So there we were, reading through the raw data of the 2013 Women’s Health Sex Survey, when we saw the stat. In response to the question, Which secret are you keeping from your partner?, 26% of the female respondents said: “I fantasise about other people.”

(Psst! Want to find out what Her Top Five Fantasies are?)

Now we know what you’re thinking: Who’s she fantasising about, and how badly can I kick his ass? Truth is, though, it’s completely normal for women – and men – to have fantasies. In fact, we’ve been doing it all our lives: depending on which research you read, we either start doing it from birth, or even a few weeks before that, during the third trimester of pregnancy.
Our earliest fantasies aren’t about sex, but about experiences that make us feel good: like milk to relieve our hunger, or the warmth of our mothers when something scares us. But as we get older, our fantasies become more complex, and we begin to dream about things we desire to have or to become. And most of those have to do with love and sex.

Sexual fantasy can be a fleeting thought, like:
I wonder what she looks like naked? or What would it feel like to kiss those lips? But they can also be longer and more detailed. A fantasy can be realistic, or completely fantastical.  Unique, or a recollection. “Some people return to the same fantasy over and over, without variation,” says Brett Kahr, psychologist and author of Who’s Been Sleeping In Your Head: The Secret World of Sexual Fantasies (R240, Kalahari). “Others generate a broad range of fantasies, and still others will use the same structure – domination, submission, humiliation, pain  – but vary the details. One night they might fantasise that they’re being dominated by a strict schoolteacher, the next night by a subordinate at work.”
Kahr divides sexual fantasies into two main types: the ones we’re willing to talk about, and the ones we’d never, ever share – even with a partner or psychologist.
The first type includes the tamer stuff. After all, your buddies wouldn’t look at you skeef if you happened to share over a quiet beer that you’d had impure thoughts about Jessica Alba, and it’s not like you’d fly into a jealous rage if your partner’s eyes lingered too long on a picture of a shirtless Channing Tatum. “About 25% of adults fantasise about sex with a celebrity,” says Kahr. “And men tend to do it more often than women. It’s also notable that most fantasies about celebs are heterosexual and involve ‘normal’ intercourse, followed by oral sex and romantic scenes.”
Trouble is, while your partner might not feel too uncomfortable telling you about her celebrity fantasies, they’re not what’s at the top of her list. According to Kahr’s research, we – both men and women – most enjoy fantasising about people we know, or characters we’ve created in our heads.
The Women’s Health survey stats back up that research: 15% of WH readers admit they’ve fantasised about sex with a stranger and another 
15% confess to having fantasised about sex with a friend or colleague.
Which leads us to the second type of sexual fantasy. We don’t like to talk about these, because many of them don’t involve our partner at all.
That’s right: her dirtiest dreams aren’t about you. And the chances are, you’re cheating on her in your dreams, too. It’s like Sigmund Freud once wrote: any sexual act involves at least four people: the two in bed together, and whoever else each of them is thinking about.
The fact is, most people “cheat” on their partners in their fantasies. And as hard as it may be for you to accept that (admit it: you’ve spent the past two paragraphs wondering which of your partner’s colleagues have been starring in the porno that’s playing in her head), this mental infidelity 
allows us to quench our desires – no matter how extreme – without 
hurting our partners.
In a study published in Psychological Bulletin, researchers Harold Leitenberg and Kris Henning concluded that the people who have the healthiest sex lives and the fewest sexual problems are also the ones who fantasise the most. Men do it slightly more than women. Guys enjoy up to five sexual fantasies a day; while the estimates for women range from three times a day to less than one a month.
This ties in with research by sexologist Ellen Laan, which suggests that testosterone is a prerequisite for sexual fantasies – even (or, perhaps especially) among women. Women with low levels of testosterone had, according to the research, far fewer sexual fantasies and desire for sex than women with normal T-levels.
Male and female fantasies aren’t that different: we dream about the same things. But men tend to fantasise mostly about acts, while women 
are more passive and tend to fantasise about something being done to them. Women are also far more reluctant to talk about their fantasies.
Sexual fantasies can foster feelings of pleasure, but also feelings of 
guilt and confusion. Because our fantasies can come from – and take 
us to – some pretty dark places.
“Sometimes we have sexual fantasies to bring more variation into our lives, or simply for the pleasure and the excitement,” says Kahr, “but often they also serve as a confirmation of a childhood trauma or even as a protection against the flood of emotional and physical demands of our partner.”
Kahr continues: “Our human behaviour is not only driven by the desire to satisfy our erotic desires, but also by the desire to fulfil our aggressive 
desires.” Translation? We get excited by sex as well as by violence. And here’s a disturbing thought: millions of people are aroused by the idea of inflicting pain on others or of being dominated themselves. About 30% of people appear to have fantasies of a masochistic nature. And that’s a conservative estimate.
“I think many women and men prefer sex in the dark, not just because it hides our wobblier parts,” Kahr says. “It’s also because a darkened bedroom allows our fantasy world to come to life.”
A study in Analysis and Modification of Behaviour reveals that sharing your fantasies with your partner can increase your libido. But is that really a good idea? “Honestly, I wish I knew the answer,” says Kahr. “I know cases where couples have shared their fantasies, with amazing results, but I also know of cases where similar attempts ended in tears. Either way, be careful.”  It’s not beyond the realms of reason that your partner might not enjoy hearing that you’ve been thinking about other women while masturbating… In some cases, your dirty dreams are probably safest when they’re locked away in your head – and hers in hers.
Our research narrowed down five of the most common female fantasies. Find out what she wants, why she wants it and how to make it work for you.