Previous researchers have suggested that people in collectivistic societies are connected with others through relational ties and are intragroup-oriented (e.g., Yuki & Takemura, 2013). There have been virtually no studies focusing on how these individuals may view disadvantaged groups to which their significant others belong (i.e., extended disadvantaged group members). Applying the cultural analysis of intergroup dynamics in activism for disadvantaged groups, we investigated whether people engaged in past activism due to their direct and extended group membership with disadvantaged groups. According to a survey data (N = 743 adult participants in Taiwan and N = 717 adult participants in the U.S.) targeting activism for sexual minorities and mental patients, respectively, we separated people into four groups according to how they relate to sexual minorities or mental patients: Directly disadvantaged groups (those who consider sexual minorities or mental patients as their important ingroups), extended disadvantaged groups (those who have significant others who are sexual minorities or mental patients), other disadvantaged groups (those who have disadvantaged group memberships other than sexual minorities or mental illness), and advantaged group (those who report not having any disadvantaged group membership). We found that in comparison to other disadvantaged group members, directly disadvantaged group members are more likely to engage in past activism for their own groups. This tendency is much stronger in the U.S. than in Taiwan for sexual minorities. In addition, in comparison to those with other disadvantaged group memberships, extended disadvantaged group members are more likely to engage in past activism for sexual minorities and mental patients, respectively, in Taiwan, but not in the U.S. We plan to conduct another survey study refining some of our measures. We expect to explore potential mediators to shed light on how activism may be similarly or differentially motivated in Taiwan and the U.S..