This study investigated the relationship between adult self-efficacy in science and scientific competencies of Taiwanese citizens. Probability proportional to size sampling was used to select 1830 participants between the ages of 18 and 70. The research methods employed was survey research; analysis was conducted using hierarchical regression analysis. The results indicated that gender and self-efficacy have an explanatory power for scientific competencies. The predictive power of self-efficacy for scientific competencies was different for male and female groups. This study proposes several suggestions regarding science education policies in accordance with the research findings.