I’ve only ever been attracted to a woman once. It was over 10 years ago but I still remember how electrifying her presence was. Whenever I was around Natalie, it was clear that I wasn’t the only one who thought that she was incredibly sexy. Sexiness seemed to ooze out of Natalie’s pores and she exuded an energy that was noticeable by everyone in her presence. In fact, the mutual friend who introduced us later confessed that Natalie was the only woman she had ever had a crush on too.

It’s worth noting that Natalie doesn’t look anything like what the marketing world tells us sexy should look like. Being half-Samoan and half-Caucasian, she had a curvy build that is a far cry from the model-thin figures we often see in ads. She also had a head full of frizzy hair — the kind people spent hundreds trying to straighten. While she didn’t tick any of the boxes for conventional standards of beauty, she had an incredible presence that was impossible to miss.

The French have a phrase — “Je ne sais quoi” — which translates to “I don’t know what.” It is generally used to describe someone who has that special “something” that you just can’t put your finger on but is irresistibly appealing. If ever there was a person who was the human embodiment of that phrase, it was Natalie.

For a long time, I couldn’t figure out exactly why I found Natalie so sexy. However, as my interest in behavioral psychology and social sciences grew, it all started to make sense. While no one will dispute that physical attractiveness contributes to sexiness, psychologists have uncovered other traits that have nothing to do with looks that people find irresistible. Here are some of the traits of desirable people that the marketing world will never tell you about.

1. They’re completely comfortable in their own skin.

Natalie always defined beauty by her own standards. She once said to me, “I have not found a single culture in the world that finds my small boobs, thick body, dark skin, and frizzy hair beautiful. So, I can choose to believe them or I can set my own standards.”

By setting her own standards, she never felt the need to buy makeup that made her look whiter or shapewear that made her look skinnier. She would wear the most outrageous outfits and somehow pull it off. When Natalie and I later became housemates, I would often catch her smiling at herself in the mirror. It was clear that she reveled in her individuality and it was hard for someone watching not to do the same.

It wasn’t that Natalie didn’t care about the physical appearance of her body. She did. She exercised, ate well, and pampered herself with spa treatments. The difference was that she did it to make herself feel good and not to gain validation from other people.

Why this is sexy — according to research

Psychologists have known for decades that simply believing that you are attractive increases your desirability by other people regardless of your physical looks. Research predicts that one of the reasons confidence is so sexy is because it’s hard for us to judge the value of a potential partner right away. But we assume that people know themselves best, so we use their self-confidence (or lack of it) as a gauge of how valuable they actually are as a partner. Conversely, if you went out of your way to disguise parts of yourself with excessive makeup or Instagram filters, it implies that there is something about you that needs to be hidden and is not valuable.

2. They know how to make other people feel good.

If Natalie had a superpower, it was that she could always make someone feel good. As soon as she saw you, her eyes would light up and a cheeky smile would creep up her face. It always felt like she was coming over to tell me a secret only the two of us shared. She had a way of focusing on you like you were the only person in the room. But the most powerful way she made people feel good was her uncanny ability to help people see themselves in a better light. Not only did she never judge someone, but she was also very good at helping people remove the judgment that they placed on themselves.

I recall a time when I had felt stupid because of a bad career decision that didn’t pan out. After listening intently, she reminded me of how brave I had been to take that step when most people would have played it safe. She also reflected on how expertly I had used that situation to gain a better role. By the end of the conversation, I felt empowered. I had started the conversation feeling stupid but ended it feeling courageous and adaptable.

Here’s the important part — Natalie never feigned interest or fawned over anyone excessively. She just naturally thought that everyone was interesting in their own way. The Roman poet, Publilius Syrus, describes why this trait is so alluring:

“We are interested in others when they are interested in us.”

Why this is sexy — according to research

Researchers believe that the reason that creating positive emotion is so important at the beginning of a connection is that it expands the self-other overlap. Positive interactions that elicit feelings of happiness and joy promote intimacy by making someone want to share more of themselves with the other person. Another study hypothesized that creating positive emotions promoted closeness by activating the dopaminergic system in our brain. So, someone who can make us feel good lights up the reward pathways in our brains and motivates us to want more.

3. They know their worth.

I recall a conversation once among our friends where someone had mentioned, “Dating seems so effortless for Natalie.” It was true. Natalie was never obsessed with whether someone had texted her the next day or worried about how to impress a romantic interest. She didn’t wait to make plans just in case someone she liked might text her.

