Diffusion is the process by which a particular phenomenon — such as a contagious disease, a technological innovation or even an idea — is spread from person to person over a period of space and time. One method of diffusion is known as expansion diffusion, which is broken down into three distinct types.
Strictly defined, expansion diffusion is the process of spreading something from one place to another in an ever-expanding "snowballing" process. Expansion diffusion is used to explain a variety of phenomena in numerous disciplines, from the spread of disease in medicine to the process of human settlement in the study of geography. Expansion diffusion is distinguished from regular diffusion when something spreads outward from a central point. Technology such as television and the Internet, for example, have been instrumental in spreading ideas from place to place, while the advent of air travel has had a similar effect on contagious diseases.
As its name suggests, contagious diffusion occurs when a particular characteristic is rapidly transmitted throughout the population. In this form of expansion diffusion, most adjacent individuals will be affected. An example of contagious diffusion is the early spread of Christianity, which spread from the Middle East to Europe. Another example can be seen in the spread of the bubonic plague that ravaged London during the 16th century, or the widespread influenza pandemic of 1918.
Hierarchical diffusion occurs when an idea is spread from a person or organization that holds authority over others. This type of diffusion is typically seen in cases where an idea is communicated by a political leader or person of influence and spreads. This typically begins in an urban setting before eventually reaching less populated areas. An example of hierarchical diffusion can be seen in the popularity of rap and hip-hop music, which began in low-income black neighborhoods in densely populated urban areas before spreading out and gaining widespread acceptance among members of other socio-economic and geographical groups. Hierarchical diffusion also explains the widespread imitation of the hairstyle worn by actress Jennifer Aniston on the hit sitcom "Friends."
Stimulus diffusion is when an idea, principle or innovation underlying a phenomenon spreads to a small portion of a population, even though the phenomenon itself may not be diffused. This typically occurs when, due to cultural differences, certain aspects of a phenomenon become diffused as opposed to the phenomenon as a whole. An example of this can be seen in U.S.-based fast-food restaurant McDonald's expanding its operations to India, a country in which the chain's primary product --- beef hamburgers --- are culturally repellent to the country's millions of Hindus. As a result, McDonald's serves no beef in its Indian restaurants, offering vegetarian patties instead. In this way, the phenomenon of McDonald's has spread to India although the fundamental principle underlying the company's success has not.