Civil liberties campaigners are preparing to legally fight San Francisco's ban on public nudity. Does the US Constitution guarantee the right to walk the streets naked?

No sooner had San Francisco's lawmakers voted to ban nakedness in public places than a lawsuit was filed. It argued that the free speech of nudists was being restricted.

It's a dispute that typifies the famously liberal city, but it also reveals much about the importance of freedom of expression to Americans.

Opponents of the move say that, by limiting the ability to express themselves, the law violates their rights. They cite the First Amendment to the US Constitution, which forbids the government from "abridging the freedom of speech".

But legal experts say previous court rulings suggest that being nude does not, in itself, constitute an act of expression.

"It's taken as a given by the Supreme Court that a general restriction on public nudity is constitutional," says Frederick Schauer, professor of law at the University of Virginia.

The answer The US Supreme Court does not regard public nakedness as a right under the First Amendment

Nudity in the form of artistic expression is protected

But authorities can ban nude performances for "secondary" reasons

San Francisco's Board of Supervisors voted 6-5 in favour of banning anyone over five from exposing "his or her genitals, perineum or anal region" in most public locations.

It came after a growing number of naked men gathered regularly in a plaza in in the city's Castro district, an area well known for its large gay community.

Ken Paulson, president of the First Amendment Center, which promotes the study of free-speech issues, says he does not believe the legal bid on behalf of the Castro nudes is likely to succeed.

Image caption A nude backpacker walks the street of San Francisco last year

"There's no First Amendment right to be naked in a public place because you are not saying anything," he adds.

But while the Supreme Court has been consistent that nudity is not in itself a form of expression, that does not mean that nudity cannot be expressive.

"Being naked in the Castro could be seen as expressing a point of view about LGBT rights," says Amy Adler, professor of law at New York University.

"I'm not sure a court would buy it but that's the argument I would expect them to make."

It's possible that the nudists could argue their nakedness was a political act.

Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof; or abridging the freedom of speech, or of the press; or the right of the people peaceably to assemble. From the First Amendment to the United States Constitution

The Supreme Court has, in fact, held that nude dancing, as practiced in strip clubs, is a form of free speech, Schauer says.

However, it ruled that establishments which offer displays of erotic dancing can be regulated.

As long as the regulations are intended to deal with the side effects of such establishments - if they are causing a rise in crime, for instance - it is legal to impose a public nudity ban. It was a principle established in law in the 2000 case Erie v Pap's AM, which set a Pennsylvania city council against a nude dancing establishment.

Of course, strip bars operate indoors, on private property, while San Francisco's nudists express themselves in full view of the public.

San Francisco supervisor Scott Wiener, one of the lawmakers who passed the nudity ban, said it was "appropriate to have some minimal standards of behaviour" in the city and "our public spaces are for everyone".

According to law professor Adler, case law suggests the city was acting within its rights.

"Even if the nudists could prove they were engaging in nudity for expressive purposes, the state has the right to regulate it," she says, as long as the regulation is not about what's being expressed, but the side effects that expression has on the rest of the city - for instance, heavy traffic or lowered property values.

But all may not be lost for the naked men of the Castro.

One option open to them would be to move somewhere such as Oregon, where public nudity is legal so long as there is no "intent to arouse".

"There's nothing in the constitution that requires a state to regulate against nudity," says Schauer.

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