In 1937, John Maynard Keynes gave a lecture on "Some economic consequences of a declining population". Many at the time felt the world was overpopulated and fewer people could only be a good thing, a view that Keynes himself shared. But the purpose of his lecture was to issue a warning: declining populations come with nasty economic side effects.
Keynes, it turned out, was worrying a couple of generations too soon. Births jumped in the post-war baby boom.