A man's eyebrow and eye
The eyebrow is an area of thick, delicate hairs above the eye that follows the shape of the lower margin of the brow ridges of some mammals. Their main function is hypothesized to prevent sweat, water, and other debris from falling down into the eye socket, but they are also important to human communication and facial expression. It is not uncommon for people to modify their eyebrows by means of hair addition, removal, makeup, tattoo, or piercings.
It is believed[by whom?] that the main function of the eyebrow is to prevent moisture, mostly salty sweat and rain, from flowing into the eye. Greek physician Herophilos, in his treatise "Anatomy," was among the first to hypothesize such a theory, proposing that the eyebrows are "adorned with hair, so that if copious perspiration came [down], it would be contained by this 'check-point' of hair placed in its way until it is wiped off, so that it could not obstruct the eyes." The typical curved shape of the eyebrow (with a slant on the side) and the direction in which eyebrow hairs are pointed, make sure that moisture has a tendency to flow sideways around the eyes, along the side of the head and along the nose. The slightly protruding brow ridges of modern humans could also be used as a defense structure against the floods of moisture. Desmond Morris, discussing the possible function of the eyebrow in human evolution, criticized this suggestion as non-effective  and suggested that the primary function of the eyebrows is to signal the changing moods of their owners. Together with the brow ridges, the eyebrows also shade the eyes from sunlight. Eyebrows also prevent debris such as dandruff and other small objects from falling into the eyes, as well as providing a more sensitive sense for detecting objects being near the eye, like small insects.
Eyebrows also have an important facilitative function in communication, strengthening expressions, such as surprise, anger or sexiness. Many makeup artists view eyebrows as a major feature in defining the face. The eyebrows shape the human face and give definition to one's eyes and forehead. Depending on the shape of the eyebrows, it sometimes can cause what is known as a false facial expression (such as a person's eyebrows shaped to seem as if the person was angry, but really isn't). Some people can also move their eyebrows up and down alternately.
Eyebrows as eyespots
Joseph Jordania suggested that the primary evolutionary function of clearly visible eyebrows was safety from predators during the vulnerable nighttime sleep, when early hominid groups started sleeping on the ground, away from the trees. Stealth predators (like big cats) as a rule stop hunting if they notice that prey animals are watching them. Cheap plastic masks, placed on the back of the head, saved many human lives in India and Bangladesh, where man-eating tigers claimed many victims in Sundarbans national park. Many predators (particularly big cats) are very sensitive to eyespots, and all the big cats have eyespots on the back of their own ears. According to Jordania, at night, when hominid eyes were closed, the eyebrows, arched upwards, and eyelashes, arched downwards, formed clearly definable oval eyespots on a ‘sleeping’ human face, creating an illusion that the eyes were still open and watching (and therefore could deter predators from attacking sleeping hominids).
In a study published by Massachusetts Institute of Technology (MIT), where subjects were asked to identify celebrities with either their eyes or their eyebrows digitally edited out, the subjects were able to recognize the celebrity 46% of the time with their eyebrows edited out, compared to 56% of the time with their eyes edited out. The findings indicate the importance of eyebrows in providing clues to an individual's identity.
Eyebrows are a major facial feature. Cosmetic methods have been developed to alter the look of one's eyebrows, whether the goal is to add or remove hair, change the color, or change the position of the eyebrow.
Several options exist for removing hair to achieve a thinner or smaller eyebrow, or to "correct" a unibrow, including manual and electronic tweezing, waxing, and threading. The most common method is to use tweezers to thin out and shape the eyebrow. Waxing is becoming more popular. Lastly, there is threading eyebrows, where a cotton thread is rolled over hair to pull it out. Small scissors are sometimes used to trim the eyebrows, either with another method of hair removal or alone. All of these methods can be painful for some seconds or minutes due to the sensitivity of the area around the eye but, often, this pain decreases over time as the individual becomes used to it. In time, hair that has been plucked will stop growing back. Some people wax or shave off their eyebrows and leave them bare, stencil or draw them in with eye liner, or tattoo them on. In Western societies, it has become more common for men to pluck part of their eyebrows.
To create a fuller look, eyebrows can be cloned in an eyebrow transplant: Individual strands of the eyebrow are mimicked to create a natural-looking eyebrow of the desired shape. Eyebrow brushes and shaders are used to further define the eyebrow. Other methods of achieving a fuller brow is by using brow pencils, which create an outline for the brows or mimick hairs where there are sparse areas. Brow gels are also used in creating a thicker brow; it allows for the hairs to be more textured, which gives the appearance of thicker and more fuller brows. Lastly, brow powders or even eyeshadows are used for those who want a fuller and more natural look, by placing the brow powder or eyeshadow (closest to the natural hair color) in areas where there is less hair.
A fairly recent trend in eyebrow modification is in eyebrow tinting: Permanent dye, similar to hair color, is used on the eyebrow, often to darken them.
An eyebrow lift, a cosmetic surgery to raise the eyebrow, usually to create a more feminine or youthful appearance, is a new phenomenon. They can be affected during a face lift or an eye lift. More recently, doctors inject patients' eyebrows with botox or similar toxins to temporarily raise the eyebrow.
From the 8th century, many Japanese noblewomen practiced hikimayu, shaving or plucking the eyebrow hair and painting smudge-like ones higher on the forehead. Shaving lines in eyebrows is another cosmetic alteration, more common among younger people in the 1990s and 2000s.
Eyebrows often thicken with puberty.
Eyebrows can be used to convey emotion, such as surprise.
One of Audrey Hepburn's trademarks was her thick eyebrows.
A young adult with hair between the eyebrows - a unibrow
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- Anti-eyebrow, a body piercing placed below the eyebrow
- Eyebrow piercing, a body piercing done through the eyebrow
- Trichotillomania, a disorder where sufferers have an impulse to pull out body hair, including eyebrows
- Unibrow, the presence of abundant hair between the eyebrows
- Eyebrow restoration, surgical procedure to change the look of one's eyebrows
- Desmond Morris, The Naked Man: A Study of the Male Body. London: Jonathan Cape, 2008
- Joseph Jordania, Why do People Sing? Music in Human Evolution, chapter "I Can See you! Eyespots in Humans." Logos, 2011, pg. 96
- Javid Sadrô, Izzat Jarudi, Pawan Sinha. The role of eyebrows in face recognition. Perception, 2003, volume 32, pages 285-293. doi:10.1068/p5027
- "Advanced Tips for Eyebrow Plucking". Makeup-product.com. Retrieved 2012-06-23.
- "Eyebrow Waxing 101 - Makeup - Skin & Beauty - Daily Glow". Hair-n-skin-care.com. Retrieved 2012-06-23.
- "What You Should Know About Eyebrows". //www.sheilasfashionsense.com. Retrieved 2012-06-23.
- "Brow Lift Procedure Recovery, Before and After on". Medicinenet.com. Retrieved 2012-06-23.
- "Access". Medscape. Retrieved 2012-06-23.
- Bruce Ely, The Oregonian (2008-04-28). "Teens shaving lines in eyebrows raise gang concerns in schools". OregonLive.com. Retrieved 2012-06-23.
- IEEE Xplore: About the relationship between eyebrow movements and Fo variations
- Dallas / Fort Worth Local News: Granbury ISD Forbids Eyebrow Shaving