12 subtle signs your casual fling is heading towards a relationship

• Casual relationships don't always stay casual. 
• If someone begins wanting to hang out outside of your hookups, they could be falling for you. 
• If you've developed pet names, that could be another sign. 

Casual relationships are pretty commonplace nowadays, but even if you're both trying to keep it simple, there are certain and unsuspecting times where it can actually become just the opposite, Helen Fisher, anthropologist, a senior research fellow at The Kinsey Institute and co-director of Match.com's annual Singles in America survey, told INSIDER.

According to Match.com's Singles in America site, out of the 55% of singles who have had a friends-with-benefits relationship, 45% of them have had it evolved into a full-fledged relationship. Likewise, out of the 40% of singles who have had a "hanging out" relationship with someone, 29% have gotten into a relationship from it. 

Whether you call them flings, situationships, or friends with benefits, here are 13 subtle signs that it could be turning into something a bit more serious.

You've begun exchanging thoughtful text messages with them.

Casual flings usually have limited communication through text messages unless it involves setting up an encounter. According to a forensic sexologist, chief of sexology, and director of clinical research programs at Felnett Health Research Foundation Damian J. Sendler, Ph.D. though, if the text messages with your fling have started to increase, you may be moving toward a real relationship.

"Typical flings are not meant to last long. Therefore, even the quality of messages that you are exchanging with your fling is poor," he said. "When you notice that the person you're talking to invests considerable time to ask meaningful questions, it means that they want to know more about you. It's the chance for both of you to get to know each other in a non-obligatory way. Plus, things like asking how your day is going is another sign that you're heading into more serious direction."

Either or both of you have started seeking opportunities to hang more often.

Casual flings aren't usually the type of situations that require you to spend time with one another outside of having sex. If it's beginning to turn into something more than that, however, you may find yourself looking for more reasons to hang out with your fling.

"It's obvious that when we like someone, we want to spend as much time with them as possible," Sendler said. "There's also a general principle that when someone is not that receptive to our offers to hang out, that's the sign to move on, and never bother this person again. The same applies to relationships. It's an especially strong sign of affection and romantic interest when the person you're seeing cancels other plans to spend time with you. This is especially relevant when you just met this individual, who's made some plans in the past, and they want to risk their time and energy to get to know you."

You're touching each other when there's no sex involved.

Engaging in a casual fling usually finds you and the other party having sex and nothing more. When it's starting to become more than just a fling though, being touchy-feely during times when you're not having sex could become a normal part of your interaction.

"This is not about sex. It's more about going back to the basics of dating," he said. "Nowadays most people start with sex, and then they figure out what's romantic about each other. When someone essentially begins to gently explore your physical body without strong sexual innuendo, it means that they want to make a good impression."