A friend is someone we turn to
        when our spirits need a lift.
A friend is someone we treasure
        for our friendship is a gift.
A friend is someone who fills our lives
        with beauty, joy, and grace.
And makes the whole world we live in
        a better and happier place.

The prospect of catching a disease may be most likely to cross your mind during a one-night stand, or perhaps when you see a scantily clad woman cruising cars in the red-light district. In reality, though, STDs occur predominantly in the context of ongoing relationships.

Cindy Masaro, research coordinator at the University of British Columbia in Canada, recently conducted a survey with fascinating results. She wanted to understand how people are sizing up their partners. When it comes to assessing STD risk, what information do people rely on?

“What I found was that familiarity, trust, and assumed knowledge of the partner’s sexual history were the three major things people used in assessing partners,” says Masaro.

While that sounds all warm and fuzzy—intimacy, after all, is built on familiarity and trust—Masaro’s study made it clear that the qualities that help make up a good relationship don’t necessarily translate to partner safety. The 317 people she surveyed had all relied on partner attributes and the nature of their relationships—and all had sought treatment at an STI clinic.

Nearly all the subjects’ STD trouble could be traced back to assumptions. Masaro found that people were deciding about STD risk based on whether the partner had a similar background, shared in the same circle of friends, or seemed intelligent. More than one in five said they’d assume a sex partner was safe if he/she was physically attractive, and more than two in five if the partner seemed like a “good” person.

Seventy percent said they would be confident a partner was safe “if I felt I could trust them.” Clearly they did trust the partner if they let him/her in their pants. Trust is virtuous, but it turns out to be a lousy method of prevention.

“We all make inaccurate assessments, and feelings of attraction and affection interfere with risk assessment,” Masaro says. “When you have that person sitting in front of you, everything else goes out the window. You’re attracted to them so you don’t want to see anything wrong with them.”

And who wants to ask, anyway? A conversation about STDs is bound to be uncomfortable, and nothing kills a moment like an impromptu interrogation under the sheets.