In two classic studies, Stephen Harkins and Richard Petty compared three different ways of presenting an argument: 1) one source presents three different arguments, 2) three sources all present the same argument, and 3) three sources each present a different argument.
What they found is that when it comes to persuasion, #3 is best. It’s most effective to have three sources presenting three different arguments.
This has several important implications for your presentations.
First, if it makes sense to do a group presentation, go for it! By having a different voice present each of your arguments, you can significantly increase the impact of them all. This is something that my wife Melissa and I have observed quite clearly since we started co-teaching our social dance classes. Previously, when only I had a microphone, we were doing #1: while we both contributed equally to the planning of the class, all of our ideas were expressed in a single voice. What we’ve recently discovered is that simply by giving Melissa a microphone and having her present half of the content, our students pay greater attention, report greater enjoyment, and perform better. Even though the content of the class is the same, adding a second voice improves every aspect of it.
That’s great if you have a group of people who can present, but what if you’re the only one? Can you still benefit from the multiple source effect? Indeed, you can, by citing multiple sources that each present a different argument. In The Science of Speaking, I discuss the power of citing experts in general. These studies add on to this advice by suggesting specific tips for applying it: where possible, cite multiple experts who each provide a unique perspective. For example, if I’m trying to sell The Science of Speaking, it would be a good idea to cite multiple experts (or better yet, customers) who each speak to a different reason they like the book. This will likely be more effective than citing one person who speaks to all of these benefits, or citing multiple people who all speak to the same benefit.