While nobody could possibly tell you with accuracy exactly how many muscles you use when you smile (43? 17? 26?), it's possible to tell you the minimum number of muscles that are used in the most insincere, subtle, restrained, mouth-only smile or frown.
If we analyze a smile that only raises the corners of the lips and the upper lip (the smile you give when you bump into your former boss in the grocery store, perhaps), then there are five muscle pairs (or 10 total muscles) that accomplish this. Two muscle pairs primarily raise the upper lip, while three other muscle pairs are tasked mainly with raising the corners of the mouth.
If we reduce a frown only to the lowering of the corners of the mouth along with a slight downward pouting of the lower lip, we're dealing with only three muscle pairs (one pair to drop the lower lip, and two pairs to lower the corners).
Counted individually (as you might count your biceps to be two different muscles, instead of one muscle pair), we reach a tally that very well may turn our understanding of the universe completely on end: 10 muscles to smile, and six muscles to frown.
But before you abandon your smile for a look of mild disappointment in order to conserve energy, consider that we can reduce both a smile and a frown even further, so that each is produced merely by raising or lowering the corners of the mouth into a robotic expression. In this case, we have a tie: two muscle pairs (for a total of four) to "smile," and the same number to "frown."
While such expressions would hardly be recognized as a proper smile or frown, the fact that the same amount of effort is used to produce one or the other means that the scientific minds of this generation and the next will have to continue searching for a good reason for humans to put a smile on their faces -- and not a frown of equal but opposing effort.
Want to know whether laughter's the best medicine and how smiling can change your mood? Try the links to the HowStuffWorks articles on the next page.