*cough* Snopes.com *cough*
From the article (since embedding isn't working):
Origins: It's become fashionable to portray outdated societal behaviors and attitudes -- ones we now consider desperately wrongheaded -- to be worse than they really were as a way of making a point about how much we've improved. When we despair over the human condition and feel the need for a little pat on the back, a few startling comparisons between us modern enlightened folks and those terrible neanderthals of yesteryear give us that. We go away from such readings a bit proud of how we've pulled ourselves up by our bootstraps and with our halos a bit more brightly burnished.
The juxtaposition of wonderful modernity with a tawdry past also serves to reinforce the 'rightness' of current societal stances by making any other positions appear ludicrous. It reminds folks of the importance of holding on to these newer ways of thinking and to caution them against falling back into older patterns which may be more comfortable but less socially desirable. Such reinforcement works on the principle that if you won't do a good thing just for its own sake, you'll surely do it to avoid being laughed at and looked down upon by your peers.
A typical vessel for this sort of comparison is the fabricated or misrepresented bit of text from the "olden days," some document that purportedly demonstrates how our ancestors endured difficult lives amidst people who once held truly despicable beliefs. Want to prove that American slaveholders were even more vile than we could possibly imagine? Just point people to the apocryphal Slave Consultant's Narrative. Remind someone what easy lives we lead these days by showing him an alleged list of rules for teachers from 1872. Or poke fun at Victorian sexual attitudes (or modern day feminism) by trotting out a piece of Advice to Young Brides.