Many studies have confirmed the positive effects of comprehensive sexuality education on the development of children's sexuality, such as the acquisition of sexual health knowledge. However, little is known about the impact of comprehensive sexuality education on children's social development, although several core aspects of the approach stress social concepts such as fairness, respect and equality. This study examined whether comprehensive sexuality education could weaken the effect of in-group bias on social decision-making towards friends and strangers. Compared to the students in a control group, who never received comprehensive sexuality education, we found that students who received six years (72 lessons) of comprehensive sexuality education had a less in-group bias towards strangers regarding trust and fairness as reflected in a trust game, an ultimatum game and a dictator game. These results suggest that comprehensive sexuality education weakened the effect of in-group bias in the experimental group and encouraged egalitarianism in the experimental group's interpersonal cooperation and social decision-making. These findings suggest that comprehensive sexuality education could positively impact children's interpersonal attitudes and contribute to their social development.