It is not at all uncommon for you to be exposed to persuasion attempts by multiple sources. At political rallies, a number of speakers argue in favour or against some position or candidate to convince you. In advertising, a number of consumers extol the virtues of a product upon you. Opposing attorneys attempt to persuade in court of the merits of their cases by using a parade of witnesses to bolster their positions.

In 1981, two researchers, Stephen Harkins and Richard Petty decided to understand the effect of arguments from multiple sources upon users. They compared three different ways of convincing a group by providing plausible arguments against a statement:

  1. Different arguments were presented from a single source,
  2. One common argument was presented from three different sources, and
  3. Different arguments were presented from three different sources.

They inferred that when it comes to persuasion, #3 is the best — it’s most effective to have three sources presenting three different arguments.

Information from multiple sources is generally perceived to be from different perspectives and independent pools of knowledge and thus more worthy of consideration.

If most of the news channels you watch say the same thing, for e.g. why the next candidate is the best, you are highly likely to believe it. You are heavily influenced by which news channels you watch, and how many news channels you watch.

This pattern of multiple sources delivering multiple arguments being persuasive is because of a phenomenon called message elaboration. When you see multiple sources you engage in more argument-relevant processing. Each time a news source appears, you gear up to process the new message.

If the arguments presented are new and compelling, the enhanced processing elicited by multiple sources results in the generation of additional thoughts favourable to the advocacy of the statement.

But if you are exposed to multiple sources presenting multiple unconvincing arguments it leads to less persuasion and to the production of counterarguments instead, even more than it does when only one source presents unconvincing arguments. It’s a double edged sword.

Multiple source effect is undoubtedly a potent weapon of persuasion. There’s only a small caveat. The persuasive advantage of multiple sources presenting strong arguments is likely to be eliminated if the sources are perceived to have formed a committee rather than being independent. If you know that a couple of news channels have grouped together to rally against a certain political candidate, you won’t budge.

If you know this beforehand, you don’t engage in enhanced processing, and thereby aren’t as easily convinced. You simply treat them as a single source, or perhaps would consider the whole thing to be fishy. However, this works only when you have this information before argument exposure. It has no affect upon you if you find out that the sources have grouped together after you’ve been persuaded. You prefer to remain fooled.

Multiple source effect is highly effective in politics, love, sales & marketing, and any other field that demands a lot of persuasion. It also acts as a corollary to confirmation bias (more on this later) and social proof.

If you’re trying to sell your product to a group of users, or yourself to a person of interest, it would be a good idea to cite multiple customers/people who each speak of a different reason why they like your product/you. This is more effective than citing one person who speaks about all of these benefits, or citing multiple people who all speak of one same benefit.

So next time you plan to woo that girl, don’t just have your best friend go to her and shower false praises about you. Make multiple people do that. Independently. Later on, to show her how truthful a person you are, you can simply tell her that you planned the whole thing ahead. Research says, once persuaded, she won’t change her mind.