Why do some people make us hot and bothered while others leave us cold? It can be hard to put your finger on the specific reason, but the scientists of sex appeal have some answers.
They've looked at all the human senses, from smell to hearing, and have come up with a surprising set of "laws" that govern what we find attractive. Here are 17 you should know:
FACT #1. It matters how old your parents were when they had you.
Are you attracted to older faces, or do you go for the younger-looking ones? It turns out that both men and women born to "older" parents (age 30 and above) are less "impressed by youth," and react more positively to signs of age in people's faces, according to a study from the University of St. Andrews.
FACT #2. It's best to have a face that's symmetrical.
You probably aren't conscious of comparing the two sides of a person's face. But around the world, facial symmetry is considered a sign of beauty. Why is that? Symmetry signals good genes for reproductive health.
"If you choose a perfectly symmetrical partner and reproduce with them, your offspring will have a better chance of being symmetric and able to deal with perturbations," evolutionary biologist Randy Thornhill told LiveScience, referring to offspring having a better chance of survival.
FACT #3. It's better to be average.
No, not average-looking, but mathematically average. People with "mathematically average" features advertise a more diverse set of genes and better reproductive health, evolutionary biologists say.
"Basically what our brain does is we go around in our environment, picking up people’s faces and making the average out of these faces we see on a daily basis," Dr. Kang Lee, psychologist at the University of Toronto, said in an episode of The Huffington Post's Talk Nerdy To Me. "And because of that, then, we actually have in our head... a representation of the average of the face. So there’s something we have genetically that’s driving us to prefer to look at something that’s average."
FACT #4. Your parents' looks affect what you're looking for.
Ever heard the saying that you're likely to marry your mother or father? That may not be so off-base. In another University of St. Andrews study, psychologists found that people tend to be with partners who have the same hair and eye color as their opposite sex parent.
FACT #5. It's all in the details -- like your chin size, "brow prominence," and your limbal ring.
Did you know your chin and brow serve as advertisements for your reproductive health? According to evolutionary biologists, women tend to go for men with a large jaw and prominent brow -- shaped by high levels of testosterone. Men are attracted to women with a smaller chin and less prominent brow, which signal higher estrogen levels.
And your limbal ring -- the area where your iris meets the white of your eye -- might signal youth and health. In a 2011 study, men and women with a dark limbal ring were perceived as more attractive.
FACT #6. There's a "golden ratio" for faces.
Ever wonder if there's some magical formula for being beautiful? According to a 2010 study, faces were considered most attractive when the features were set apart according to certain ratios. Ideally, you want the distance between your eyes and mouth to be about 36 percent of the length of your face. And, ideally, the distance between your eyes should be 46 percent of your face's width.
You can't change these distances, but you can change how others look at you -- for instance, getting bangs can shorten the apparent length of your face.
FACT #7. The color of your shirt can make a difference.
In a University of Rochester study, men found women wearing red to be more attractive and sexually desirable, compared to women in blue. The study suggests that red serves as a signal for sexual excitation, since the effect also occurs in nonhuman primates.
And in another study, women perceived men clad in red as more powerful and more high-status -- traits highly associated with male attractiveness.
FACT #8. The nose knows.
When sniffing t-shirts saturated with men's sweat, women preferred the smells of men with high levels of testosterone, at least when the women were at peak fertility.
But what about those elusive pheromones scientists talk about? While animals have specialized organs for detecting pheromones, the jury's still out on whether they mediate sexual attraction in humans.
FACT #9. Your voice can betray you.
Ever notice your voice change when you find someone extra sexy? A study conducted in the U.K. found that women spoke with a higher pitched voice when they found a man attractive.
Many studies have found that men prefer women with higher voices, associated with higher levels of estrogen -- and women prefer men with lower voices, linked with more testosterone.
FACT #10. It matters how hungry you are?
A 2006 study linked men's hunger to the traits they find desirable in women. During an experiment, men who were hungry preferred women with higher body weights. These hungry men found heavy women more attractive than did the men who were not hungry at the time.
Why does this happen? The researchers suspect it might be an evolutionary response to resource scarcity -- that a heavier woman advertises access to resources, like food.
FACT #11. It matters what time of the month it is.
A woman's menstrual cycle affects how attractive she finds a man. When a woman is ovulating -- and at the most fertile period in her cycle -- she's more likely to prefer a masculine man.
Men also find ovulating women to be more attractive. "There could be changes in blood flow that would result in color changes in the face, changes in acne, or changes in puffiness due to water retention," psychologist Nathan Pipitone of Adams State University in Colorado told The Huffington Post in an email.
FACT #12. Contraceptives can toy with preferences.
Researchers found that women taking the pill preferred less masculine male faces, compared to before they were on the pill. Women who were on the pill before they met their partner were more likely to choose a mate with a less masculine face, compared to women not taking the pill.
FACT #13. Your waist-hip ratio matters.
For centuries, men have chased after women with the hourglass figure. And that may be a product of evolution, as waist-hip ratio (WHR) might serve as an indicator of reproductive health in both women and men.
A WHR of 0.7 on women was rated as most attractive in European cultures, while a ratio of 0.8 or 0.9 was found most attractive in parts of South America and Africa. And a ratio of 0.9 in men's bodies has been considered most attractive to women.
FACT #14. There's one kind of facial hair that's just right.
An Australian study revealed that women prefer men with heavy stubble -- over those with a clean-shaven face or a full beard.
What could possibly explain such a specific hankering? "Our findings confirm that beardedness affects judgments of male socio-sexual attributes and suggest that an intermediate level of beardedness is most attractive while full-bearded men may be perceived as better fathers who could protect and invest in offspring," the researchers wrote in their abstract.
FACT #15. It doesn't make a difference if you're gay or straight.
Gay or straight, men seemed to be wired to go for certain features.
“Our work showed that gay men found highly masculine male faces to be significantly more attractive than feminine male faces.," Harvard psychologist Aaron Glassenberg said in a written statement. "Also, the types of male faces that gay men found attractive generally did not mirror the types of faces that straight women found attractive, on average."
FACT #16. It makes a difference if you're looking for a fling, or something more long-term.
In a recent study, men found women with feminine faces more attractive -- but only when these men were looking for a short-term relationship.
FACT #17. Smiling might not make you more attractive.
Men are attracted to women who smile, but it's not true the other way around. Research found that smiling females were rated as more attractive, whereas men showing happy emotions were rated as less attractive. Displays of pride -- indicated by a slight smile and raised fists -- were perceived as most attractive in men.