"Whether people are persuaded by spokespeople in advertisements depends on their ability and motivation to think about the relation between the spokesperson and the advertised product," explains Paul M. Herr (University of Colorado). "When consumers are either unable or unwilling to consider the spokesperson's credibility, they rely on the spokesperson's attractiveness."
The researchers present the first unified theory explaining how we respond to spokespeople and models, outlining the major processes involved in our perception of this type of advertising. We first gauge the attractiveness of the spokesperson and use this as information about the product. We then evaluate the spokesperson's relevance to the product before correcting our biases.
"When consumers are focused on the ad and they believe their thinking to be unduly influenced by something about the spokesperson (other than that spokesperson's credible product claims), attractive spokespeople may be less persuasive than relatively unattractive spokespeople," write the authors.
In other words, though she may very well be an expert, a woman in a bikini does not seem credible as an authority on computer processing speed. A consumer concerned primarily with the quality and features of the computer may suspect the undue influence of the model on his/her perception and compensate with a negative judgment.
Yong-Soon Kang and Paul M. Herr. "Beauty and the Beholder: Toward An Integrative Model of Communication Source Effects" Journal of Consumer Research. June 2006.