Accurate, rational, and scientific decision making is now considered to be the most important skill in science education. Many studies have found that overconfidence bias is one of the cognitive biases hindering people from achieving such decision making. Gender and country play crucial roles in overconfidence bias. For instance, some particular cultures and genders tend to be more overconfident than others. However, whether or not the two variables interact to influence overconfidence bias also indirectly influences decision making, especially in the context of science education. The purpose of this study is to identify the effects of country and gender on performance, confidence, and overconfidence bias in the samples of Indonesian and Korean high-school students while doing on a biology exam. The twenty-one American Association for the Advancement of Science (AAAS) questions on the topics of genetics and evolution were administered to 297 Indonesian and 235 Korean high-school students, in their first and second years. Every question was featured with a question asking students how confident they are in answering the question correctly. The two-way Analysis of Variances (2-way ANOVA) test was used to answer the research questions. Based on the analyses, we found no significance interactional effects of gender and country in test scores. In contrast, we found a significant interactional effects in both confidence in genetics and evolution. Regarding overconfidence bias, for which that we merged both concepts, we found that country had a higher influence on students' overconfidence bias than did gender. Additionally, we found the hard-easy effect phenomenon followed overconfidence bias phenomenon. The relationships between country, gender, science education, cognitive bias, and overconfidence bias are discussed. Suggestions for reducing overconfidence bias are also provided.