Women have a greater variety of bacteria on their hands than men do, possibly because men have more acidic skin. Other possible explanations relate to differences in sweat and oil production, the frequency of moisturizer applications, skin thickness variations, or hormonal differences.

Researchers took samples from both palms of 51 college students and tested them using a highly detailed system for detecting bacteria DNA. They identified 4,742 species of bacteria, but only 5 species were found on every hand. The average hand harbored 150 species of bacteria. Each person's left hand and right hand shared only about 17% of the same bacteria types.

Although recommending regular hand washing, researchers noted that hand washing-at least as practiced by the students-didn't remove most of the bacteria found on hands.

Study results were posted in the online edition of Proceedings of the National Academy of Sciences on November 3, 2008.