When most people fantasize about having sex, they think about a range of partners—or even multiple partners at once, such as in a threesome or an orgy. There's one person, however, who is more likely to appear in your sexual fantasies than anyone else: your current romantic partner. (Or if you're single, it's one of your exes.)

For my new book, Tell Me What You Want, I surveyed more than 4,000 Americans about their sexual fantasies. Among other things, I found that nine out of ten participants said they had fantasized about a current romantic partner before and, further, among those who were in relationships, nearly two-thirds said they fantasized about their current partners often. By contrast, fewer than one in ten participants in relationships said they fantasize frequently about Hollywood celebrities, porn stars, politicians, or other famous people.

It’s worth noting that this was true for persons of all genders and sexual orientations. Many people would probably find these results surprising: There’s a widespread assumption that, when it comes to sex, we want what we can’t have—like a one-night stand with Channing Tatum or Scarlett Johansson. (Incidentally, I found that these two were the most fantasized-about celebrities). These results, however, suggest that people in relationships have fantasies that are more grounded in reality than people assume, at least when it comes to who they're having sex with.

Why do people in relationships disproportionately fantasize about their romantic partners? A new set of studies offers a reasonable explanation: Fantasizing about our partners seems to be good for our sex lives and our relationships. In one study, for instance, heterosexual college students who were currently in monogamous relationships were instructed to think of a sexual fantasy involving either their partner or someone else and to write out this fantasy in narrative form. Afterwards, they were asked to report how much sexual desire they felt for their partner, as well as how interested they were in doing something nice for them.

Compared to participants who fantasized about other people, those who fantasized about their partner expressed more desire for sex and had the most interest in making their partner happy. A follow-up study sought to clarify the direction of this effect: Did fantasizing about one’s partner increase desire, or did fantasizing about someone else decrease desire? Two conditions were added to the experiment in which participants were asked to think about non-sexual activities involving their partner or someone else.

More from Tonic:

Adding in these baseline conditions revealed that fantasizing about one’s partner increased desire; however, fantasizing about someone else had no effect on desire. Translation: Fantasizing about other people isn’t harmful, but it doesn’t help your relationship in the same way as fantasizing about your partner, either.

In another study, heterosexual couples completed a daily diary for three weeks in which they reported on all of their sex fantasies as they had them. Researchers then looked at how fantasies about one’s partner were related to relationship behaviors.

Consistent with the findings from my book, most fantasies (66 percent) involved one’s partner. And when people fantasized about their partner, they tended to be more kind and considerate toward them afterwards, such as by being affectionate and offering more compliments. Fantasizing about other people was unrelated to participants’ relationship behaviors.

In the fourth and final study, researchers again collected daily diary data on couples' sex fantasies, but they also asked questions regarding how people felt about their relationships. What they found was that when people fantasized about their partners, this increased positive perceptions of the relationship while decreasing negative perceptions.

Basically, people felt their relationships were more valuable to the extent that they fantasized about their partners, and this seemed to motivate them to behave in ways that bolstered their relationship, such as by going out of their way to do nice things for each other.

What this pattern of results suggests is that sexual fantasies about our partners seem to have adaptive value for our relationships—they may not only stimulate sexual passion, but also bring us closer. In light of this, it shouldn’t be surprising that, for people in relationships, they fantasize about their partners more often than anyone else.

That said, it’s worth repeating that fantasizing about other people isn’t inherently problematic. Most of us will have fantasies about people other than our partners from time to time, but this isn’t necessarily a sign that your partner is inadequate or that you’re headed for breakup.

Where fantasies about others could potentially lead to trouble is when they effectively replace fantasies involving your partner. Indeed, as I report in Tell Me What You Want, people who fantasized most often about unattainable celebrities with perfect bodies were the least satisfied with their sex lives and relationships.

In short, if you’re fantasizing about other people to the total exclusion of your partner, that’s a different matter, and just might be the sign of a troubled relationship.