The interplay between perceived stress, psychological flexibility, and interpretation biases in undergraduate mental health.

Stress is a well-established risk factor for psychological burden, yet individual vulnerability varies, suggesting that stress intensity and cognitive-psychological factors might be implicated in this process. This study examined how perceived stress at two different phases (moderate vs. high stress) relates to psychological symptoms and positive mental health through lower levels of psychological flexibility and whether interpretation biases in response to social ambiguity moderate this effect. A total of 228 healthy undergraduate students were assessed once at the beginning of a semester (T0: moderate stress phase) and 12 weeks later during exams (T1: high stress phase). Psychological flexibility mediated the relationship between stress and both psychological symptoms and positive mental health at both time points. Notably, baseline negative interpretation biases moderated this mediation effect for depressive symptoms at T1 (high stress phase), such that lower psychological flexibility was linked to higher depression levels only among individuals with moderate to high levels of negative interpretation bias. In turn, the absence of negative biases buffered against these effects. No such moderation effects were observed under moderate stress (T0) or for positive interpretation biases at either time point. These findings highlight the importance of interventions that combine techniques to foster psychological flexibility and reduce negative interpretation biases, particularly for individuals under high stress or vulnerable to depression.