The Silver Age of Comic Books was a period of artistic advancement and commercial success in mainstream American comic books, predominantly those in the superhero genre. Following the Golden Age of Comic Books and an interregnum in the early to mid-1950s, the Silver Age is considered to cover the period from 1956 to circa 1970, and was succeeded by the Bronze and Modern Ages. A number of important comics writers and artists contributed to the early part of the era, including writers Stan Lee, Gardner Fox, John Broome, and Robert Kanigher, and artists Curt Swan, Jack Kirby, Gil Kane, Steve Ditko, Mike Sekowsky, Carmine Infantino, John Buscema, and John Romita, Sr. By the end of the Silver Age, a new generation of talent had entered the field, including writers Denny O'Neill, Mike Friedrich, Roy Thomas, and Archie Goodwin, and artists such as Neal Adams, Jim Steranko, and Barry Windsor-Smith. The popularity and circulation of comic books about superheroes declined following the Second World War, and comic books about horror, crime and romance took larger shares of the market. However, controversy arose over alleged links between comic books and juvenile delinquency, focusing in particular on crime and horror titles. In 1954, publishers implemented the Comics Code Authority to regulate comic content. In the wake of these changes, publishers began introducing superhero stories again, a change that began with the introduction of a new version of DC Comics's The Flash in Showcase #4 (Oct. 1956). In response to strong demand, DC began publishing more superhero titles including Justice League of America, which prompted Marvel Comics to follow suit beginning with Fantastic Four #1. Silver Age comics have become collectible; as of 2008 the most sought-after comic of the era is Spider-Man's debut in Amazing Fantasy #15.