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Diffusion of responsibility is a social phenomenon which tends to occur in groups of people above a certain critical size when responsibility is not explicitly assigned.

Diffusion of responsibility can manifest itself:

• in a group of peers who, through action or inaction, allow events to occur which they would never allow if alone (action is typically referred to as groupthink; inaction is typically referred to as the bystander effect) or
• in hierarchical organizations as when, for example, underlings claim that they were following orders and supervisors claim that they were just issuing directives and not doing anything per se.

This mindset can be seen in the phrase "No one raindrop thinks it caused the flood".

• Kitty Genovese, a New York woman, was stabbed to death near her house. More than 30 of Genovese's neighbors heard her screaming for help, yet no one helped her, each thinking that somebody else would.
• In a firing squad, one random shooter is traditionally given a blank bullet, allowing all members of the firing squad to believe that they only fired a blank.
• In some electric chairs there are many switches, only one of which is connected. The executioners may then choose to believe that they pulled a non-functional switch.
• This phenomenon also applies to much more mundane circumstances, such as cleaning and maintenance of shared space/items or unassigned work in large organizations getting neglected.

The latter definition of diffusion of responsibility was used as a legal defense (unsuccessfully) by many of the Nazis being tried at Nuremberg. It has been used with varying degrees of success in other situations.

• Groupthink
• Milgram experiment
• Design by committee
• Bystander effect