OverviewA mirror argument is any argument where accusations which might legitimately be made against one party are instead made by that party against the other.
In other words, if:
- person A is arguing with person B,
- and person B is possibly guilty of X, Y, and Z,
- but A clearly is not guilty of those,
if B accuses A of X, Y, and Z before A has a chance to (legitimately) accuse B of the same thing, then person B is using a mirror argument.
This type of argument seems to have a lot in common with the phenomenon of psychological projection; they may be the same thing.
 Origins & Theory
This phenomenon was originally described to me by someone who later used it against me (accusing me of a wide variety of stuff which I had definitely not done and which he had either actually done or tried to do, or which sounded more like his kind of behavior than my kind), so if true it's highly ironic; I'm also therefore a little suspicious of it because of the source. I've had at least one or two other anecdotal confirmations of similar behavior, however, and I'm also seeing the pattern popping up in other contexts such as political debate.
The person who described it called it simply "mirroring", but Wikipedia's entry for mirroring describes a rather different phenomenon.
Some theories for why people might do this:
The Conscious Theory: Accusing someone else of one's own transgressions has benefits in an argument, especially if personal integrity is not a priority. These benefits include:
- Strategy: If your opponent now legitimately and truthfully makes the same accusation of you, it sounds much weaker – more like a "you too, stupid!" than a legitimate accusation. You come across sounding legitimate either way, and possibly your opponent will seem foolish as well.
- Opportunism: You might get lucky, unknowingly identify something that the other party is guilty of, and trigger a confession (typically as part of their defense rather than as a surrender) or a concession.
The Unconscious Theory: This theory is a bit more complicated, and has three parts to it.
- First: If someone has a long history of committing certain kinds of transgressions, they have probably heard others accuse them of those transgressions. Some of those accusations may be particularly well-worded and stick in the memory; others may have simply been repeated so much that the transgressor has memorized them. Either way, they are words which come to mind in an argument situation.
- Second: It has been shown  that many adults are "stuck" at a non-abstract level of thinking. Although they can manipulate the same symbols as abstract thinkers, they don't really understand what it means to construct a logical argument. Their strategy in an argument, then, may be geared entirely towards winning the argument, rather than in arriving at the truth or a reasonable compromise. Their prime criterion for judging the value of an argument may be emotional effectiveness (does it "sound good"?) rather than reasonableness or accuracy (does it make sense?).
- Third: If Person X is someone whose thinking is not really on the abstract level, has memorized a number of arguments that were used effectively on X at an earlier time, X may honestly believe that those arguments are effective by themselves – like physical weapons, or magic incantations. Hence X may pull out the "best" of the arguments s/he has heard used against her/him and return fire, believing that this is acceptable and normal in such a situation.
If this theory is true, it would seem to have a wide range of interesting implications (most of which I have yet to work out). It could well explain a lot of hostile attitudes towards the ideas of "negotiation" and "compromise", for example. To someone for whom words are blunt instruments whose effectiveness is determined largely by the degree of force used, social interaction must seem to consist largely of dominance games rather than cooperation for mutual benefit.
"Pot Calling the Kettle Black" theory: A mirror-arguer may simply be understanding someone else's outward actions solely in the context of what motivations the mirror-arguer might have for doing the same thing, and then jumping to the conclusion that this explanation is correct. (This explanation is essentially a milder form of the "psychological projection" phenomenon.)
- Is mirror-arguing usually done deliberately (the conscious theory), or is it the result of some form of self-deception (either the unconscious theory above, or some other unconscious theory yet to be identified)?
 Related Pages
Thanks to Vee for verbalizing the "Pot Calling the Kettle Black" theory and for finding the term "psychological projection" in Wikipedia.