I don't know what's been in your mouth...

Considering I eat bunch in communal Chinese settings where dishes are shared rather than hoarded to oneself, I don't get too worked up about the "double dipper." However, some people do, and inspired by George (Costanza, not Firecracker) and that classic episode of Seinfeld, a group from Clemson University studied whether such practices truly are bad for you. From The New York Times:

The team of nine students instructed volunteers to take a bite of a wheat cracker and dip the cracker for three seconds into about a tablespoon of a test dip. They then repeated the process with new crackers, for a total of either three or six double dips per dip sample. The team then analyzed the remaining dip and counted the number of aerobic bacteria in it. They didn’t determine whether any of the bacteria were harmful, and didn’t count anaerobic bacteria, which are harder to culture, or viruses.

There were six test dips: sterile water with three different degrees of acidity, a commercial salsa, a cheese dip and chocolate syrup.

On average, the students found that three to six double dips transferred about 10,000 bacteria from the eater’s mouth to the remaining dip.

Each cracker picked up between one and two grams of dip. That means that sporadic double dipping in a cup of dip would transfer at least 50 to 100 bacteria from one mouth to another with every bite.

The kind of dip made a difference in a couple of ways. The more acidic water samples had somewhat fewer bacteria, and the numbers of bacteria declined with time. But the acidic salsa picked up higher initial numbers of bacteria than the cheese or chocolate, because it was runny. The thicker the dip, the more stuck to the chip, and so the fewer bacteria were left behind in the bowl.

I'm still not convinced that the transfer of bacteria is an adequate end point to study. They admit that you probably should be studying whether those are harmful bacteria that are going from one mouth to the next, or just normal flora. Don't worry, though--I don't double dip.