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The Multiple Source Effect

The Multiple Source Effect

Ah, multiple sources. When I was first learning to write term papers in school, you were the bane of my existence. Even though these were the days before Wikipedia or anything close to a widely available high-speed Internet, and we were dragging our bodies physically to college libraries, navigating a card catalog (although if you were lucky and it was a nice, high-tech college library, you might have rudimentary computerized search available), and digging up hard copies, you still had to have multiple sources.

For the paper in general of course. But also for any particularly important or controversial points you wanted to make.

It could be irritating at times when you wanted to ray off into undiscovered territory to need so much intellectual backup. But the teacher was never satisfied unless you cited multiple sources.

I found myself wondering many times while in the midst of digging up yet another source to try to support a contention… what about the quality of these sources? I mean, they were all ostensibly quality sources, due to their presence at the college library (which tended to reserve more space for well-supported work), but did my teacher actually know that?

Wouldn’t it be better to prioritize quality of sources over quantity? Couldn’t I just pull three shoddy sources (or material only tangentially related to my point) out of my back pocket where one single, extremely relevant and very well-supported source would have made a stronger statement?

Multiple Source Effect

It turns out that whiny teenage me might have been on to something (I’m as shocked as you are) — although I must admit, so was my teacher.

In research conducted by Harkins and Petty in 1981, three separate conditions were tested arguing against a statement:

  1. Multiple different arguments against were presented from one source.
  2. The same argument against was presented from three separate sources.
  3. Multiple different arguments against came from three separate sources.

The researchers found that the most persuasive effect came in the third condition — when a variety of different arguments came from three separate sources.

It’s quite unsurprising that it was important to provide a multiple arguments against something to be persuasive. Typically, an effective debate platform is made up of multiple contentions, several smaller points supporting the overall argument.

If you have just one contention, you might feel like you have a case, but it terms of effective persuasion or argument, you really don’t.

What was interesting, however, is when that same varied case was presented as coming from three separate sources, it was perceived as more credible/persuasive.

So it would seem that there’s definitely a quality (variety/complexity of argument) AND a quantity (multiple sources) issue when it comes to citing sources.


In the spirit of multiple source effect, here are a few other followup studies on multiple source effect:


This post is part of an ongoing Poly Land feature called Psyched for the Weekend, in which I geek out with brief takes about some of my favorite psychological studies and concepts. For the entire series, please see this link.

Featured Image: CC 0 – Pixabay