Unintended consequences are situations where an action results in an outcome that is not (or not only) what is intended. The unintended results may be foreseen or unforeseen, but they should be the logical or likely results of the action. For example, it is often conjectured that if the Treaty of Versailles had not imposed such harsh conditions on Germany, World War II would not have occurred. As such, war was an unintended consequence of the Treaty of Versailles.
Unintended consequences can be classed into roughly three types:
- a positive unexpected benefit, usually referred to as serendipity or a windfall
- a potential source of problems, according to Murphy's law used in Systems engineering
- a negative or a perverse effect, which is the opposite result of what is intended
Discussions of unintended consequences usually refer to the third situation of perverse results. This situation often arises because a policy has a Perverse incentive and causes actions contrary to what is desired.