BOOK REVIEW / NONFICTION : If You Take Privacy for Granted, Don’t : CROSSING OVER THE LINE: Legislating Morality and the Mann Act <i> by David J. Langum</i> ; University of Chicago Press $24.95, 304 pages

David J. Langum makes no effort to conceal the righteous fury that burns at the heart of “Crossing Over the Line,” his scholarly monograph on the so-called Mann Act. Langum’s book is dedicated to “the victims of the Department of Justice,” and the very first line is an expression of the author’s exasperation and indignation over the government’s role in “legislating morality.”

“It is hard to believe,” he writes, “how things once were.”

What has gotten Langum so worked up is an odd relic of American jurisprudence formally titled the White Slave Traffic Act, but more commonly known as the Mann Act.

The law was enacted in 1910 as part of a rather bizarre international crusade against white slavers who supposedly lured vulnerable young women into prostitution --but Langum argues that the Mann Act was routinely used to criminalize even “purely consensual relationships, noncommercial in nature, between unmarried consenting adults.”

All that was required for a conviction under the Mann Act in its original form was proof that a female, regardless of age, marital status or state of mind, was transported across state lines “for immoral purposes.”

As Langum points out, the law criminalizes intent rather than conduct: “ ‘Orwellian badthought,’ plus transportation, defined the essence of the crime.”

Among the more celebrated victims of the Mann Act, as Langum shows us, was were Charlie Chaplin, whose radicals politics drew the attention of crafty federal prosecutors, and Frank Lloyd Wright, whose estranged wife used the law to leverage a favorable divorce settlement. But Langum argues that the Mann Act was (and is) a threat to everyone’s freedom.

“Almost every adult American male . . . has violated the Mann Act at least once in his life,” Langum writes. “Any man who has ever driven his girlfriend from New York to New Jersey, with the hope of sexual activities that evening . . . has committed a federal felony.”

Langum explains how Americans whipped themselves into what he calls a “moral panic” over the imagined evils of a vast international trade in young women destined for service in brothels.

And he suggests that the hysteria was actually prompted by a kind of collective anxiety over “immigration, urbanization and changing sexual mores,” all of which encouraged women to play a more liberated role in American life.

Whatever the original intent of the Mann Act, Langum demonstrates that a law intended to protect women from sexual predators was quickly turned into a weapon to punish “interstate adulterers or boyfriends and girlfriends”--and, ironically, a tool for blackmailers who lured men into sexual liaisons and then threatened them with criminal prosecution if they did not pay hush money.

Of course, the Mann Act is something of a curiosity nowadays, but Langum insists that the outrages of the not-so-distant past are a healthy caution against more recent (and current) government attempts to punish people for how they conduct their private lives.

Langum, a law professor and historian, reminds us that the Mann Act is still on the books, although it has been modified and updated, and the government is still perfectly willing to prosecute men and women for purely private acts ranging from one’s choice of reading matter to one’s choice of sexual partners and sexual acts.

“Congress has never had the courage to repeal an absurd law that has been used, and still could be used, to interfere with inoffensive activities or to persecute selected individuals.”

“Crossing Over the Line” is a work of scholarship as wrought by a civil libertarian, and the text--which is often quite technical--sizzles with the passion of an ardent believer in real liberty under reasonable laws. And so Langum’s book is a kind of wake-up call for Americans who may take their privacy and their personal freedom too much for granted.