Final answer:
A message is very effective when the source is both an expert and trustworthy. Expertise, consensus among experts, and lack of bias enhance source credibility. However, the effectiveness also depends on how well the source aligns with the research purpose or argument.
Step-by-step explanation:
Research shows that a message is very effective when the source is both an expert on the topic and trustworthy. Features such as the credibility of the speaker and their expertise significantly influence the persuasive power of a message. Trustworthiness includes factors like honesty, being unbiased, rational, well-informed, and clearheaded. When we evaluate a source for reliability, we look for expertise, consensus among experts, and the absence of bias. The physical attractiveness of the speaker also plays a role in persuasion. This is why famous actors and athletes are often chosen to advertise products.
However, it's also important to consider how the source fits the purpose of your research or argument. Even the most credible and attractive sources can be used ineffectively if they don't align with the research requirements or argument being made. The use of testimonials in advertising, for example, relies on the authority of the source to persuade the audience.