In soap operas, if an infant gets switched at the hospital or given up for a secret adoption, you can guarantee that the infant will grow up to fall in love with a sibling. (They never consummate their love, though—some evildoer always gets a pang of conscience and interrupts the wedding ceremony just in time to preserve the standards of network television.)
Other social primates have more sense than daytime TV stars. But just how do they manage to avoid breeding with their close relatives? They recognize the relatives they live with, of course. (A study in the Proceedings of the National Academy of Sciences to be released later this week, for example, shows that capuchins monkeys can distinguish between photos of strangers and photos of capuchins from their own group.) Primates also disperse; either males or females leave their natal group as they reach reproductive age. That makes inbreeding less likely but not impossible: close kin could bump into each other in the wild without having grown up together.
The most fool-proof method for maintaining an incest taboo is scent, and an exhaustive new study of ring-tailed lemurs shows that closely related males and females smell a lot alike. At Duke University, Christine Drea (full disclosure: I worked on a project with her years ago when I was a grad student at Berkeley and she was a post-doc) and colleagues analyzed hundreds of smelly compounds in the genital secretions of ring-tailed lemurs, they report in BMC Evolutionary Biology. Related lemurs had similar chemical profiles—and the similarity was even more pronounced during breeding season, when it's most important to identify and decide not to mate with your sibling.
As for the rest of the year? It might be useful to identify close relatives for purposes of nepotism, the scientists suggest. Lemurs have complex, hierarchical, family-based societies that are pretty soap-operatic in their own right, as detailed in a story on lemurs in Smithsonian magazine a few years ago.