When you're in love, your eyes light up, your face lights up -- and apparently, so do four tiny bits of your brain. "It is the common denominator of romantic love," says Andreas Bartels, a research fellow at University College London.

Bartels used functional MRI scanning to examine eleven women and six men who said they were truly in love -- statements backed up by psychological tests. When the subjects were shown photographs of their sweethearts, different areas of the brain scan lit up -- indicating higher blood flow -- than when they were shown pictures of friends. Theses "love spots" were near, but not the same as, sections that become active when someone is feeling simple lust. Looking at pictures of their dearest also reduced activity in three larger areas of the brain known to be active when people are upset or depressed.