When you try to persuade people to accept your position on a divisive issue, including opposing opinions in your argument can make your message more persuasive.
You might be reluctant to make your audience aware of other viewpoints for fear you could lose their support, but social psychology research shows that the opposite is more likely to be true.
Whether you are presenting your argument in a written message or in a speech, if there is an opposing view, you could address it in two ways. You could make the audience aware that they might hear another argument intended to persuade them, and then leave it at that. But a more effective approach would be to mention the opposing view, explain what it is, and then present a counterargument.
A theory of behavioral psychology, called innoculation, comes into play, and it works the same way as it does in medicine. When you are innoculated against a disease, you are less likely to contract the illness. Similarly, research shows that if you present an opposing view and then give a compelling explanation of why the other side’s position is inferior, it forewarns what people might hear later. According to research, the audience is more likely to form an opinion consistent with yours and will be more resistant to opposing points of view when they are exposed to them later.
Psychologist Robert Gass says that by forewarning people, you give them time to think about their position, and they are more likely to develop their own counterarguments against opposing viewpoints.
With a one-sided argument, you can devote more time to your own position, but readers or listeners are left thinking that you conveniently omitted the opposing perspective. Consequently, you are viewed as less credible.