People's health could be harmed by social networking sites because they reduce levels of face-to-face contact, an expert claims. Dr Aric Sigman says websites such as Facebook set out to enrich social lives, but end up keeping people apart. Dr Sigman makes his warning in Biologist, the journal of the Institute of Biology. A lack of "real" social networking, involving personal interaction, may have biological effects, he suggests. He also says that evidence suggests that a lack of face-to-face networking could alter the way genes work, upset immune responses, hormone levels, the function of arteries, and influence mental performance.