Abstract
Suicide in Japan remains elevated and uneven across regions. We hypothesized that (H1) unemployment and (H2) living alone each increase suicide mortality, and that (H3) their combination yields more-than-additive risk, especially among middle-aged men. Using specially tabulated mortality data (2018-2022) from the Japan Suicide Countermeasures Promotion Center, we cross-classified deaths and denominators into 24 strata by sex, age (20-39, 40-59, ≥60), employment (employed/unemployed), and cohabitation (with others/alone). Five-year average rates per 100,000 were computed; between-group differences were tested with chi-square (Holm-adjusted contrasts). Additive interaction between unemployment and living alone was quantified with the Interaction Contrast (ICR) and Synergy Index (SI), and Akita rates were benchmarked against national strata. Prefecture-level quantification and national benchmarking are rarely reported in Japan. Rates differed significantly across employment-by-cohabitation groups in every sex-by-age stratum (p < 0.001). Unemployment and living alone each elevated risk, with the highest rate reported among unemployed men aged 40-59 who were living alone (317.1; >14× employed, cohabiting peers at 22.1). Additive interaction was strongest in men aged 40-59 (ICR = 198.3; SI = 3.05) and present in men aged 20-39 and ≥60; among women, interaction was most evident at the ages of 40-59 and sub-additive at ≥60. Compounded effects among men were consistently larger in Akita than nationally, whereas the largest absolute burden fell on unemployed men aged ≥60 who were living with others (203 deaths). The novelty of this investigation lies in quantifying additive interaction with national benchmarking and contrasting per capita risk with absolute burden to guide dual-track prevention. The findings are ecological.