From Rowland: Back in the 1960s/early 70s I did a Masters' degree (University of Sydney) on The Diffusion of Innovations in Religious Organizations.

I've just come across a Wikibooks article which summarizes this field of social psychology:

According to Rogers (1996), diffusion refers to "the process by which an innovation is communicated through certain channels over time among the members of a social system. An Innovation is an idea, practice or object perceived as new by an individual or other unit of adoption. The diffusion of innovations involves both mass media and interpersonal communication channels" (p. 409). That is, by sharing communication channels such as interpersonal communication or mass communication people can get information of an innovation and perceive its innovation as useful. Lasswell (1948) presented a well-known model of communication that is analyzed as five parts, S-M-C-R-E (e.g., sender-message-channel-receiver-effect). Rogers (1995) mentioned, "this S-M-C-R-E communication model corresponds closely to the elements of diffusion" (p. 19). Specifically, (1) sender can be inventors or opinion leaders, (2) message can be a new idea or product, (3) channels can be interpersonal or mass communication, (4) receivers can be members of a social system, and finally (5) the effects can be individual's adoption or social change. In the diffusion theory, 'Time' variable is a very important factor. According to Rogers (1995), time variable is involved in diffusion in (1) the innovation-decision process; (2) innovativeness; (3) an innovation's rate of adoption.

Most innovations have an S-shaped rate of adoption. Diffusion research has attempted to explain the variables that influence how and why users and audience adopt a new information medium, such as the Internet. According to evolution of media technology, interpersonal influences are important even though in the past the individual is usually the unit of analysis. Also, critical mass becomes an important factor in adopting new media because new media are interactive tools and thus are required by many users to gain efficiency. That is, the more people use, the more people get benefits. In this sense, diffusion theory not only can apply to practical things, but also can be related to digital divide.

There are five different types of adopters in the diffusion process, according to Innovativeness: "(1) Innovators (venturesome), (2) Early Adopters (respectable), (3) Early Majority (Deliberate), (4) Late Majority (skeptical), and (5) Laggards (traditional)" (Rogers, 1995, pp. 183-185). Rogers defined this term as "the degree to which an individual is relatively earlier in adopting new ideas than other members of his social system" (Rogers, 1995, p. 40). Figure 1 shows the relationships between types of adopters divided by innovativeness and their place on the adoption curve. Also, these categories follow a standard deviation curve which is bell-shaped.

Source by www2.gsu.edu/~wwwitr/docs/diffusion/

More...http://en.wikibooks.org/wiki/Communication_Theory/Diffusion_of_Innovations