SANTIAGO, Chile — For years, so many twins have been born in the small southern Brazilian town of Cândido Godói that residents wonder whether something mysterious lurks in the water, or even if Josef Mengele, the Nazi physician known as the Angel of Death, conducted experiments on the women there.

But a group of scientists now says it can rule out such long-rumored possibilities. Ursula Matte, a geneticist in Porto Alegre, Brazil, said a series of DNA tests conducted on about 30 families since 2009 found that a specific gene in the population of Cândido Godói appears more frequently in mothers of twins than in those without. The phenomenon is compounded by a high level of inbreeding among the population, which is composed almost entirely of German-speaking immigrants, she said.

“We analyzed six genes and found one gene that confirms, in this population, a predisposition to the birth of twins,” Dr. Matte said.

She was the first scientist to document, in a study published in the 1990s, that the rate of twin births in the town was unusually high. It was especially high in São Pedro, a village of about 350 residents that is part of Cândido Godói. Dr. Matte found that from 1990 to 1994, 10 percent of the births in São Pedro were twins, compared with less than 1 percent for Brazil as a whole.