Kiera Knightley once said that a dramatic lip "is for when you've had a s--t day and you need red lipstick to face the world."

No lies detected. Because, as the English actress pointed out, nothing exudes more power and confidence than swiping on the bold shade. But the makeup classic does more than just change your attitude. It's known to spark arousal as well, almost acting as a mating call.

"Red, the color of blood, of blushes and flushes, of nipples, lips, and genitals awash with sexual excitement, is visible from afar and emotionally arousing," evolutionary psychologist Nancy Etcoff wrote in a particularly passionate section of her book Survival of the Prettiest: The Science of Beauty.

And while that is certainly one way to put it, cosmetics historian Gabriela Hernandez offered more insight to E! News about how its roots are naturally tied to sex.

"It has to do with deep associations in our brains that make that color more attractive in a mate," she explained. "Red also livens up the face, calls attention to itself and will always have a deep connection to sex since arousal is usually associated with blushing."

Another reason why the fiery look often makes people get in the mood, according to Hernandez, is that for centuries it's been "associated with good health."

"Red cheeks and lips signified a good prospect," she explained of the ancient courting ideals. "Being in good health gave women better odds of having children and surviving childbirth, which was particularly difficult since medicine was not advanced to help if anything went wrong."

Back then, the Bésame Cosmetics founder explained, red implied, "youthfulness, which was always more desirable since people did not live very long."

Ironically, up until about the 18th century, many red lip and cheek rouges were made with poisonous ingredients such as cinnabar (derived from red mercuric sulfide), lead, rubric, orchilla weed, red chalk and alkanet, according to Lisa Eldridge's Face Paint: The History of Makeup. So, while the wearer might have applied a pinch of red on their lips and cheeks to appear more youthful, healthy and beautiful, it would've had the opposite effect and deteriorated them from the inside out.

Moreover, in ancient times, you had to carefully tread the line between looking sexy enough to find a partner but not too sexy that you were considered damaged goods.

In ancient Greece, for example, sex workers were required by law to wear red lipstick and obvious face paint to mark their status in society. Otherwise, they'd be punished for not only deceiving the public but potential spouses for posing as "respectable" women.

And the idea that the more provocative among us prefer red lipstick over other shades continues to be imbued into our social fabric.

In the late 1930s, according to Rachel Felder's Red Lipstick: An Ode to a Beauty Icon, the defunct company Volupté sold two lipsticks: Hussey, a vibrant red, and Lady, a soft pink. Apparently, many wore their red with pride, as Hussey outsold Lady by over 80 percent.

And beauty brands today still tend to play up red's sex appeal. Too Faced Cosmetics' crimson liquid lipstick is named "Nasty Girl," while Rihanna's Fenty Beauty calls its universal red "Uncensored."

And there's no denying that we've related the classic lip color to women who come across as both seductive and successful. Marilyn Monroe, Queen Elizabeth I, Cleopatra, Selena Quintanilla, Sade and many others have armored themselves with the tantalizing hue.

One could even argue that red lipstick has become a sex symbol in its own right. 

As Rihanna perfectly put it, "If you have on a strong, good lipstick, it changes everything."

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