In 1996, Jeff Bigler was trying to help a friend at MIT better understand her co-workers and why she and they weren't communicating well. The analogy he presented worked for her and Jeff posted it to the Web.
All people have a "tact filter", which applies tact in one direction to everything that passes through it. Most "normal people" have the tact filter positioned to apply tact in the outgoing direction. Thus whatever normal people say gets the appropriate amount of tact applied to it before they say it. ...
"Nerds," on the other hand, have their tact filter positioned to apply tact in the incoming direction. Thus, whatever anyone says to them gets the appropriate amount of tact added when they hear it. ...
(from Tact Filters, Copyright © 1996, 2006 by Jeff Bigler.)
Ten years later, the "Tact Filter" theory has stood the test of time. Jeff still gets "quite a bit of fan mail" about it and the Tact Filter has been mentioned in many other sites, most recently on 43Folders where I first heard about it.
I think the theory works well to explain a lot of personal interactions. Discovering it gave me one of those AhHa! moments. (I'm one of Jeff Bigler's "nerds" by the way. :-)
However, there's one part of Jeff's theory that I don't agree with.