Zhang, XBiswas, PKRanasinghe, DRoberts, H2026-05-222026-05-222026-03-14Accounting in Europe, ISSN: 1744-9480 (Print); 1744-9499 (Online), Informa UK Limited, ahead-of-print(ahead-of-print), 1-27. doi: 10.1080/17449480.2026.26220791744-94801744-9499http://hdl.handle.net/10292/21194This paper examines the relationship between private firm investment efficiency and board gender diversity in the UK from 2006 to 2019. The findings indicate that board gender diversity has a positive impact on private firm investment efficiency, with the effect primarily driven by reductions in overinvestment rather than underinvestment. Contrary to the critical mass hypothesis, even the presence of a single woman director is associated with improved investment efficiency, and this positive effect becomes more pronounced as women's representation increases. Further analysis reveals that the effect of women directors is stronger in financially constrained firms and more pronounced in non-family firms. The findings support regulations to increase the representation of women directors in private firms.© 2026 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. The terms on which this article has been published allow the posting of the Accepted Manuscript in a repository by the author(s) or with their consent.3502 Banking, Finance and Investment35 Commerce, Management, Tourism and Services3507 Strategy, Management and Organisational Behaviour5 Gender Equality9 Industry, Innovation and Infrastructure1501 Accounting, Auditing and Accountability1503 Business and Management3501 Accounting, auditing and accountabilityinvestment efficiencyUK private firmsboard gender diversitycritical masswomen directorsJournal ArticleOpenAccess10.1080/17449480.2026.2622079