Omission bias
We judge harm caused by doing something as worse than the same harm caused by doing nothing.
What it means
Omission bias is the tendency to judge harmful actions as worse, or less moral, than equally or more harmful inactions that produce the same outcome. Faced with two options that lead to identical results, people often prefer the one that comes about through omission, feeling less responsible and less culpable for harm they allowed than for harm they caused. The proposed mechanisms include a sense that acts carry more causal agency and intention than failures to act, greater anticipated regret and blame for commissions, and a default toward the status quo. The bias can be normatively defensible in some moral frameworks but becomes costly when inaction itself is the riskier choice, as when fear of a small action-linked harm blocks a far larger benefit. It matters in medicine, public policy, and ethics — most visibly in vaccination, where reluctance often reflects weighing a vaccine's rare side effect more heavily than the larger risk of the disease left unaddressed.
Examples
Parents who would accept a small risk of a child dying from a disease refuse a vaccine with an even smaller risk of a fatal side effect, because a death caused by their action feels worse than one they merely failed to prevent.
An investor keeps a fund quietly losing money rather than switching, because a loss after selling would feel like their doing, while sitting still feels like nobody's fault.
A referee swallows the whistle in the final minute: awarding a wrong penalty would be blamed on him, while missing a real one merely lets the game decide itself.
First described in Ritov & Baron (1990).