I have always wondered how the Japanese got the idea of making animated productions including underage girls and the obsession with them. When did it start? Did the idea originate in Japan? Was it welcomed by the actual anime public?
Appears to have started in the 1970's, but over time the definition got a little cloudy and changed from being pedophilia to more of an obsession. There was also controversy similar to violence in video games linked to violence in real life, where one party suggested that lolicon could fuel sex crimes toward children, while the other party maintained that if anything it decreased the chances by letting people live out their fantasies through manga/anime.
There is a lot of referenced material here.
Lolicon (ロリコン?), also romanised as lolikon or rorikon,is a Japanese portmanteau of the phrase "Lolita complex". In Japan, the term describes an attraction to underage girls (whether prepubescent, pubescent, or post-pubescent)or an individual with such an attraction. It is also commonly used when referring to lolicon manga or lolicon anime, a genre of manga and anime wherein childlike female characters are often depicted in an "erotic-cute" manner (also known as ero kawaii), in an art style reminiscent of the shōjo manga (girls' comics) style. Outside Japan, "lolicon" is in less common usage and usually refers to the genre. The phrase is a reference to Vladimir Nabokov's book Lolita, in which a middle-aged man becomes sexually obsessed with a twelve-year-old girl. It was first used in Japan in the 1970s and quickly became used to describe erotic dojinshi (amateur comics) portrayals of young girls. Laws have been enacted in various countries, including in Japan, which regulate explicit content featuring children or childlike characters. Parent and citizens groups in Japan have organized to work toward stronger controls and stricter laws governing lolicon manga and other similar media. Critics say that the lolicon genre contributes to actual sexual abuse of children, while others say that there is no evidence for this claim. Studies of lolicon fans state that lolicon fans are attracted to an aesthetic of cuteness rather than the age of the characters, and that collecting lolicon represents a disconnect from society
Adding onto @Angelo's answer, the rise of the trend seems to coincide with the rise of idol culture in Japan.
The idol phenomenon began during the early 1970s, reflecting a boom in Japan for the musician Sylvie Vartan in the French film Cherchez l'idole in 1963, with Japanese title (アイドルを探せ Aidoru wo sagase?) in November 1964.
The term came to be applied to any cute actress or female singer, or any cute male singer. Teenage girls, mostly between 14 and 16, and teenage males, mostly between 15 and 18, began rising to stardom. One in particular, Momoe Yamaguchi, was a huge star until her marriage and retirement in 1980.
Idols dominated the pop music scene in the 1980s, and this period is known as the "Golden Age of Idols in Japan".
Perhaps somewhat as a result of this, kawaii-ness soon came into fashion - especially with schoolgirls. Schoolgirls are commonly fetishized in the sleazier section of society anyway, so this could be where the trope slipped into anime's fanservice.
It's not really a definitive answer, but I'm of the opinion that television reflects society, rather than life reflecting television (although, there is some of that) - So I would say that it came from the gradual shift in attitudes in Japan, rather than a new idea that was introduced to anime.
And I suppose it wasn't too much of a shift either as many shows would have had young female characters already because they were targetted at a younger audience and it was just a case of moving them to more "mature" shows, which by their nature contained fanservice.
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