Subthreshold depression is associated with altered facial expression and impression formation via subjective ratings and action unit analysis

Depression is often linked to reduced facial expressivity and to biases in recognizing others' emotions. Whether subthreshold depression (StD)-a putative prodromal stage-shows comparable alterations remains unclear. We recorded 10‑second self‑introduction videos of Japanese undergraduates (ratees; n = 64) and obtained subjective impression ratings from a separate group (raters; n = 63). Both groups completed the Beck Depression Inventory‑II (BDI‑II). Raters' depressive tendency was not associated with their impression ratings (p > .10, Benjamini-Hochberg corrected). In contrast, ratees with StD (BDI‑II = 11-20) received significantly lower scores on positive items-expressive, natural, friendly, likeable-than healthy ratees (BDI‑II = 1-10; partial η² = 0.18-0.70). Automated analysis with OpenFace 2.0 showed higher presence/intensity of AU01 (Inner Brow Raiser), AU05 (Upper Lid Raiser), AU20 (Lip Stretcher), AU25/26/28 (mouth‑opening AUs) in StD faces; five of these AUs correlated with BDI‑II after false‑discovery‑rate correction (q < 0.05). Subthreshold depression was associated with muted positive expressivity and distinct eye‑ and mouth‑movement patterns, but did not bias observers' first‑impression judgements. The observed AU signatures may aid early identification of individuals at risk for clinical depression. Together, our findings suggest that subthreshold depression is associated with alterations in facial expressivity, particularly in positive expressions, while not significantly influencing how others perceive those expressions.