Every year Pornhub, the world’s largest pornography website, releases annual statistics detailing the trends in online porn. Some takeaways from 2018? A staggering 4,403 petabytes of data transferred, the United States is the largest porn consumer (by a huge margin), and Stormy Daniels is the most searched for person (just brushing up on current events).

Nestled among the categories and search terms is a word that may seem oddly foreign: hentai.

If you’ve never heard of hentai, you’re not alone. This loanword from Japan is less well-known than other Japanese words like sushi, samurai, tsunami, and typhoon, yet produces more Google results than any of them. In its mother tongue, the word denotes a perverse or extreme sexual situation. After the word leapt the Pacific, it came to represent erotic comics and animations in the Japanese style.

Despite its unfamiliarity to many, hentai was Pornhub’s second most searched for term of 2018 and one of its most popular categories. Some may dismiss this new trend with a snide, “Yeah, but Japan, amiright?” But they are wrong.

Japan certainly has a history of illustrated erotica — shunga, such as “The Dream of the Fisherman’s Wife” by Hokusai, is perhaps the most famous example — but it is hardly the only culture to compose drawings meant to stimulate more than the imagination.

Western culture has produced plenty of sexually-charged cartoons. Examples include Marge Simpson’s turn as a Playboy playmate, 1950s pin-up girls, and Tijuana bibles, pulpy porn comics popular during the Great Depression.

Nor is this trend limited to the modern era. Medieval artists produced many ribald paintings, the Mughal Empire commissioned illustrated editions of the Kamasutra, and sensual frescas have been unearthed among the ashes of Pompeii. Artistic history, it seems, has quite the carnal cache tucked beneath its mattress.

Attraction to the illustrated human form clearly extends deeper into our psyches than some newfangled millennial kink. But before we look at why people are attracted to hentai, we need to take a slight detour to discuss songbirds.

Songbirds and supernormal stimuli

Nikolaas Tinbergen’s long and celebrated career changed how we understand animal instincts and behaviors, discoveries for which he was awarded the 1973 Nobel Prize in Physiology/Medicine alongside Karl von Frisch and Konrad Lorenz. Among his many insights was a theory that evolution may not have imbued animals with an innate kill switch toward instinctual responses. Nikolaas Tinbergen’s long and celebrated career changed how we understand animal instincts and behaviors, discoveries for which he was awarded the 1973 Nobel Prize in Physiology/Medicine alongside Karl von Frisch and Konrad Lorenz. Among his many insights was a theory that evolution may not have imbued animals with an innate kill switch toward instinctual responses. To test his theory, he created fake eggs that were large, saturated blue, and covered with black polka dots. He then placed these eggs in the nests of songbirds instinctually driven to sit on speckled, pale blue eggs. The birds quickly abandoned their natural brood to nurture the new arrivals, despite the artificial eggs being too big for them to lay on without sliding off. He called this a “supernormal stimulus” — a phenomenon that occurs when an artificial object triggers an animal’s instinctual response more intensely than the natural object the instinct evolved to seek out. Because nature could never produce eggs like Tinbergen, the songbirds could not adapt evolutionary defenses to prevent the fake eggs from pulling so strongly at their instincts. Tinbergen devised several other experiments to show supernormal stimuli affecting other species: Herring gull chicks beg for food by pecking at their mother’s long yellow bill with contrasting red patch. When presented with a fake bill sporting three red patches, the chicks pecked much more furiously at it. Male stickleback fish will ignore real rivals if presented with a wooden fish flourishing a brighter red ventral. Male grayling butterflies will attempt to mate with fake butterflies more than real females if the dummies are larger, darker in color, and flutter “enticingly.” Shape does not matter. Graylings will try to make it with a rectangle if it flutters with enough come-hither. Supporting Tinbergen’s experiments are supernormal stimuli we’ve created accidentally. Turns out, beer bottles are exactly what an Australian jewel beetles looks for in a mate (and then some). These beetles treat trash piles like a singles bar and can become so enamored with the bottle of their dreams that they will die trying to mate with it. Some animals have even evolved ways to use supernormal triggers to their advantage. Studies have suggested that the cuckoo chick, a brood parasite, acts as a supernormal stimulus to its host parent. The cuckoo chick’s gape-colored skin patch is thought to trigger the host parent’s visual instinct, causing it to favor the parasitic chick over its natural offspring.