The omission bias, a cognitive bias, is the tendency to judge overt, harmful actions more severely than harmful omissions. This is thought to occur because harmful actions are more obvious than omissions, even though they may be equally harmful.
For instance, one person might set a fire that does serious damage. This is an example of a harmful action. Another person might see the fire set but not bother to report it. This is an example of harmful omission. Are they equally responsible for the fire damage? Although the consequences of this are similar (the fire caused great damage) the omission bias suggests that the individual who started the fire would be judged more harshly than the person who saw it and didn't report it.