Young couples have long signaled their devotion to each other by various means - the gift of a letterman jacket, or an exchange of class rings or ID bracelets. Best friends share locker combinations. The digital era has given rise to a more intimate custom. It has become fashionable for young people to express their affection for each other by sharing their passwords to email, Facebook and other accounts. Boyfriends and girlfriends sometimes even create identical passwords, and let each other read their private emails and texts.

They say they know such digital entanglements are risky, because a souring relationship can lead to people using online secrets against each other. But that, they say, is part of what makes the symbolism of the shared password so powerful.

"It's a sign of trust, " Tiffany Carandang, a high school senior in San Francisco, said of the decision she and her boyfriend made several months ago to share passwords for e-mail and Facebook. "I have nothing to hide from him, and he has nothing to hide from me. "

"That is so cute, " said Cherry Ng, 16, listening in to her friend's comments to a reporter outside school. "They really trust each other. " We do, said Carandang, 17. "I know he'd never do anything to hurt my reputation, " she added.

It doesn't always end so well, of course. Changing a password is simple, but students, counsellors and parents say that damage is often done before a password is changed, or that the sharing of online lives can be the reason a relationship falters.

The stories of fallout include a spurned boyfriend in junior high who tries to humiliate his ex-girlfriend by spreading her email secrets;tensions between significant others over scouring each other's private messages for clues of disloyalty or infidelity;or grabbing a cellphone from a former best friend, unlocking it with a password and sending threatening texts to someone else.

Rosalind Wiseman, who studies how teenagers use technology and is author of Queen Bees and Wannabes, a book for parents about helping girls survive adolescence, said the sharing of passwords, and the pressure to do so, was somewhat similar to sex.
Sharing passwords, she noted, feels forbidden because it is generally discouraged by adults and involves vulnerability. And there is pressure in many teenage relationships to share passwords, just as there is to have sex. "The response is the same: if we're in a relationship, you have to give me anything, " Wiseman said.

In a 2011 telephone survey, the Pew Internet and American Life Project found that 30 per cent of teenagers who were regularly online had shared a password with a friend, boyfriend or girlfriend. The survey, of 770 teenagers aged 12 to 17, found that girls were almost twice as likely as boys to share. And in more than two dozen interviews, parents, students and counsellors said that the practice had become widespread.

In a recent column on the tech-news Web site Gizmodo, Sam Biddle called password sharing a lynchpin of intimacy in the 21st century, and offered advice to couples and friends on how to avoid missteps.

"I've known plenty of couples who have shared passwords, and not a single one has not regretted it, " said Biddle in an interview, adding that the practice includes the unspoken notion of mutually assured destruction if somebody misbehaves.

Students say there are reasons, beyond a show of trust, to swap online keys. For instance, several college students said they regularly shared Facebook passwords - not to snoop on or monitor each other, but to force themselves to study for finals.