Separation into discrete time periods was done by Brent Bellamy, "based on perceptible shifts in form or style and historical changes." Bellamy credits the massive increase in publishing during "The Contemporary" not necessarily with a cultural increase in demand for such stories, but easier access to niche publication has allowed authors of these stories to reach interested audiences. That said, it's hard to ignore a correlation between world-shaking events and a subsequent boom of post-apocalyptic fiction: the World Wars preceding the Long Fifties, the rise at the end of the Cold War, and 9/11 preceding the Contemporary.