"Yes, we can. Yes, we can. Yes, we can."

This was a speech given by Obama during his electoral campaign in America in 2008. Clinton defeated Obama by 39.1% to 36.5% in the New Hampshire primary on January 8, 2008, and Obama gave this concession speech.

The following is a transcript of part of the speech:

“For when we have faced down impossible odds, when we've been told we're not ready or that we shouldn't try or that we can't, generations of Americans have responded with a simple creed that sums up the spirit of a people: Yes, we can. Yes, we can. Yes, we can. 

It was a creed written into the founding documents that declared the destiny of a nation: Yes, we can. 

It was whispered by slaves and abolitionists as they blazed a trail towards freedom through the darkest of nights: Yes, we can.

It was sung by immigrants as they struck out from distant shores and pioneers who pushed westward against an unforgiving wilderness: Yes, we can.

It was the call of workers who organized, women who reached for the ballot, a president who chose the moon as our new frontier, and a king who took us to the mountaintop and pointed the way to the Promised Land: Yes, we can, to justice and equality. 

Yes, we can, to opportunity and prosperity. Yes, we can heal this nation. Yes, we can repair this world. Yes, we can.”

Quite clearly Obama has repeatedly stated the phrase “Yes, we can.”
Repetition is a classic persuasive technique which increases how believable the message is. Research by Boehm (1994) has shown that repetition of a message can increase the perceived validity of its content (this is known as the validity effect). In this study, 2 statements with neutral initial validity (rated as 4 on a 1-7 validity scale) were selected to be included in posters which would be displayed throughout a university for 3 days. One statement was true and the other false. One week later, students completed validity, familiarity and source recognition questionnaires containing the true and false statements seen on the posters (repeated statements) and 4 new statements (2 non-repeated true statements and 2 non-repeated false statements). 

As can be seen in the table above, repeated statements were rated as more valid (5.45 out of 7) than non-repeated statements (4.07 out of 7). The statements were chosen due to their neutral ratings of 4 in the pilot test, meaning the repeated statements therefore increased in perceived validity, whereas the non-repeated statements showed minimal increase. Simply repeating the statements increased the perceived validity of both true and false statements. It was also found that repeated statements were rated as more familiar (6.58 out of 7) than non-repeated statements (2.43 out of 7), and that controlling for familiarity eliminated the validity effect. This suggests that familiarity is a mediator of the validity effect. This research therefore shows that repetition of a message, regardless of whether the message is true or false, increases its perceived validity. However, this effect is mediated by familiarity, meaning that a repeated statement may be perceived as valid indirectly through its familiarity.

This means that Obama’s repetition of the phrase “Yes, we can” should increase its perceived validity among voters, indirectly through its familiarity. This shows that repetition is an effective persuasive technique, as through repeating this phrase Obama would likely have persuaded many voters that “Yes, we can” make America a better place is a valid statement.

References
Boehm, L. E. (1994). The validity effect: A search for mediating variables. Personality and Social Psychology Bulletin, 20(3), 285-293.