There's one brain bias that affects 80% of adults and it has a familiar name you may not expect: optimism. Not always thought of as a cognitive mechanism, the optimism bias leads people to overestimate the likelihood of positive outcomes and to underestimate the likelihood of negative outcomes. It can be hugely helpful in our social lives and in keeping us motivated even if the trade off is, at times, the denial of reality. So where does this cognitive bias come from? Are we born with it, or do we develop it as we grow? Developmental psychologist Lori Markson compiles research about how optimism works in babies and young kids, and how that may help us to understand why we adults are the way we are. This video was filmed at the Los Angeles Hope Festival, a collaboration between Big Think and Hope & Optimism.