A still from The Shining. Not all twins are this creepy. Just most.
Wiki WormholeWe explore some of Wikipedia’s oddities in our 5,664,405-week series, Wiki Wormhole.  

This week’s entry: Twins

What it’s about: This week’s topic was suggested by The A.V. Club’s resident twin, Caitlin PenzeyMoog, who insists she’s the good twin. Twins are, of course, humans who shared a womb with a sibling before being born. As this only happens once in every 90 or so births, twins have been a subject of fascination around the world. There are two types of twins—identical, in which a fertilized egg splits into two genetically identical embryos, and fraternal (sometimes called sororal when both are girls), in which two separate eggs are fertilized, and are no more similar than any other set of siblings.

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Strangest fact: There are sorta-identical twins. In some cases, identical twins can have different genes activated in utero, causing them to develop differently, although the differences are usually slight. In rare cases, an egg splits before fertilization (not after, in the case of fraternal twins), and the halves are fertilized by different sperm, resulting in “half-identical” twins, who share the same set genes from their mother, but have different genes from their father.

Biggest controversy: Not only are fraternal twins not identical, sometimes they aren’t full siblings. In rare instances, a woman who has unprotected sex with multiple men can have have two eggs fertilized by two different men, producing fraternal twins who are half-siblings. A study of fraternal twins involved in paternity suits (an admittedly small cross-section) found 2.4 percent had different fathers. Fraternal twins can also be different races. What we think of as race is determined by a very small number of genes, and in rare instances, each twin will get all of the genes from one ancestry or the other. Wikipedia cites a German/Ghanan couple whose twins each inherited all of the racial characteristics of a different parent. There are also documented cases (not cited here) in which two biracial parents have had twins whose genes split along racial lines.

Thing we were happiest to learn: Twins actually do sometimes have their own private language. Idioglossia is the scientific term for a language invented and spoken by a small number of people, but twins have a more specific version, cryptophasia. As many as half of all sets of twins have at least some private words and phrases in early childhood. The bad news is, this often slows their development in the language they have to share with everyone else.

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Thing we were unhappiest to learn: We have no idea what makes a woman more or less likely to have identical twins. Weirdly, geography may be involved. There are certain twin hot spots in the world, mostly in Nigeria or India, where a town or region has a disproportionate number of twins. In the small Brazillian town of Linha São Pedro, one in five pregnancies results in twins. Hausa women, another ethnic group in Nigeria, have twins 4 percent of the time, and a quarter of those are identical, for reasons we don’t understand at all.

Twins can also appear as a result of bad choices while browsing Netflix.

Also noteworthy: Twins don’t appear strictly at random. There are several factors behind fraternal twins: Women over 30, above-average in height and weight, or who have had several children already are more likely to have twins. Different ethnic groups are also more or less likely than others to produce twins. The Yoruba, an ethnic group mostly found in Nigeria and Benin, have fraternal twins 4.5 percent of the time. Throughout south and southeast Asia and Latin America, the rate is 1 to 1.5 percent. Fertility drugs are also more likely to produce twins, which is part of the reason the U.S. twin rate has shot up, from 1.89 percent to 3.33 percent. The other reason is more women having kids later in life. Eating dairy may also be a factor, as vegan mothers are only one-fifth as likely to have twins, possibly because of the growth hormones now present in most milk.

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Best link to elsewhere on Wikipedia: Twins unsurprisingly leads to siblings, but that page’s section on three-quarters siblings (half-siblings for whom the different parents are themselves siblings, making them half-sibling, half-cousin), links to a historical footnote on the Deceased Wife’s Sister’s Marriage Act of 1907, a British law that undid a strict prohibition that had treated a relationship between a widower and his sister-in-law as incestuous as one between a biological brother and sister.

Further down the wormhole: Similar to the twin is the doppelgänger, a common trope in fiction (and a near-impossibility in real life), in which two completely unrelated people are identical. The word comes from the German words Doppel (double) and Gänger (walker), and was used for the first time by German Romantic writer Jean Paul in his 1796-97 three-volume book Siebenkäs. We’ll look at other newly coined words next week.