We often use the word "creepy" to caution our friends about people to avoid. "Watch out for him," we warn a person we care about, "he's a total creep." But what, exactly, makes someone seem that way?

According to a study published in New Ideas in Psychology — the first-ever empirical study of "creepiness," its authors say — analyzed the perspectives of 1,029 women and 312 men with an average age of 29 to determine the most common markers of a "creepy" individual.

The results of the research show that men are more likely to be seen as creepy than women, and women who feel creeped out are more likely to also feel sexually threatened than men are.

Furthermore, unpredictability is often associated with creepiness, which we can see a lot in these tweets.

And it makes sense. Think about it someone laughing at seemingly random times or making sudden and unexpected attempts to hug you. A big part of the uneasiness they make you feel comes from not knowing what to expect from them next.

The researchers highlighted that these findings support the idea that being “creeped out” is a response that we evolved to pay more attention to something that could turn out to be a threat.

When your gut feeling tells you that something is off about someone, don’t instantly dismiss that reaction, even if you can’t pin down exactly why you have it — more often than not, it's there to protect you.

To identify the behaviors likeliest to make someone seem creepy, researchers instructed respondents to rate 44 different patterns on how likely they thought it was that a "creepy" individual would demonstrate them.

1. He watches you before interacting. Being watched by someone before he introduces himself or joins the conversation can make your skin crawl — you're wondering, What is he thinking? while dreading that whatever it is, it's inappropriate.

2. He touches you frequently. Even a "friendly" hug or kiss on the cheek is unacceptable if it's not invited.

3. He steers the conversation toward sex. Unwanted intimacy can be verbal too.

4. He asks to take a picture of you. Unless you are balancing 10 plates on top of your head while also riding a unicycle, there is no reason a rando should want a photo of you.

5. He asks for personal details about your family. Hey, dude, stick with complaining about the weather or asking me if I have any restaurant recommendations. That's what we call "small talk" — leave my family out of it.

6. He has greasy hair. Well, there are many possible explanations for having greasy hair, but let's just say it generally isn't a turn-on. Bad hygiene can be a sign that someone isn't aware of or doesn't care about the impression they make on others, a definite creepiness red flag.

7. He never looks you in the eye. This can come off shady and disengaged, as though he's plotting something.

8. He's significantly older than you. To be fair, this one doesn't make someone "creepy" in and of itself, but combined with other behaviors, such as frequent touching or sex talk, it can make someone seem predatory.

9. He shows little emotional expression. A deadpan stare is disconcerting. Again, what is he thinking — or hiding, for that matter?