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In the human population, if you meet someone how likely are they to be a twin with someone of opposite sex? That includes non-twins, not just people who are twins.

Ben Bolker
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Haydee
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The answer will vary a lot by time and location because of changes in the use of fertility treatments, which increase the proportion of twins enormously, and changes in maternal age: from 1980 to 2009 "[twinning] rates [in the US] doubled among non-Hispanic white mothers, rose by about one-half among non-Hispanic black mothers, and by one-third among Hispanic mothers"(US CDC). Globally, "Researchers analysed records from more than 100 countries and found a substantial rise in twin birthrates since the 1980s, with one in 42 people now born a twin, equivalent to 1.6 million children a year. According to the study, the global twin birthrate has risen by one-third, on average, over the past 40 years." (Guardian, Monden et al 2021).

A twinning rate of 15/1000 looks like a reasonable global average. plot of global twinning rates

Taking this information as true: given 15 pairs of twins and 985 single births, (2 × 15/(985 + 2 × 15)) ~ 3% of babies born are twins. Identical (monozygotic) twins are always same-sex, but are at least 10x less common than dizygotic twins, so we can ignore them for a rough estimate. Given a 50/50 sex ratio (I think this is true/close enough for dizygotic twins), 1/2 of twins have an opposite-sex twin, so 1.5% of people are part of an opposite-sex twin pair.

So about 1 out of every 68 people is a member of a male-female twin pair.

Monden, Christiaan, Gilles Pison, and Jeroen Smits. 2021. “Twin Peaks: More Twinning in Humans than Ever Before.” Human Reproduction 36 (6): 1666–73. https://doi.org/10.1093/humrep/deab029.

Ben Bolker
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  • Of every 30 births, there will be 31 babies if 1 in 30 is a twin gestation. You also ignore the fact that this number reflects fertility treatments and IVF which is more common in higher income brackets of wealthier nations. Many of the more successful fertility treatments were not available, say, 40 years ago, so the age of the individual matters. Taking that into account, it depends on a lot more than one or two numbers. It depends on where the individual is from, their economic background, their age, and more. – anongoodnurse Sep 25 '22 at 23:04
  • I feel like I've accounted for your first sentence (I said that 2/31 babies were twins). I know that this number will vary a lot, but I don't expect that it's really possible to get more than an approximate answer to this question anyway ... – Ben Bolker Sep 26 '22 at 00:03
  • I've updated. (I don't understand where the 1/30 number came from - I looked at the original source, but it seems out of sync with the Monden paper - maybe a twins vs births confusion somewhere along the line) – Ben Bolker Sep 26 '22 at 00:30
  • I didn't see the calculation that you made in either of your sources. "...but I don't expect that it's really possible to get more than an approximate answer to this question anyway..." Neither do I, but you did anyway: "So about 1 out of every 68 people is a member of a male-female twin pair." The better answer is, it depends on too many factors to say with any degree of even speculation. – anongoodnurse Sep 27 '22 at 01:44