Check out this fascinating article about sarcasm and how the ability to detect sarcasm develops in the brain ...
Here's an excerpt ...
"The smirking put-down that buries its barb by stating the opposite, requires a nifty mental trick that lies at the heart of social relations: figuring out what others are thinking. Those who lose the ability, whether through a head injury or the frontotemporal dementias afflicting the patients in Rankin's study, just do not get it when someone says during a hurricane, "Nice weather we're having."
http://www.iht.com/articles/2008/06/04/healthscience/03sarc.php