You may not have noticed, but you are more likely to buy a product if the person in the advertisement is like you. You’re also more likely to trust a testimonial if it is provided by somebody who resembles your traits and demographics.
We can go all the way back to the 1870s and find advertisements like this one, which includes endorsements from Hop Bitters customers who have used the product. In one case, the testimonial included the full address of the author, increasing the authenticity of what was written.[*]