The omission bias is an alleged type of cognitive bias. It is the tendency to judge harmful actions as worse, or less moral than equally harmful omissions (inactions) due to the fact that actions are more obvious than inactions. It is contentious as to whether this represents a systematic error in thinking, or is supported by a substantive moral theory. For a consequentialist, judging harmful actions as worse than inaction would indeed be inconsistent, but deontological ethics may, and normally does, draw a moral distinction between doing and allowing. The bias is usually showcased through the trolley problem.
Examples & Applications 
Spranca, Minsk and Baron extended the omission bias to judgments of morality of choices. In one scenario, John, a tennis player, would be facing a tough opponent the next day in a decisive match. John knows his opponent is allergic to a food substance. Subjects were presented with two conditions: John recommends the food containing the allergen to hurt his opponent's performance, or the opponent himself orders the allergenic food, and John says nothing. A majority of people judged that John's action of recommending the allergenic food as being more immoral than John's inaction of not informing the opponent of the allergenic substance.
The effect has also held in real world athletic arenas: NBA statistics showcased referees called 50 percent less fouls in the final moments of close games.
An additional real world example is when parents decide not to vaccinate their children because of the potential chance of death - even when the probability the vaccination will cause death is much less likely than death from the disease prevented.
See also 
- Frances Howard-Snyder, Doing vs. Allowing Harm (Stanford Encyclopaedia of Philosophy)
- Spranca, M., Minsk, E., & Baron, J. (1991). Omission and commission in judgment and choice. Journal of Experimental Social Psychology, 27, 76-105.
- Wertheim, Jon, and L. Tobias Moskowitz. Scorecasting: The Hidden Influences Behind How Sports Are Played and Games Are Won. N.p.: Random House Digital, 2011. Print.
- Ritov, I., & Baron, J. (1990). [http://www.sas.upenn.edu/~baron/papers.htm/vac.html Reluctance to vaccinate: omission bias and ambiguity]. Journal of Behavioral Decision Making, 3, 263-277.
- Baron, Jonathan. (1988, 1994, 2000). Thinking and Deciding. Cambridge University Press.
- Asch DA, Baron J, Hershey JC, Kunreuther H, Meszaros JR, Ritov I, Spranca M. Omission bias and pertussis vaccination. Medical Decision Making. 1994; 14:118-24.