There’s something about a one-night stand—the romped-in sheets, the clothes strewn about the room, the streaks of last night’s makeup under the eyes—that, depending on whether you’re a guy or a girl, can speak to two overwhelming emotions: satisfaction and serious regret. 

Impulsively shagging someone you don’t know too well—and might not see again—isn’t all that uncommon an occurrence. About half of all Americans will have at least 1 one-night stand in their lifetime, according to one Norwegian University of Science and Technology study. And in some Western European countries, that number climbs to 7 in 10.

But which partner wakes up satisfied with a shit-eating grin, and which wakes up under a cloud of dread, breaks fairly consistently along the lines of what we already know about evolution and gender, a new study has revealed. 

This isn’t the first time results like these have been uncovered. A previous study, conducted in the U.S., showed that, while 35% of women regretted their most recent episode of casual sex, only 20% of men felt the same. Norwegian researchers wanted to see if this also rang true in their country—which is alleged to be more sexually liberal—and, if so, why emotions conflict so greatly across genders. 

To find out, they questioned 263 students ranging in age from 19–37, all of whom had had at least a single one-night stand.

The quick and dirty: Women regretted having a one-night stand far more than men. And men regretted passing up a one-night stand far more than women.

Only 30% of the Norwegian women said they were “unequivocally happy” with their most recent casual sex, compared with more than 50% of men who said they were content with their choice. And nearly 80% of women were pleased they’d skipped casual sex, while only 43% of men were happy they’d turned it down.  

To try to understand why men seem so gung-ho about casual flings and women seem so dissatisfied, researchers worked with evolutionary psychologist David Buss, from the University of Texas at Austin, to create an in-depth analysis. 

Here, listed from least to most important, are the top 7 reasons why she may be a handful in the morning.

7. A woman worries more

Overall, females stress more than men over pretty much every facet of life, according to lead study author Leif Edward Ottesen Kennair. Men, on the other hand, are more prone to "do stupid things that they die from." (His words, not ours.)   

6. Unwanted pregnancy is always a concern

One-night stands aren't always associated with late-night booze sessions, but it's often the case. And with looser inhibitions and greater intoxication comes poorer decision making—like not using a condom. As long as pregnancy is a possibility, it will always be a potential cause of post-sex regret. 

5. She's afraid she'll get gonorrhea, chlamydia, or something like it

As with unwanted pregnancy, the thought she might contract an undesirable STI (which includes...all of them) won’t be far from her mind—and, she’s thinking, even the best sex in the world isn't worth that.

4. She doesn't want to end up with a bad reputation

No girl likes (or, we should add, deserves) to wear that label. Even if she doesn't tell her friends what she did, any judgment she places on herself can make her feel remorseful. 

3. She probably won't be able to get off

Women often wish they'd skipped casual sex because it wasn’t as sexually gratifying as it was for the guy, researchers say. In fact, in the Norwegian study, far more men had orgasms (surprise!) than women during their last sexual encounter.

2. You're not her ideal partner  

One of the main reasons women regret one-night stands more than men is due to a basic difference in evolutionary psychology, research shows. Men usually regret saying no to casual sex because they're hard-wired to have as many sexual partners as possible: quantity over quality. Guys may not consciously think this, but, as with most humans, the biological “end goal” is to pass your genes on to the next generation. Eons ago, the men who impregnated the most women scored "best in the evolutionary race." (No, this isn’t your green light to go on a sex frenzy; but it does explain the customary male attitude toward casual sex. )

In the long run, partner quality—the desire to end up with the man who’s best fit to father a child—is way up there on the list for women. It’s a mindset that runs unconsciously in her thoughts pretty much all the time, and there’s not much you can do about it.

1. Women have more to lose

If you don’t measure up to her standards, aren’t willing to invest your heart and soul in her and in a potential family, and make her think you'll waste her time by fooling around with other women, then she's going to regret going to bed with you. It sounds like way more thought and pressure than a night of casual sex should have, but it's ingrained in her. It’s biology.

"Female choice—deciding when, where, and with whom to have sex—is perhaps the most fundamental principle of women's sexual psychology," Buss says. Even with changing societal norms—today’s women regularly support themselves financially and raise their own children—evolutionary reasoning remains fixed. 

Sorry if we just opened the floodgates for you to overthink your next potential one-night stand; but, hey, the more you know, right?

In any case, here's how to make sex with a new woman less awkward and how to handle "the morning after" with a new girl like a gentleman.