Optimism bias is the tendency for individuals and groups to overestimate the likelihood of positive outcomes and underestimate the likelihood of negative ones. This bias can manifest in many ways, from underestimating the time and resources required to complete a project to overestimating the potential success of a new product or business venture.

In group decision making, optimism bias can lead to a phenomenon known as "groupthink," where members of a group become so convinced of the correctness of their initial assumptions that they fail to consider alternative perspectives or options. This can be particularly dangerous when the group is making high-stakes decisions, as it can lead to poor outcomes or even catastrophic failure.

One reason for the prevalence of optimism bias in group decision making is that people tend to want to be seen as positive and forward-thinking, and they may be reluctant to voice dissenting opinions. Additionally, group dynamics can create pressure to conform to the opinions of the majority, further exacerbating the bias.

To combat optimism bias in group decision making, it is important to actively encourage dissenting opinions and to create a culture where it is safe to speak up. Additionally, tools such as decision analysis and scenario planning can be used to help groups consider multiple perspectives and potential outcomes.