Not only is left-handedness twice as common among twins as among regular siblings, but left-handers are twice as likely as right-handers to produce twins. This eerie link lies at the heart of another modern theory (and Smits’s favorite): that "being a monozygotic twin is a precondition of being left-handed." In other words, only someone who has had a twin in utero can be truly left-handed. The twins are mirror images of one another; one is left-handed, and the other right-handed. Of course, left-handedness doesn’t require that one ultimately be born with a twin. If only one fetus results at the end of term, that means the other died in the womb and was absorbed by the mother: a "vanishing twin."