偏見不只影響遭受者的身心健康，也違背社會正義，為了改善這樣的狀況，研究者們致力於發展降低偏見的方案。不過這些降低偏見的方案通常針對種族或性別群體，卻甚少關心肥胖者與愛滋感染者。肥胖者、愛滋感染者不僅受研究社群忽視，他們在生活中亦不受重視（如醫療體系），因此本論文著眼降低人們對肥胖者、愛滋感染者的偏見。研究一發展降低偏見方案，透過撰寫支持肥胖者或愛滋感染者的短文，引發人們言行不一的認知失調感，可能進而改變其原本對肥胖者或愛滋感染者的偏見。研究一的結果顯示，只有撰寫愛滋主題的參與者其愛滋偏見分數低於其他組別，也就是說他們的行為違反本身對愛滋感染者的既有態度而產生認知失調，透過降低愛滋偏見來化解認知失調之不適感。不過，撰寫肥胖主題的參與者，其肥胖偏見分數則與各組無異，也就是說這種降低偏見的方案對肥胖者的偏見無影響。預試二發現，相對於愛滋感染者，肥胖者通常被視為較可控制自己身屬於該社會群體的命運（可控制性），且其較需為自己的遭遇負責。為了檢視人們認為肥胖者與愛滋感染者可控制性的差異，是否影響偏見方案的效果，研究二除了改良研究一的降低偏見方案外，更操弄可控制性的高低。研究二發現可控制性會調節降低偏見方案的效果－當人們認為成為愛滋感染者是自己可以控制時，降低偏見方案的效果較弱；反之，若人們認為成為愛滋感染者是自己不能控制的，則降低偏見的效果較強。不過，撰寫肥胖主題文章的參與者則無此效果。這樣的差異或許是因為人們對肥胖者與愛滋感染者的熟悉程度有所不同。相較於愛滋感染者，人們對肥胖者較為熟悉，對其態度不容易改變，不論是接受高可控或低可控的訊息操弄，都無法改變人們對肥胖者的想法。根據本論文結果，認知失調適合降低人們較不熟悉與視其較為不可控的社會群體，如愛滋感染者，不適合應用在人們較熟悉的社會群體，如肥胖者。本論文進一步討論降低偏見方案執行困難的原因，並提出可能的建議與後續的研究方向。 / Prejudice and discrimination may have negative consequences on subordinate group members’ physical and mental health, and prejudice and discrimination also violate social justice. To deal with these problems, researchers have dedicated to develop various interventions to counter prejudice and discrimination. However, most of these interventions focus on racial and gender minorities, and researchers largely ignore people with obesity and people with HIV/AIDS. The purpose of this thesis is to develop an intervention that could reduce prejudice against people with obesity, as well as to reduce prejudice against people with HIV/AIDS. According to cognitive dissonance theory, when people’s attitudes and behavior are inconsistent, they feel uncomfortable and in turn change their attitudes to eliminate the inconsistency (Festinger, 1957). Following the basic ideas in cognitive dissonance theory, in Study 1, I randomly assigned participants to write presumably counter-attitudinal essays, in which they write about people with obesity or people with HIV/AIDS (to advocate allocating more resources to them), or school policy change (a control group). The results showed that after writing about people with HIV/AIDS, participants displayed significantly lower prejudice against people with HIV/AIDS than participants in other conditions. However, after writing about people with obesity, participants’ prejudice against people with obesity remained at similar levels as participants in other conditions. According to findings in the second pilot study, the diverging findings in Study 1 may be due to that people with obesity were considered having more controllability and responsibility than people with HIV/AIDS. To further explore the effects of levels of controllability on intervention, I revised Study 1’s intervention and added a manipulation of controllability on people with obesity or on people with HIV/AIDS. The findings showed that controllability manipulation moderated the intervention effect on people with HIV/AIDS, but had no effect on people with obesity. Because participants were more familiar with people with obesity than with people with HIV/AIDS, it is possible that attitudes toward a familiar social group are more difficult to be changed than the attitudes toward an unfamiliar social group, The results suggested that interventions based on cognitive dissonance principle have effects on unfamiliar social groups, such as people with HIV/AIDS. Obstacles with prejudice reduction interventions were further discussed; suggestions and future research directions were offered.