Wikipedia claims that American actors Mary Kate Olsen and Ashley Olsen are fraternal twins. They have also both claimed this. Other sources claim the same: Washington Post, St. Petersburg Times, and the LA Times.

Are they fraternal twins, or identical twins?

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    I can't see how this question could be properly answered. If they had genetic tests, they would be private, and the only people who could release them would be the very people making the claim. If they wanted to pointless lie about something like this, they could also fake those. What sort of evidence would you accept?
    – Oddthinking
    Commented Aug 3, 2015 at 15:38

1 Answer 1


In my opinion, the best evidence that I can find is testimony from the girls' father, Dave Olsen. The testimony was received via telephone by Slate.com author, Angela Tchou:

"We were just told by the doctors" that the girls were fraternal, says their dad, Dave Olsen, when reached today by phone. (Can you tell Mary-Kate from Ashley).

It is possible that the doctors were wrong ("as many as twenty percent of all twin births are misidentified as fraternal or identical"), or that Dave misremembers or is for some other reason inaccurately portraying the situation.

Other purported evidence (1 inch height difference, handedness difference) is not dispositive. 20.7% of monozygotic twins had different handedness in this study (see Table 5). Height is affected by childhood illness and nutrition.

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