Pornography has been around since the dawn of history. It's taken many forms -- from ancient Greek vases depicting copulating couples to grainy photos advertised in the back of 1950s men's magazines.

But today, America faces an epidemic of pornography that differs radically from everything that's gone before. Thanks to the Internet, you no longer need to sneak off to a grungy X-rated theater. With the click of a mouse, you can bring a stream of hyperrealistic, hard-core sexual images into your bedroom around the clock.

As a result, America is now a porn-saturated society. In 2005, for example, 13,585 hard-core porn video/DVD titles were released here, compared with 1,300 titles in 1988. A recent study of college and graduate students found that 69 percent of men and 10 percent of women view porn more than once a month.

As a society, we tend to see the issue of pornography through a libertarian lens. Sure, some may find it morally objectionable, we say, but it's a free-speech issue -- victimless entertainment that involves only the user and the consenting adults who participate in it. If you don't like it, don't use it.

Now a report from the Witherspoon Institute in Princeton, N.J., urges us to think again. "The Social Costs of Pornography" musters new social science and brain research, and finds voluminous evidence that pornography causes multifaceted harms for which our entire society is paying a price.

Porn is far more than private, passive entertainment. It "functions as a teacher, a permission-giver and a trigger of ... negative behaviors and attitudes," according to Mary Anne Layden, coauthor of the report and director of the Sexual Trauma and Psychopathology Program at the University of Pennsylvania.

What's more, today's universally available, intensely realistic porn creates a serious risk of addiction. Recent advances in neuroscience make clear that frequent exposure can actually alter the brain by hyperstimulating its appetitive pleasure system.

"Men at their computers looking at porn" are "uncannily" like "rats in cages ... pressing the bar to get a shot of dopamine or its equivalent," according to a neuroscientist quoted in the report. "They [have] been seduced into pornographic training sessions that [meet] all the conditions required for plastic change of brain maps."

Like alcoholics, porn addicts need increasingly novel or bizarre sexual images, over time, to reach the same level of arousal. As a result, they often seek out acts that initially disgusted them -- from genital mutilation to bestiality. Frequent porn viewing tends to stoke aggression: a 2007 study found that 88 percent of 50 best-selling adult videos contain physical violence. For some men, porn is a slippery slope to child pornography -- the subject of 116,000 online searches every day.

Pornography can have a profoundly detrimental effect on frequent users' social lives, productivity and happiness, according to the report. Paradoxically, Internet porn "can render the chronic user incapable of the very sexual satisfaction that he is seeking," undermining his ability to relate to women and even his sexual competence with a real partner. In the words of one expert, frequent porn users "risk the loss of love, in a world where only love brings happiness."

Pornography's ill effects aren't restricted to adult users. For example, 65 percent of 16- to 17-year-old boys say they have friends who regularly download pornography. Even pre-teen boys are being treated for porn addiction. Boys who use pornography are much more likely than their peers to sexually harass other youngsters. One study found that 29 of 30 juvenile sex offenders had been exposed to pornography, with 7 being the average age of first exposure.

Girls also face risks. "I am ... witnessing more female adolescents tolerating emotional, physical and sexual abuse in dating relationships," says one therapist quoted in the report. "[They are] normalizing sexual abuse ... because they see the same acts eroticized in pornography."

Pornography is seeping into our society at every level. It plays a role in many divorces, according to a recent survey of family lawyers. It has spilled into popular culture through songs, movies and music videos. The number of TV sex scenes nearly doubled between 1998 and 2005. Porn may even have influenced rogue American soldiers' abuse of prisoners at Abu Ghraib. The acts they perpetrated included weird elements of sexual humiliation.

We Americans like to think we take public health very seriously. We stigmatize smoking, and wring our hands over schoolyard bullying. But isn't Internet pornography a public health menace that dwarfs these?

Pornography teaches us to view other human beings not as ends in themselves, but as means to our own sexual pleasure. We can only guess at the future costs our unprecedented consumption of it will bring.

Katherine Kersten is a Twin Cities writer and speaker. Reach her at kakersten@gmail.com.