Abstract: Stress is a pervasive experience that affects individuals across multiple domains of daily life. A survey in the Lancet estimated that more than 500 million individuals worldwide experience stress-related mental health problems annually. Consequently, public concern has grown substantially, making the identification strategies to enhance mental health and resilience in the context of stress an urgent priority. Dose-response theory posits a linear relationship where stress negatively impacts mental health—a perspective widely held by both the public and academia. However, over the past 15 years, research in positive psychology has suggested that moderate stress may foster resilience and adaptive psychological outcomes. According to stress inoculation theory, stress and mental health follow an inverted U-shaped trajectory. Thus, no consensus exists between the linear and curvilinear models.
A systematic search was conducted across Web of Science, ProQuest, EBSCO, and PubMed, as well as the Chinese databases CNKI and WanFang Data to identify empirically studies reporting quadratic effects of stress on mental health. Data were analyzed in R using the metafor package. A three-level meta-analysis synthesized linear (r) and semi-partial (sr) effect sizes and tested for moderators. Publication bias was assessed using funnel plots, Egger-MLMA regression, and the trim-and-fill method. A total of 23 studies (yielding 59 linear and 59 quadratic effect sizes) comprising 31, 422 participants were included to compare the two hypotheses.
Results indicated a moderate negative correlation for linear stress terms and a small negative partial correlation for quadratic terms. Overall, the linear association between stress and mental health remains dominant. However, given statistical power constraints, these findings offer some support for the curvilinear hypothesis, suggesting a potential curvilinear relationship between stress and mental health. Meta-regression analyses further revealed that stress measurement methods significantly moderated the curvilinear association, while population characteristics and types of mental health outcomes moderated the linear relationship. Specifically, stress was negatively associated with mental health in healthy populations, but this effect was non-significant in trauma-exposed groups. When outcomes reflected negative mental health, stress consistently predicted poorer outcomes, but showed no significant linear effect on positive mental health outcomes. By contrast, no statistically significant moderation was detected in association between the quadratic component of stress and mental health. Nevertheless, substantial heterogeneity was observed, indicating that future research should pay greater attention to nonlinear patterns linking stress and mental health and the conditions under which such patterns emerge. In particular, more attention should be given to the potential for individuals to fulfill their potential and experience positive growth under moderate levels of stress.
These findings indicate a small but significant negative curvilinear relationship between stress and mental health, suggesting that moderate levels of stress may promote more favorable psychological outcomes. The variability of the linear relationship across populations underscores the importance of considering sample characteristics and outcome measures in future research. A more precise understanding of these mechanisms and boundary conditions is critical to clarifying the complex role of stress on mental health.

Key words: stress, mental health, curvilinear relationship, three-level meta-analysis