Matthew Breuer has shared the passwords to his computer, email and social media accounts with every girlfriend he’s ever had. It’s a matter of convenience — she can check his email when he can’t access it or get into his phone to change the song playing on the speakers. But it’s also symbolic.

“I feel like it’s so much easier to live in a relationship where you know you have nothing to hide and are entirely 100 percent honest about who you are and what you’re doing,” he says. “Times in my life when I’ve realized that something wasn’t working in my relationship coincided with times when I would be worried, ‘Oh, do I really want to say this on Facebook to somebody else?’ It’s such a red flag if there’s something you’re concerned about your partner seeing. That means there’s some fundamental issue with your relationship beyond privacy.”

Breuer, a 22-year-old student at Yale University, has most American couples on his side. According to a recent Pew study, 67% of Internet users in marriages or relationships have shared passwords to one or more of their accounts with their partner.

Though we don’t feel comfortable exchanging passwords with perhaps more trustworthy family members and long-term friends, we do feel comfortable exchanging access to our personal information with boyfriends and girlfriends. It’s an exercise in trust, the logic goes. If you have nothing to hide, why would you want to hide your password? And, as Breuer point out, knowing someone may look over your shoulder can keep you honest.

For Jasmine Tobie, a 29-year-old graduate student at the University of Oklahoma in Tulsa, seeing someone else’s transgressions via email has saved them from a toxic relationship. After finding some receipts that proved her boyfriend was lying to her about being on a business trip one weekend, she decided to look at his email to be sure before she pulled the plug on the relationship. “Once I found that I just had to have more evidence.”

She didn’t know his password, but was able to guess correctly using clues on his desktop. “He was still ‘communicating’ with his exes. He had taken a trip to visit an ex and told me it was a work trip. He was still signed up with dating sites and other ‘hookup’ sites and actively communicating with those people…I found some pictures of him and people he swore were ‘friends’ in the act.” The two had dated for a year and lived together for about nine months. “I was trying to find some way to give him the benefit of the doubt. In the end, it did clarify for me that he was not it for me at all and that there were issues I couldn’t fix.”

Tobie says those were extraordinary circumstances, and she wouldn’t read someone else’s emails again. She doesn’t share passwords with her current boyfriend.

In most circumstances, psychologists suggest keeping passwords private.  ”In relationships, we depend on each other for a lot of things, but it’s good and healthy to have some independence too,” says Kelly Campbell, PhD, an Associate Professor of Psychology at California State University. “The more you self-disclose, the happier you are. But the happiest couples have some degree of secrecy and privacy.”

Unsurprisingly, sharing passwords can cause some serious problems during a relationship or after it ends.

Rosalind Wiseman, author of Queen Bees and Wannabes — the book that inspired Mean Girls — advises the teens she talks to for her research to not share passwords because “the relationships can change so quickly, and the emotions behind the breakups can be so strong.” She says that one high schooler she worked with was blind-sided when his ex-girlfriend found his phone. “She knew where he charged his pone during class and knew his password, so she went in and sent all sorts of texts to friends, to another girl he was talking to — it really created a lot of problems for him.”

Though one might assume that teens and 20-somethings are the ones foolishly sharing passwords — and suffering from the resulting drama — the survey found that the practice of password-sharing is pretty equal across age groups, and that 18-29-year-olds were actually the least likely to share passwords. Sixty-four percent of 18-29-year-olds share passwords, compared with 70% of 30-49-year-olds, 66% of 50-64-year-olds, and 69% of those over 65.

And you don’t have to be a teenager to have password problems with your significant other. Suzy*, a 46-year-old mother living in Brooklyn, got into a dangerous situation years ago when her then-boyfriend started reading her emails. She hadn’t given him her password, but one day she forgot to log out and he checked her email. The couple had been on-again-off-again, and she hadn’t told him that she had created an online dating profile while they were apart. She had since deleted the profile and deleted most of the email exchanges with the men she met through the site. “But he went through all my emails, including ones that I had thrown away. He went into every folder. He got really mad and basically attacked me,” she says. “I ended up having to call an ambulance.”

Since, she says she’s never even considered sharing passwords with a significant other. “I now have this paranoia where I wouldn’t even share it even if I trusted someone. You never know what’s going to upset someone,” she says. ‘I don’t know if that makes me less trusting or just wiser.”

Still, optimists like Breuer are undeterred by such horror stories. Breuer says he has always developed friendships with the girls he has dated before dating, and therefore felt they could be honest with one another. ”I think sharing passwords honestly ends up affording you the privacy you want,” Breuer says, pointing to a password etiquette that has developed between him and his partners in recent years. “Just because you tell somebody your password to things doesn’t mean they actually end up looking through your stuff.” Breuer says he’s never changed his password after a breakup since he’s always trusted and respected those he has dated.

Campbell says the best way to determine if you’re ready to share passwords with your significant other is to check and see if you’re on the same page. “If you have any question in your mind, the answer is no,” says Campbell. “I would say that it should be reciprocal. You shouldn’t be sharing something if your partner also didn’t share it…People are happiest when they have a match. You and your partner should be a match in that respect too.”

But much of the tough negotiating about privacy goes out the door once you have kids. “Sure, a lot of people have found out about their significant other’s indiscretions by looking at the texts on that person’s phone,” says Wiseman. “But once you have children, the constant checking of logistics with the other person to just get through the day—to get everyone to basketball practice on time—blows all of this privacy stuff out of the water.”

Interestingly, the attitude about privacy seems to change when it’s the child’s, not the partner’s, text messages in question. Both Wiseman and Suzy admitted that they’ll often try to figure out their children’s passwords or have their kids show them text exchanges to make sure they’re not getting into any trouble.

But presumably, by 18, you’ve earned the right to some privacy if you choose to have it.