Testing Continuous Age as an Effect Modifier of the Association Between Diminished Ikigai and Psychological Distress in Two Long-Term Disaster Contexts in Japan

Background/Objectives: Mental-health research often represents chronological age as a threshold or adjustment variable, although psychosocial associations may vary across adulthood. We tested whether continuous age modified the association between a context-specific single-item indicator of diminished ikigai and psychological distress in two Japanese communities with long-term disaster consequences. The ikigai indicators differed by context and were not treated as equivalent multidimensional measures. Methods: Secondary cross-sectional analyses used Miyakejima 2020/21 (N = 158; complete-case n = 147) and Okuma 2024 (N = 134; 111 valid 28-item General Health Questionnaire (GHQ-28) classifications; complete-case n = 110). Age-by-ikigai interactions were primary; fixed <65/≥65 analyses and robustness checks were secondary or exploratory. Results: In Miyakejima, the estimated interaction odds ratio (OR) was 2.00 per 10 years (95% confidence interval [CI] [1.03, 3.91], p = 0.042; likelihood-ratio p = 0.029), with substantial uncertainty at age extremes; the threshold interaction was unsupported. In Okuma, low/absent ikigai was associated with GHQ-28 high-risk status (OR = 12.94, 95% CI [4.96, 33.8], p < 0.001), while age modification was not detected (interaction OR = 0.88, 95% CI [0.44, 1.76], p = 0.711). The historical Miyakejima reanalysis also did not provide detectable evidence of corresponding modification. Conclusions: Within these context-specific samples, continuous-age modelling yielded preliminary evidence of heterogeneity in Miyakejima not captured by a 65-year threshold. Larger longitudinal studies should evaluate this signal’s precision and transferability.