As part of Slate’s twins issue this week, Barry Harbaugh writes about DNA tests that twins can take to determine whether they are fraternal or identical siblings. He found that most twins who believe they’re fraternal aren't wrong about being dizygotic—it’s the identical twins who are more likely to be mistaken about their twinly identity. When multiples can describe themselves as being "like two peas in a pod," he writes, they almost always end up having come from the same fertilized egg.
That got us thinking about the most famous twins in the United States: Mary-Kate and Ashley Olsen. Despite the fact that they look eerily similar—similar enough to have found fame playing a single character—the pair has always claimed, without citing scientific evidence, that they are, in fact, fraternal twins. ("We were just told by the doctors" that the girls were fraternal, says their dad, Dave Olsen, when reached today by phone. "We never thought about it much. They are good kids.")
Profile writers often cite physical differences to support the claim that they are a dizygotic set. Ashley, for example, is right-handed, while Mary-Kate is a lefty. But it turns out that this is shabby evidence, since about 21 percent of identical twins do not share hand dominance. So in order to further challenge the claim that the Olsen twins are a fraternal pair, we’ve created a quiz to test just how identical they really are.
Click through to see if you can tell Mary-Kate apart from Ashley—then let us know how you did in the comments.
Quiz compiled by Lauren Hepler.