Gender Bias in ASD Diagnostic and Screening Tools: A Systematic Review and Meta-Analysis

(1) Background: ASD diagnostic and screening instruments were historically developed on predominantly male samples, raising concerns about their differential validity across sexes. (2) Objective: This meta-analysis quantified the magnitude of gender bias in ASD diagnostic and screening tools and examined its consistency across instruments, age groups, and clinical contexts. (3) Method: A systematic search was conducted in APA PsycINFO, ERIC, Dialnet, and PsicoDoc (2015–2025) following PRISMA 2020 guidelines. Fourteen empirical studies were included (N = 55 to ~3,000,000). Effect sizes were computed as Fisher r-to-z transformed coefficients and pooled using a random-effects model. Heterogeneity was assessed via Cochran’s Q and I2; methodological quality was assessed via the CRF-QS. (4) Results: The pooled effect size was μ^ = 0.29 (95% CI: 0.11–0.47, z = 3.11, p = 0.001), indicating systematic bias favoring detection of externalizing, prototypically male profiles. Substantial heterogeneity was observed (I2 = 99.93%, Q(13) = 11,417.61, p < 0.001, τ2 = 0.11), with a prediction interval of −0.40 to 0.97. Funnel plot asymmetry suggested possible publication bias (p = 0.01). (5) Conclusions: ASD diagnostic instruments systematically underperform in identifying female presentations of autism. Bias operates across screening and formal evaluation stages, is not corrected by specialist assessment, and is associated with diagnostic delay and increased psychopathological burden. Revision of diagnostic tools and clinical training is urgently warranted.