Update: According to the Guardian, there is a superstition about werewolves and seventh sons: "According to some versions of the myth, the seventh son of the seventh son is particularly prone to fall victim to the curse." But that seventh son myth is not connected to the seventh son adoption tradition:

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That custom began in 1907, when Enrique Brost and Apolonia Holmann, Volga German emigrés from south-eastern Russia asked then-president José Figueroa Alcorta to become godfather to their seventh son, said the historian. “The couple wanted to maintain a custom from Czarist Russia, where the Tsar was said to become godfather to seventh sons, and Argentina’s president accepted.”

Christina Fernández de Kirchner, the president of Argentina, got a new addition to her family last week when she adopted a 13-year-old Jewish boy named Yair Tawil as her godson, the Independent reports. Her motivations for doing so are based on a centuries-old legend that seventh sons will transform into werewolves.

In Argentina, the werewolf is referred to as el lobison; in Paraguay it goes by the name Luison, and in Brazil it's called the Lobisomem. The Independent elaborates on the South American legend:

The werewolf-like creature shows its true nature on the first Friday after boy’s 13th birthday, the legend says, turning the boy into a demon at midnight during every full moon, doomed to hunt and kill before returning to human form. As well as feeding on excrement, unbaptized babies, and the flesh of the recently dead, the lobison was said to be unnaturally strong and able to spread its curse with a bite.

In Guaraní mythology, the lobison is the offspring of Tau, an evil spirit, and Kerana, a mortal woman. In the cultures that believe in the lobison, that creature acts as a sort of Grim Reaper, whose mere presence means that death will soon befall those it comes into contact with. As the Independent explains, by the 19th century, fear of this creature was so acute in Argentina that families sometimes murdered their seventh sons to prevent the legend from coming true. So in 1907, in an attempt to stop this practice, the Argentinean president began adopting seventh sons, which the president insisted would stop the curse. In 1973, the Independent adds, the presidential adoption tradition was also extended to seventh daughters.

In addition to being able to say he's the president's godson, Tawil will receive a gold medal and enjoy a full educational scholarship. In modern times, the curse, it seems, is more aptly described as a blessing.