In the fickle world of modern-day advertising, there is one certainty we can always count on: Sex sells.

Or so we've been trained to believe. Indeed, that age-old mantra of consumerism is repeated so often by marketing executives that it’s accepted with pious conviction, but past research on the effectiveness of sexual imagery in advertising has been at best conflicting and at worst downright murky.

Now three new studies published in the journal Organizational Behavior and Human Decision Processes suggest that the picture is even more nuanced than previously thought, with blatant sexual images often having the opposite of their intended effect -- at least when it comes to female consumers.

The studies, conducted at the University of Manitoba and the University of Michigan, found that women tend to deploy a subconscious defense mechanism when they are exposed to advertising in which sexy models are blatantly featured. That defense mechanism, the researchers say, is a natural response, but, for advertisers, it comes with a price, namely a negative association with whatever product the sexy model is hawking.

“In those cases, the women tended to rate the product negatively,” Dr. Tamara Ansons, an assistant professor of marketing at Warwick Business School and one of the researchers who conducted the studies, said. “They experience the model as a threat.”

Ansons said the negative reactions to the advertising were subtle and that women typically didn’t even realize they were having them. “It’s automatic,” she said. “I think it’s just the natural way that we deploy defensive strategies to restore our positive self-perceptions. It was only after they were asked to evaluate the models did we see that they were having negative associations.”

This is not to say that sexual advertising is completely ineffective on female consumers. Ansons and her colleagues found that sexual images, if used subtly, can generate positive brand associations among female consumers if the sexy models in question are not the focal point of the ad. In their third study, the researchers exposed women to various ads for vodka -- some featuring blatant sexual imagery and others featuring sexy models off to the side. The blatant ads, Ansons said, tended to elicit the defensive strategy.

Sexual advertising, whether it’s effective or not, is more prominent than ever. Last year, a research study out of the University of Georgia revealed that, in 1983, sexually related magazine ads were used only 15 percent of the time. Twenty years later, that number climbed to 27 percent. Despite the prevalence of such ads, however, Ansons said little research has been done to gauge their effectiveness on female consumers. That’s a perplexing thought when we consider that women, by some estimates, control upwards of 80 percent of the household consumer spending.

“There is a lot of interest in how these ads affect men,” she said. “We thought it was important to see how women were reacting and to look at the difference between a subtle and blatant presentation.”

So what does all this mean for advertisers keen on enlisting Megan Fox, Kate Upton or Gisele Bündchen as their next brand ambassador? Ansons said her study doesn’t mean that ad agencies need to swear off sexy ads for good. “From a marketing perspective, I think it’s just a word of caution,” she said. “You shouldn’t just blindly slap an attractive model on your product without considering whether viewers might feel threatened by these images. You could be creating negative associations with your brand.”

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