Smile Baby Smile...

"It takes one more muscle to smile than to frown, according to plastic surgeon David H. Song, MD, FACS, assistant professor at the University of Chicago Hospitals.

Newspapers around the globe assure us, "Frowning takes more muscles than smiling,"
13 to smile, 33 to frown - The Washington Post.
10 to smile, 100 to frown - The New York Times,
4 to smile, 64 to frown - The Hindu. An urban myth?

A genuine smile takes two muscles to crinkle the eyes, two to pull up the lip corners and nose, two to elevate the mouth angle, and two to pull the mouth corners sideways. Total smile: 12.

On the other hand, a frown needs two muscles to pull down the lips and wrinkles in the lower face, three to furrow the brow, one to purse the lips, one to depress the lower lip, and two to pull the mouth corners down. Total frown: 11.

A fake smile, however, only takes two muscles. We detect the fake because "the eyes aren't smiling."

Source: http://wiki.answers.com/Q/How_many_muscles_does_it_take_to_form_a_smile