Background: There is rising public concern over the widening health inequalities in many countries. The aim of this study was to clarify the associations of socioeconomic status (SES)-related variables, such as levels of household disposable income and employment status, and lifestyle factors with mental health conditions among Japanese adults aged 40 to 69. Methods: A cross-sectional study of 3085 participants (1527 males and 1558 females) was undertaken by using a self-administered questionnaire that included the Japanese version of the 12-item General Health Questionnaire (GHQ-12) and questions related to socioeconomic and lifestyle factors. Results: The prevalence of poor mental health conditions, represented by a GHQ-12 score of 4 or more, was 33.4% among males and 40.4% among females. Males whose annual household disposable income was less than 2 million yen had significantly higher GHQ-12 scores than those with an annual household disposable income above 2 million yen. As per binary logistic regression analyses, short sleep duration and the absence of physical exercise were significantly related to poor mental health conditions among both males and females. Among females, a household disposable income of less than 2 million yen could be a risk factor for poor mental health conditions. Age and habitual drinking were inversely associated with poor mental health conditions. Conclusions: Low levels of household disposable income and unhealthy lifestyle factors were significantly associated with mental health conditions. These results suggest the importance of improving unhealthy lifestyle behaviours and developing effective health promotion programmes. In addition, there is a need for social security systems for people from different socioeconomic backgrounds.
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