Natalie valued herself and that included valuing her own time and attention. Because she never waited around for a guy or tried to fit her life around someone else, people dating her knew that they needed to get their act together if they wanted to see her. It’s important to note that Natalie wasn’t playing “hard-to-get”, she was simply too busy living her own life to be available to them at a drop of a hat.

Dave later told me that one of the things that made Natalie more desirable was the fact that she never sought validation from him. She didn’t need him to text her constantly and she never felt the need to talk about how awesome she was. He also knew right away that Natalie would never put up with any disrespect or mind games. Knowing that he had to work a little for her attention and affection made him want to chase her more. As dating expert Matthew Hussey says:

Show a girl you don’t care and she will chase you. Show a woman you don’t care and she’ll replace you.

Why this is sexy — according to research

Researchers who showed that uncertainty and the lack of availability made romantic partners more desirable explained that this was a core human trait that is consistent with economic models where lack of supply increases demand. When you don’t know when the opportunity to see that person will be or even if they will continue to see you, you will make more effort to get them to value you. People who know their own worth are selective about who they give their time and attention to which means they will naturally be less available than someone with low self-esteem.

4. They’re having a great time.

One of my friends, Dave, had come out one night with me because he had been dying to meet another friend of mine, Lisa. However, when he arrived at the club that night, he found himself repeatedly drawn to Natalie. While Lisa would scroll through her phone with a bored expression all night, Natalie would be busting moves on the dance floor or happily chatting with people. Though he got both phone numbers that night, he only asked Natalie out.

When I asked Dave about his decision later, he said, “Natalie just always looks like she’s having a blast in life. Now, that’s sexy. It’s hard not to want to be around that.” Lisa was his physical ideal but in the end, he found Natalie far sexier. It wasn’t that Natalie was a party girl, it was that she clearly had a passion for life. She always seemed fully present in whatever she was doing.

Why this is sexy — according to research

Studies found that people who were fun and had a great sense of humor were more successful romantically because they created positive emotions in the opposite sex. In addition to making us feel good, another reason we are drawn to fun people is that we might perceive them to be better at communication and conflict resolution. It turns out, this gut feeling is correct. Research validates that people who choose a partner that is more affable than them cope better with difficult life circumstances and have higher levels of relationship satisfaction.

5. They have the courage to be vulnerable.

Natalie was one of those people that someone would ask for directions from and then end up spilling their entire life stories to. Her dates frequently ended with deep and meaningful conversations that lasted until 4 am in the morning — which was exactly how my first encounter with her went as well. I remember being incredibly drawn to her after that conversation and really looking forward to seeing her again.

While being a complete hot mess who vomits emotions tends to come across as needy and be a turn-off, being authentically vulnerable at the right times and with the right people can create a strong attraction. Natalie had a way of inviting intimacy by being open and relatable. When the connection was right — she was willing to share things she struggled with, powerful events in her past, or how much she cared about someone. For many men, the ability to go deep was such a rare occurrence that they found someone who could create that kind of trust incredibly captivating.

Why this is sexy — according to research

We often feel that showing any kind of vulnerability makes us appear weak and flawed but researchers who conducted a series of experiments on hundreds of participants found that vulnerability can be very alluring. They named this “the beautiful mess” effect.

Their observations align with the work of renowned vulnerability researcher and author, Professor Brene Brown. In her book Daring Greatly, Professor Brown explains that when people have an emotional armor on, it makes it hard for others to be able to relate and authentically connect with them. In contrast, we love seeing raw truth and openness in other people because vulnerability is incredibly humanizing. It reminds us that at the core of it, there are experiences that we all share and we can bond over that.

The two-word advice by the sexiest person I know

I once asked Natalie where she thought the self-assuredness that was the root of her sexiness came from. Her response was only two words: “Don’t compare.”

As I thought about it over the years, I came to realize exactly how powerful this piece of advice is. We feel insecure about our bodies when we compare them with what advertisements tell us is desirable. We stop being able to have fun in the present when we compare ourselves with whether other people are having more fun. We stop feeling worthy when we compare ourselves to other people’s achievements or other people’s ideas of value. We stop making the people in our lives feel good when we compare them with other people we think we would rather be with. We are unable to be vulnerable when we constantly compare how we feel with what we think we should feel.

At the end of the day, though there were many traits that contributed to Natalie’s sexiness, they all stemmed from the way she thought about herself. Her sexiness was fueled by the fact that she was completely, unapologetically herself.