowing 23% in breadth and tilting toward computational work. A solo paper is proposed as an observable behavioral trace of AI substituting for a human collaborator — but the design is an interrupted time series with no untreated unit, roughly half the pooled break is venue composition, and the disciplinary ordering, not any single number, is the actual argument