(CNN) We all fantasize, whether that means daydreaming about a tropical island or telling off your boss. Although you might not act on your fantasies, they can provide a healthy outlet for stress and even inspire you to make positive changes in your life.

Sexual fantasies are no different -- and they can offer a glimpse into your personality. Of course, their main purpose is to turn you on, but the types of sexual adventures that you imagine can also tell you a lot about how you view yourself and the world around you.

To learn more about what Americans fantasize about and what it means, Justin Lehmiller, a social psychologist and research fellow at the Kinsey Institute, conducted the largest and most comprehensive study of its kind on this topic. He asked more than 4,000 Americans ranging in age from 18 to 87 and spanning a spectrum of occupations, sexual and gender identities, and political and religious affiliations about their sexual fantasies.

The results, which he details in his new book , "Tell Me What You Want: The Science of Sexual Desire and How It Can Help You Improve Your Sex Life," offer plenty of insight into the sexual thoughts we all have but often keep to ourselves. Here's a look at some of his most intriguing findings.

We're more alike than we think

Although every fantasy is unique, Lehmiller found that the majority fall into one of three categories: group sex, BDSM (bondage, dominance/submission, sadomasochism, etc.) and novelty or adventure. Within these classifications, threesomes, bondage and trying new positions or having sex in new locations were the most popular fantasies.

Surprisingly, there were even more similarities between men's and women's fantasies than you might expect. "I found that many of the fantasies people tend to stereotype as being as masculine, like threesomes, and feminine, like emotional fulfillment, were actually things that a majority of men and women alike were fantasizing about," Lehmiller said.

But there are differences, too. His research found that women were more likely than men to fantasize about same-sex experiences and BDSM, while men were more apt to have gender-bending fantasies (such as crossdressing) and more taboo thoughts. Likewise, women placed more emphasis on where they were having sex, while men focused more on who they were having sex with.

The star of the show may surprise you

"My results suggest that the one person who is most likely to appear in your sexual fantasies is -- believe it or not -- your current romantic partner," Lehmiller said. Indeed, people fantasized far less often about celebrities and porn stars than they did about their actual lovers.

This may be because fantasies are often designed to meet our emotional needs, such as feeling loved, desired and sexually competent. It's hard to meet these and other needs when our fantasy partners are unknown or unattainable. Instead, we may fantasize about our partners more than anyone else because we know that our partners are more capable of giving us what we need in that moment.

Your personality may dictate your fantasies

"Overall, our fantasies appear to reflect who we are and seem to be designed to meet our unique psychological needs," Lehmiller explained. He found that people with different personalities tend to fantasize about very different things. For example, people who are more extroverted and outgoing fantasize more about group sex and non-monogamy. This makes sense because they like meeting new people.

People who have more concern for the well-being of others tend to fantasize less about BDSM, infidelity and emotionless sex. This pattern makes sense because they don't want to hurt anyone and they want to include their partner in whatever is happening.

If you are detail-oriented in your everyday life, Lehmiller found that you'll probably pay more attention to things like where your fantasy is taking place. And if you're someone who doesn't deal well with stress, his results suggest that your fantasies will tend to include more calming emotional content and less about trying new things.

At the same time, Lehmiller's results suggest that our fantasy selves are often different from our true selves: Fantasies give us the opportunity to change things we may not like about ourselves. "People frequently change their age, body, genital appearance, personality or a combination of these," he said.

"Introverted people may be more outgoing in their fantasies, while anxious folks may be more relaxed and confident." The group least likely to change: Those sticklers for detail, who are often the same in both fantasies and real life.

From fantasy to reality?

So should you share your fantasies with your partner -- or even act on them? That depends. Sharing sexual fantasies can certainly bring you closer as a couple and have the potential to spice things up in bed.

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Lehmiller recommends starting slow by sharing a less-adventurous fantasy before delving into deeper or more taboo topics. Consider whether your fantasy is actually something you want to experience.

"I found that most people who acted on their fantasies reported that the experience was at least as good -- if not better -- than they were expecting and said that it improved their relationship," Lehmiller said. "But it isn't something you want to leap into or take lightly." There are things to be both gained and, potentially, lost by making fantasy reality, so be sure to educate yourself on the rewards and risks before taking that step.