Declinism is the belief that a society or institution is tending towards decline. Particularly, it is the predisposition, caused by cognitive biases such as rosy retrospection, to view the past more favourably and the future more negatively.

"The great summit of declinism" according to Adam Gopnik, "was established in 1918, in the book that gave decline its good name in publishing: the German historian Oswald Spengler's best-selling, thousand-page work The Decline of the West."

The belief has been traced back to Edward Gibbon's work The History of the Decline and Fall of the Roman Empire, published between 1776 and 1788, which argues that the Roman Empire collapsed because of the gradual loss of civic virtue among its citizens, who became lazy, spoiled and inclined to hire foreign mercenaries to handle the defence of state. He believed that reason must triumph over superstition to save Europe's great powers from a similar fate to the Roman Empire.

Spengler's book The Decline of the West, which gave declinism its popular name, was released in the aftermath of World War I and captured the pessimistic spirit of the times. Spengler wrote that history had seen the rise and fall of several "civilizations" (including the Egyptian, the Classical, the Chinese and the Mesoamerican). He claimed that they go in cycles, typically spanning 1,000 years. Spengler believed that Western civilization is in a decline that is inevitable.

The idea that Western civilization is declining has been a common historical constant, often repeating variations on the same themes. Historian Arthur L. Herman, in the introduction to his book The Idea of Decline in Western History, wrote that:

... intellectuals have been predicting the imminent collapse of Western civilization for more than one hundred and fifty years ... Yet when I point this out as evidence that, to paraphrase Mark Twain, reports of the demise of the West might be greatly exaggerated, I usually meet with strong skepticism.

Declinism has been described as "a trick of the mind" and as "an emotional strategy, something comforting to snuggle up to when the present day seems intolerably bleak."

One factor in declinism is the reminiscence bump in which older people tend "to best remember events that happened to them at around the ages of 10-30." As one source puts it, "[t]he vibrancy of youth, and the thrill of experiencing things for the first time, creates a 'memory bump' compared with which later life does seem a bit drab." Gopnick suggests that "the idea of our decline is emotionally magnetic, because life is a long slide down, and the plateau just passed is easier to love than the one coming up." Citing the widespread love of "old songs," he writes: "The long look back is part of the long ride home. We all believe in yesterday."