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Bishonen Jump Syndrome, also known as Shoujo Jump Syndrome, is a specific trend in Shonen (boy's) Manga (and Anime), aimed at attracting a female Periphery Demographic through gratuitously Bishounen character design.
The Manga anthology magazine Weekly Shonen Jump (on which the US magazine Shonen Jump is based) has long been the best-selling manga magazine in Japan. Although it is targeted to boys, it has a large female Periphery Demographic. Sometime in the mid-1990s, Shonen Jump realized the sales potential of this demographic, and began to deliberately court female readers via Mr. Fanservice. Specifically, they took standard Shounen all-male teams and simply made them into a Cast Full of Pretty Boys (an idea promptly picked up by Shoujo as too good to waste on the guys). Traces of Ho Yay are often also added (if not, the fangirls will see it anyway).
Among the first series to show this effect were Yu Yu Hakusho and Rurouni Kenshin (albeit with the latter it was a deliberate choice by the mangaka); compare the art in these to Slam Dunk or Great Teacher Onizuka, which have old-school character designs. Katekyo Hitman Reborn is probably the most blatant example that's currently serializing.
The fangirl-bait worked, sales went up, and the idea was promptly copied by other shonen magazines, to the extent that it is now more-or-less standard procedure. It is nonetheless often ridiculed by readers who prefer the older tough-looking art styles, who deride Shonen Jump as "Bishonen Jump" or "Shoujo Jump".