We continually interact with each other & share information to make decisions together as friends, families, committees, juries, interest groups and institutions. The question of how collective decisions are made dates back many centuries and has been vigorously studied in social psychology and political economics, but the biological basis of collective decision making in human brain is almost entirely unknown. This question is arguably at the heart of human society’s urgent need to communicate effectively & find better ways of arriving at global collective decisions. My research has provided new and potentially important computational models of collective decision-making based on empirical data from visual psychophysics. I propose to characterize the neurobiological basis of joint decisions by combining recent advances in economics and social cognitive neuroscience in a novel interdisciplinary research program that will ask 4 questions: (1)How do we learn to make better collective decisions? (2)What are the brain mechanisms underlying the different psychological components of collective decision making? (3)What makes some people better & some others worse at joint decisions? (4)What is the role of the neuro-modulatory hormones oxytocin & testosterone in collective decisions? I plan to develop a formal theoretical model of collective decision-making (Q1) & use converging evidence from complementary methodologies e.g. behavioural experiments & fMRI (Q2), structural brain imaging (Q3) and neuro-pharmacology (Q4) to test & advance the model. My findings could help understand what physiological events underlie our ability to learn from experience to contribute effectively to group efforts and also disclose the biological basis that makes some of us better and some worse at working in groups . The results may provide a clearer picture of how hormonal interactions in the brain strike a balance between trusting ourselves vs accepting the opinion of others.