Just days shy of the biggest travel day of the year, the furor over the Transportation Security Administration's new airport screening procedures shows no sign of abating. The policy, which sometimes requires a choice between posing for semi-revealing backscatter X-ray images or submitting to a vigorous pat-down of private areas, has raised hackles both online and in real life, with one man stripping off his clothes in protest at the San Diego airport on Nov. 21.

Another group of protestors, We Won't Fly, is calling for a national opt-out day on Nov. 24, the day before Thanksgiving and a boom time for travel. The group is urging passengers to jam up security lines by refusing to go through the controversial full-body scanners.

There's no single reason for the overflow of anger at the TSA: Some people cite concerns about radiation, while others worry about children being virtually stripped by scanners or patted down by strangers. Others debate how effective and necessary the TSA policies are and argue that the Fourth Amendment prevents such extensive searches.

But it's no coincidence that anger has boiled over in response to fully-body scans and full-contact pat-downs, psychologists say. Human beliefs about modesty and the sanctity of the body are influenced by culture, researchers told LiveScience, but their roots run deep.

"Physical characteristics, for men, but especially so for women, are what people are evaluated on by prospective partners," said Daniel Kruger, an evolutionary psychologist at the University of Michigan. "So it's going to be a very sensitive issue."

Society and modesty

Beliefs about what is considered modest versus immodest vary widely by culture, but most societies have some rules about what is acceptable, Kruger said. In America alone, acceptability gamut runs from the covered-up to the let-it-all-hang-out crowd, with religious groups like Muslims, Orthodox Jews, Mormons and conservative Christians advocating modest dress, while the average beachgoer is happy to bear all in a bikini or swim trunks.

Nonetheless, self-conscious emotions like shame and embarrassment develop early, said Karen Barrett, a developmental psychologist at Colorado State University. Kids start to show signs of embarrassment by about 15 months of age, Barrett told LiveScience. First, kids start to show discomfort when people stare at them; later, Barrett said, they start to learn the rules of society and feel shame when they break those rules. The taboo of nudity is one of those learned rules.

"Some kids are going to be modest at an earlier age than others, primarily because it's been emphasized in their environment," Barrett said. "It's pretty typical for 2-year-olds to feel perfectly comfortable undressing in front of whomever... but it would be unusual in our society to have someone completely unaware of it past 7 or so."

Evolution and embarrassment

The universality of these emotions has led some researchers to theorize that they're a necessary social glue, motivating us to play nice within the community. For that reason, being asked to break those rules — by stepping into a body scanner or allowing a stranger to pat your genitals — elicits a strong emotional reaction. This may be particularly true for people with medical devices or other characteristics usually kept private.

"People really do feel invaded," Kruger said.

Part of the reason, Kruger said, is that information about a person's body is integral to how other people size them up as a potential mate. People want to reveal that information strategically, Kruger said, keeping it close to the vest unless they're in the midst of courtship. Thus, being told you must reveal the pooch of your stomach or shape of your breasts to a stranger is distressing.

Another factor, said University of California, Los Angeles evolutionary psychologist Daniel Fessler, is sexual jealousy. Human fathers put a lot of resources into their offspring, so knowing that they're investing in their own genetic offspring is important. Enforcing sexual modesty is one way to try to control female reproduction.

"In pursuit of such restriction, men favor and enforce greater sexual modesty for women than for men," Fessler wrote in an e-mail to LiveScience.

Kruger sees echoes of that pressure in the TSA screening debate.

"Women specifically have said, 'My body is something only my husband can see,' " he said. "Women want to make sure they're not being seen as promiscuous, that they're seen as faithful."

Evolution aside, the screenings strike a nerve, because choosing between a full-body scan and a pat-down isn't the same as donning a swimsuit at the beach, Barrett said.

"I think part of it is the fact that it is non-volitional. This is something they are being forced to do," Barrett said of angry travelers. "If you chose to expose yourself, that feels very different than if someone forces you to do it."

This article was reprinted with permission from LiveScience.

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