Enlarge ImageDéjà vu? Subjects were shown dozens of pictures like these and tested on whether they remembered them the next day.Credit: U. Rimmele et al., J. Neuroscience, 7 January 2009 The next time you spot an old friend from across the room, thank oxytocin. Researchers have shown that the brain hormone helps us sense whether a face is familiar. Oxytocin is a powerful social chemical. In voles, for example, the hormone is key to attachment behavior: Males with higher levels of oxytocin are more likely to be faithful to their mates. Humans also make use of the hormone. Oxytocin...