It had become customary for popes to publish new writings on social issues at ten-year intervals since Rerum novarum, in order to develop the teachings in concert with the evolving social context as a result of the industrial and political revolutions of the 20th Century.
The tradition of popes publishing new writings on social issues every ten years, established after Rerum Novarum, is a response to the evolving social context shaped by the industrial and political revolutions of the 20th century.