We've been lied to.

As the above video shows, popular imagery of Vikings is filled with lots of horned helmets. It's everywhere from football mascots (like the Minnesota Vikings) to far too many New Yorker cartoons. The only problem is that those horned helmets are a complete myth.

Roberta Frank wrote the seminal paper on the subject, "The Invention of the Viking Horned Helmet" (you can find a Scribd copy here). That work not only confirms the historical consensus that Vikings never had horned helmets — it also explains how the mythical headdress landed on Viking heads.

The main culprit? Costume designer Carl Emil Doepler, who included horned helmets in his gorgeous costume designs for the 1876 performance of Wagner's classic Norse saga, Der Ring des Nibelungen. The gorgeous designs are available here, and it's easy to see why they quickly became iconic. The opera was so influential that Vikings with horned helmets became a new standard — despite the fact that they were mythical.

Doepler's horned helmets were the result of some fascinating historical transposition. Germans were fascinated by Vikings, at least in part because they represented a classical origin story free from Greek and Roman baggage. That had a lot of appeal in an age of nascent German nationalism. As a result, Doepler and other scholars intertwined German and Norse history in a surprising way: They put stereotypical ancient and medieval German headdresses — like horned helmets — on Viking heads. Norse and German legends were intertwined in the popular imagination, and we still haven't untangled them.

Vikings did have a real history — from the 8th to 11th century, the famous Scandinavians really did explore (and even plunder) Europe. But an invented "Viking Age" was a powerful mythical companion to the historical reality. As Frank writes:

Until the viking age was invented, there was no horned-helmeted viking, and vice versa: the two go together like Easter and bonnet. A "viking age" is first mentioned in 1873, in two independent Danish and Swedish articles; the period gets its first monumental write-up in Johannes Steenstrup's four-volume Norman-nerne published between 1876 and 1882. Perhaps only an expansionist, empire-building era could have thought up an age that began with naval attacks on foreign shores and ended when these attacks ceased. The horned viking helmet was just one of countless colorful items in the armoury of a fin-de-siecle Europe fascinated by war and its tools.

It's a complicated historical detective story that's resulted in a vast misconception about Viking headgear. (Though, as Frank told me, Scandinavian countries have been scrupulous in never falling for the myth.) Frank poetically assessed the strange phenomenon:

However "wrong," the horned Viking helmet has been a recurrent fantasy transmuting the desert of daily existence into contours rare and strange.

Still, the next time you see a Viking helmet, make sure the wearer knows the truth — or at least sing them some Wagner.