Diffusion theory was first studied in the early 20th century by French, German, and Australian anthropologists. It was later developed and popularized in 1962 by Everett Rogers in his book "Diffusion of Innovations", which proposed that new ideas and technologies spread through populations via communication channels over time. The theory describes the process by which an innovation is adopted by individuals in a social system, following a common pattern: knowledge, persuasion, decision, implementation, and confirmation. Key elements include the innovation itself, communication channels, time, and the social system.