Haven’t we learned from 9th grade health class that you can’t just look at someone and be like, “naaaah, he can’t have herpes!!” A new study shows that people judge their partner’s safety by their intelligence or outward appearance.

The questionnaire asked patients whether they would be “pretty sure” that a sex partner was “safe” in various situations — such as when they knew the person well, knew his or her friends, or simply felt they could trust the person.

The researchers found that people often took such subjective qualities as a sign that their partner would put them at low STD risk. For example, more than 70 percent of patients said they would probably consider a partner “safe” if he or she were generally trustworthy.