I knew this was possible in dogs (it’s pretty common when a dog in heat roams the whole neighborhood) but I didn’t know that it sometimes happens in people:

“T.M.,” the mother of the twins, was applying for benefits and told the agency her twins had been fathered by A.S., described by the New York Times as her “romantic partner.” He apparently disputed this because he did not want to pay child support, and the agency then filed a paternity suit to force him to pay. T.M. then admitted in her testimony that she had a second romantic partner (actually, the Times just calls that one an “unidentified man”) within a week of being with the first one. The court ordered a paternity test, and the results were surprising to everyone.

But not too surprising to Dr. Karl-Hans Wurzinger, a DNA expert who testified in the case [who is] known for a 1997 paper on fraternal twins with different fathers.

Basically, this can happen if, in the course of about a week, one blessed event occurs, the female ovulates again, and then she has a second romantic partner who also hits the target. Scientists refer to it as “heteropaternal superfecundation” but it is known informally as Have None of These People Ever Heard of Birth Control Syndrome. Wikipedia says this is not uncommon in animals, for reasons that probably need not be explained, but Dr. Wurzinger found that it is relatively rare in humans, occurring in one out of in 13,000 reported paternity cases. (It probably explains some regular cases of fraternal twins, but those don’t get disputed, and DNA testing wouldn’t work anyway.) Another expert quoted by the Times pointed out that DNA testing is not always necessary to detect a case of heteropaternal superfecundation, saying that the phenomenon is illustrated in medical textbooks with a picture of fraternal twins who were, shall we say, of different racial backgrounds.

From the legal blog Lowering The Bar.

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