Two wrongs fallacy
The two wrongs fallacy refers to any statement where a given action which is known to be wrong is justified or excused by the presumption that another person did, or would have done, the same thing under corresponding circumstances. The fallaciousness of such arguments is generally recognized in the common saying "two wrongs don't make a right".
This fallacy has other common forms such as "he did it first"; see also both sides do it.
- The harmful acts of one party do not justify others in committing similar acts; wrong does not become right through repetition.
- The mere fact that both parties have engaged in a particular form of bad behavior does not necessarily mean that they are equally culpable; one party may have committed far fewer offenses than another, even though both have in fact been guilty of the same bad behaviors at one time or another; see both sides do it.
lowering the bar
A legitimate use for this statement is when one is arguing mainly that the original perpetrator (the one who "did it first") has thereby lowered the generally accepted standards of behavior in some regard. If one does not agree with those new standards, however, it is inconsistent to argue that "he did it first" justifies a repeat of the action; it is only consistent to argue that the original perpetrator was in the wrong and has done harm to society.
If one believes or argues that the new standards are, in fact, reasonable, then it may also be reasonable to argue that subsequent actions along the same lines are acceptable.
Another legitimate argument applies when the accusation is being made as part of a comparison between multiple parties. Arguing for party A over party B on the basis that party B has committed some wrong is a clear example of bias or double standard if party A has in fact committed the same wrong to the same degree; a defender of party B would be correct to point out that a fair comparison can only be made in the knowledge that the act in question is not a characteristic by which one can distinguish between the two parties.
Note that this does not apply if the degree is substantially different. For example, if party A lies 5% of the time but party B lies 50% of the time, it is fair to say that party B lies much more than party A; a response that party A lies too would not be a sound counterargument.
The tu quoque ("you too!") fallacy is a subset or variation of the "two wrongs" fallacy in which the accuser, rather than a 3rd party, is accused of "doing it too".