Opinion leadership is a concept that arises out of the theory of two-step flow of communication propounded by Paul Lazarsfeld and Elihu Katz Significant developers of the theory have been Robert K. Merton, C. Wright Mills and Bernard Berelson. This theory is one of several models that try to explain the diffusion of innovations, ideas, or commercial products.
The opinion leader is the agent who is an active media user and who interprets the meaning of media messages or content for lower-end media users. Typically the opinion leader is held in high esteem by those who accept his or her opinions. Merton  distinguishes two types of opinion leadership: monomorphic and polymorphic. Typically, opinion leadership is viewed as a monomorphic, domain-specific measure of individual differences, that is, a person that is an opinion leader in one field may be a follower in another field. An example of a monomorphic opinion leader in the field of computer technology, might be a neighborhood computer service technician. The technician has access to far more information on this topic than the average consumer and has the requisite background to understand the information, though the same person might be a follower at another field (for example sports) and ask others for advice. In contrast, polymorphic opinion leaders are able to influence others in a broad range of domains. Variants of polymorphic opinion leadership include market mavenism, personality strength  and generalized opinion leadership. So far, there is little consensus as to the degree these concept operationalize the same or simply related constructs.
In his article "The Two Step Flow of Communication" by Elihu Katz, he found opinion leaders to have more influence on people's opinions, actions, and behaviors than the media. Opinion leaders are seen to have more influence than the media for a number of reasons. Opinion leaders are seen as trustworthy and non-purposive. People do not feel they are being tricked into thinking a certain way about something from someone they know. However, the media can be seen as forcing a concept on the public and therefore less influential. While the media can act as a reinforcing agent, opinion leaders have a more changing or determining role in an individual’s opinion or action.
In his article, Elihu Katz  answers the question, "Who is an opinion leader?" One or more of these factors make noteworthy opinion leaders:
- expression of values
- professional competence
- nature of his social network.
Opinion leaders are individuals who obtain more media coverage than others and are especially educated on a certain issue. They seek the acceptance of others and are especially motivated to enhance their social status. In the jargon of public relations, they are called thought leaders.
In a strategic attempt to engage the public in environmental issues and his nonprofit, The Climate Project, Al Gore utilized the concept of opinion leaders. Gore found opinion leaders by recruiting individuals who were educated on environmental issues and saw themselves as influential in their community and amongst their friends and family. From there, he trained the opinion leaders on the information he wanted them to spread and enabled them to influence their communities. By using opinion leaders, Gore was able to educate and influence many Americans to take notice of climate change and change their actions.
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