Sunk cost fallacy is a cognitive bias that occurs when individuals or groups continue to invest resources in a decision or project because they have already invested a significant amount of resources in the past. In other words, the more we invest in something, the harder it is to walk away from it, even if it is becoming increasingly clear that it is a losing proposition.

This fallacy can have a major impact on group decision making, as it can lead to a lack of objectivity and an unwillingness to cut losses. When a group has invested a significant amount of resources into a project or decision, members may feel a sense of ownership and attachment to it, making it difficult for them to objectively evaluate whether or not it is still a viable option. This can lead to a situation where the group continues to pour resources into a failing project, instead of cutting their losses and moving on to something more promising.

Sunk cost fallacy can also lead to groupthink, where members of the group are unwilling to voice dissenting opinions or consider alternative options because they don't want to appear as if they are trying to undermine the group's investment. This can lead to a lack of diversity in thinking and can make the group more vulnerable to group polarization and biases.

In order to mitigate the effects of sunk cost fallacy in group decision making, it is important for the group to establish clear decision-making criteria and protocols. This can help to ensure that decisions are made based on objective data and analysis, rather than emotions or attachment to past investments. Additionally, encouraging dissenting opinions and open communication can help to ensure that all options are considered and that the group is able to make the best possible decision.