text: Hierarchical diffusion: an idea or innovation that spreads by moving from larger to smaller places, often with little regard to the distance between places, and often influenced by social elites.
  Contagious diffusion: an idea or innovation that spreads based on person-to-person contact within a given population with no regard for hierarchies. HIV/AIDS first spread to urban neighborhoods (Hierarchical diffusion) and then spread outwards (contagious diffusion)
  Stimulus diffusion: an idea or innovation that spreads based on its attachment to another concept. Occurs when a certain idea is rejected but the underlying concept is adopted. Early Siberian people domesticated reindeer only after exposure to the domesticated cattle raised by cultures to their south. They had no use for cattle but the idea of domesticated herds appealed to them, and they began domesticating reindeer, an animal they had long hunted.
  Mechanisms
  Inter-cultural diffusion can happen in many ways. Migrating populations will carry their culture with them. Ideas can be carried by trans-cultural visitors, such as merchants, explorers, soldiers, diplomats, slaves, and hired artisans. Technology diffusion has often occurred by one society luring skilled scientists or workers by payments or another inducement. Trans-cultural marriages between two neighboring or interspersed cultures have also contributed. Among literate societies, diffusion can occur through letters, books, and, in modern times, through electronic media.
  There are three categories of diffusion mechanisms: