It's sometimes hard to pinpoint exactly what it is that attracts us to someone. It might be their confidence, or their sense of humour, or you might just like the way they look.
A lot of research over the years has gone into trying to work out what it is that makes us fancy each other. Results have varied, showing women may like the smell of men who have a particular kind of diet, and men may find women in groups more attractive.
Some research has suggested we often go for people who share some of the same characteristics we do.
We've looked at a number of studies to try and get to the bottom of what makes us like the look of one person over another.
Here are 9 face traits that can make someone more attractive to us, according to science.
Studies such as this one published in the Journal of Comparative Psychology, and this one published in the Journal of Evolution & Human Behaviour, have shown that in experimental conditions, men and women both prefer faces that are more symmetrical.
A study on identical twins found that the twin with a more symmetrical face was considered more attractive. Even macaque monkeys have been observed gazing longer at symmetrical faces than asymmetrical ones.
One conclusion scientists have reached to explain this is that in evolutionary terms, we may consider a symmetrical face a result of good health. Having a face that developed in a symmetrical way could show you have "good genes," because you developed more successfully in the face of environmental pressures when you were in the womb.
However, in 2014, research from Brunel University in London compared facial symmetry of about 5,000 teenagers, and found there was no correlation between symmetry and overall health.
It's not an absolute rule, though. In fact, you can probably think of many celebrities you fancy who don't have symmetrical faces at all. Sometimes, like in the case of Milo Ventimiglia, a crooked smile is what adds to someone's charm.
In fact, absolute symmetry can make people look pretty weird. When attractive celebrities' faces are made to look symmetrical, they don't look quite right.
People tend to like faces that are distinctly average, or those that resemble others in the general population.
In 1878, a paper in Nature first noted that a bunch of faces blended together was considered more attractive than those on their own.
One study, published in the journal Human Nature, argued it could be because average faces represent a more diverse set of genes, which is often a genetic advantage in fighting off illnesses and parasites.
4. Looking older
A study in 2012, published in the Journal of Evolutionary Psychology, found that women prefer men who look older.
The more financially independent women became, the more they said they liked older guys, according to the study.
One explanation for what researchers have coined the "George Clooney Effect" could be that women are only fertile from puberty to menopause, while men can father a child until much later. Women may get a better offer by waiting until the man has more status and resources due to being older.
5. Facial hair
Some people love a bushy beard, whereas others go for clean-shaven men. A study in 2013 recruited 177 men and 351 women to look at pictures of men who had no facial hair, light stubble, heavy stubble, or a full beard.
The results showed that women thought the most attractive beard length is heavy stubble, or about 10 days of growth.
Heavy beards, light stubble, and clean shaven were all equally less attractive than heavy stubble.
6. Facial scars
This one depends on if you're attracted to men or women. One study, published in the journal Personality and Individual Differences, found that men could be considered more attractive if they had a facial scar, whereas this wasn't found for women.
Researchers took photos of 24 male and 24 female students and manipulated them to look like they had scars on their faces. Then, another 200 students were asked to rate how attractive the photos were, and whether they thought the person would be good for a short or long-term relationship.
Men with scars were rated as more attractive for short-term relationships than men without scars. There was no difference in how attractive women were with or without scars.
According to one study, published in Current Biology, genetics might not have much to do with it.
The researchers found that the types of faces we fancy are influenced more by our personal experiences in life than anything else.
The study found that identical twins had different ideas of who they thought was attractive, suggesting a genetic predisposition wasn't a factor. Overall, genetics explained just a fifth of the variation in people's preferences.
The authors concluded that these results could explain how models and celebrities can make money from their good looks, while friends constantly disagree about who is to their taste.
A study from 2016 found that rather than men having a preference for certain features like lips or eyes, they went for more of an overall look.
The researchers recruited 169 men in France and showed them two sets of random women's faces. They were asked to rate them on a scale of zero to 20 in terms of attractiveness.
Results showed that overall, the faces the men found most attractive were "sparsely coded" — that means the ones that were symmetrical, more plain, and had no distinguishing features.
9. Looking like your parents
Some research suggests that we tend to find people who look like our opposite sex parent attractive.
According to research from St Andrews, we are attracted to the features that our parents had when we were born, possibly because we see them as our first caregiver, and associate positive feelings with their features.
In one study from 2002, researchers asked participants to rate how attractive faces of different ages were.
"We found that women born to 'old' parents (over 30) were less impressed by youth, and more attracted to age cues in male faces than women with 'young' parents (under 30)," the authors wrote. "For men, preferences for female faces were influenced by their mother's age and not their father's age, but only for long-term relationships."
Also, in a follow up study, a sample of 697 men and women showed people were more likely to have romantic partners who had the same eye and hair colour as their opposite-sex parents.
Read next on Business Insider: Women are more attractive in groups than when they're alone because of something called the 'cheerleader effect'