Abstract
AIM: Existing reviews on mental health disparities between deaf and hard-of-hearing (DHH) and hearing populations have focused predominantly on children, adolescents, or older adults, leaving a gap for working-age adults. We conducted a systematic review comparing the prevalence, incidence, and severity of any DSM-5-TR or ICD-11 mental disorder between DHH and hearing adults aged 18-60 years. We aimed to quantify disparities and examine disorder-specific patterns to inform future research, policy, and service development. METHODS: On 13 December 2025, we searched Ovid Medline, Embase, APA PsycINFO and Web of Science. We included analytical observational studies involving DHH and hearing adults aged 18-60 years, reporting mental disorder prevalence, incidence, or severity. Two researchers independently extracted data, and risk of bias (RoB) was assessed using the modified CLARITY tool. We narratively synthesised findings by aggregating outcomes at the study level using two approaches: summary and majority of the effect directions within a study. Subgroup syntheses examined outcome type, study RoB, age group and mental disorder category. RESULTS: Sixty studies (n = 8 578 466) met inclusion. In the summary-direction synthesis, 58.3% (35/60) of studies reported higher mental disorder outcomes for DHH adults, 21.7% (13/60) found no difference and 20.0% (12/60) had mixed findings; none indicated lower mental disorder outcomes for DHH. Under the majority-direction approach, 65.0% (39/60) showed higher mental disorder outcomes and 35.0% (21/60) no difference. These patterns were consistent across prevalence (62.8-72.1% higher) and severity (61.1% higher). Studies with higher RoB more often reported higher mental disorder outcomes (66.7-72.2%) than lower-RoB studies (54.8-61.9%), though both mirrored the overall synthesis. Effects were similar across younger (61.9-71.4%) and older adult samples (61.1-66.7% higher). Disorder-specific syntheses identified psychotic disorders, post-traumatic stress disorder and suicidal outcomes as having the strongest disparities (≥72.2% higher), followed by general mental disorders, anxiety and depression. Fewer than five studies examined each of the other disorders, thereby limiting conclusions for these disorders. CONCLUSIONS: Most available evidence indicates that the prevalence and severity of mental disorders are higher among DHH adults aged 18-60 years than among hearing adults, with limited evidence on incidence. No studies reported lower aggregated mental disorder outcomes for DHH adults. Addressing these disparities requires targeted intervention research, supported by population-based, longitudinal and (quasi-)experimental studies including comprehensive reporting of participant characteristics. This will inform more tailored interventions, improve screening and ultimately contribute to better mental health and quality of life for DHH adults.