People may feel stressed when engaging with contentious topics, such as migration. However, when individuals learn that their opinion-based ingroup is growing or shrinking, they may experience this stress in different ways, namely as a threat or a challenge. In a preregistered study (N = 203 Dutch university students), we examined among host society members how progressive and conservative changes (vs. stability) in public opinion on migration interacted with their political ideology to influence cardiovascular reactivity indicative of challenge and threat. Electrocardiography, impedance cardiography, and blood pressure were continuously measured during a one- to three-minute speech task in which participants reflected on the future of interethnic relations in the Netherlands. Additional self-reported outcomes, including demand and resource appraisals and prejudice towards migrants, were assessed after the speech task. As predicted, progressive change (vs. stability) in public opinion led leftists to exhibit a cardiovascular pattern indicative of relative challenge (relatively lower total peripheral resistance and higher cardiac output) and rightists to display a cardiovascular pattern indicative of relative threat (relatively higher total peripheral resistance and lower cardiac output). Additional analyses suggest that progressive change (vs. stability) increased leftists' resource appraisal regarding the speech and reduced their prejudice towards migrants, while both progressive and conservative changes (vs. stability) increased rightists' prejudice. These findings highlight that a growing opinion-based ingroup size can function as a resource for coping with the stress of forming and expressing one's opinion on a sensitive societal issue
