Hair: Difference between revisions

Hair is a filamentous biomaterial, that grows from follicles found in the dermis. The human body, apart from its glabrous skin, is covered in follicles which produce thick terminal and fine vellus hair. Most common interest in hair is focused on hair growth, hair types and hair care, but hair is also an important biomaterial primarily composed of protein, notably keratin.

Found exclusively in mammals, hair is one of the defining characteristics of the mammalian class.^[1] Hair-like structures, called cilia, but which are not classified as hair, are visible in organs such as the nose and ear and these structures also occur in many other organs of mammals, other animals, and plants. Although non-mammals, especially insects, show filamentous outgrowths, these are not considered "hair" either, in the scientific sense. These so-called "hairs" (trichomes) also are found on plants. The projections on arthropods, such as insects and spiders, are classified as bristles, which are composed of a polysaccharide called chitin. There are varieties of cats, dogs, and mice bred to have little or no visible hair (fur). In some species, hair is absent at certain stages of life.

Hair often refers to two distinct structures: 1) the part beneath the skin, called the hair follicle or when pulled from the skin, called the bulb. This organ is located in the dermis and maintains stem cells which not only re-grow the hair after it falls out, but also are recruited to regrow skin after a wound;^[2] and 2) the shaft, which is the hard filamentous part that extends above the skin surface. A cross section of the hair shaft may be divided roughly into three zones. Starting from the outside: 1) the cuticle which consists of several layers of flat, thin cells laid out overlapping one another as roof shingles, 2) the cortex which contain the keratin bundles in cell structures that remain roughly rod-like and in some cases, 3) the medulla, a disorganized and open area at the fiber's center.^[3]

Hair is composed mainly of the protein keratin. Keratin assembles into rope-like intermediate filaments. The structure of these filaments provides strength to the hair shaft.

Hair growth begins inside the hair follicle. The only "living" portion of the hair is found in the follicle. The hair that is visible is the hair shaft, which exhibits no biochemical activity and is considered "dead".^[4] The base of the root is called the bulb, which contains the cells that produce the hair shaft.^[5] Other structures of the hair follicle include the oil producing sebaceous gland which lubricates the hair and the erector pili muscles, which are responsible for causing hairs to stand-up. In humans, with little body hair, the effect results in goose bumps.^[6]

Each strand of hair is made up of the medulla, cortex, and cuticle.^[7] The innermost region, the medulla, is not always present and is an open, unstructured region.^[4][8] The highly structured and organized cortex, or middle layer of the hair, is the primary source of mechanical strength and water uptake. The cortex contains melanin, which colors the fiber based on the number, distribution and types of melanin granules. The shape of the follicle determines the shape of the cortex, and the shape of the fiber is related to how straight or curly the hair is. Asian hair typically has a round fiber and is quite straight. Oval and irregularly-shaped fibers are generally more wavy or even curly.^[4] The cuticle is the outer covering. Its complex structure slides as the hair swells and is covered with a single molecular layer of lipid that makes the hair repel water.^[7] The diameter of human hair varies from 17 to 180 μm (0.00067 to 0.00709 in).^[9]

All natural hair colors are the result of two types of hair pigment. Both of these pigments are melanin types, produced inside the hair follicle and packed into granules found in the fibers. Eumelanin is the dominant pigment in dark-blond, brown, and black hair, while pheomelanin is dominant in red hair.^[4] Blond hair is the result of having little pigmentation in the hair strand. Gray hair occurs when melanin decreases or disappears.

Hair grows everywhere on the external body except for mucus membranes and glabrous skin, such as that found on the soles of the hands, feet, and lips.

Hair follows a specific growth cycle with three distinct and concurrent phases: anagen, catagen, and telogen phases. Each phase has specific characteristics that determine the length of the hair. All three phases occur simultaneously; one strand of hair may be in the anagen phase, while another is in the telogen phase.

The body has different types of hair, including vellus hair and androgenic hair, each with its own type of cellular construction. The different construction gives the hair unique characteristics, serving specific purposes, mainly warmth and protection.

Many mammals have fur and other hairs that serve different functions. Hair provides thermal regulation and camouflage for many animals; for others it provides signals to other animals such as warnings, mating, or other communicative displays; and for some animals hair provides defensive functions and, rarely, even offensive protection. Hair also has a sensory function, extending the sense of touch beyond the surface of the skin. Guard hairs give warnings that may trigger a recoiling reaction.

While humans have developed clothing and other means of keeping warm, the hair found on the head serves as primary sources of heat insulation and cooling (when sweat evaporates from soaked hair) as well as protection from ultra-violet radiation exposure. The function of hair in other locations is debated. Hats and coats are still required while doing outdoor activities in cold weather to prevent frost bite and hypothermia, but the hair on the human body does help to keep the internal temperature regulated. When the body is too cold, the arrector pili muscles found attached to hair follicles stand up, causing the hair in these follicles to do the same. These hairs then form a heat-trapping layer above the epidermis. This process is formally called piloerection, derived from the Latin words 'pilus' ('hair') and 'erectio' ('stiffening'), but is more commonly known as 'having goose bumps' in humans.Cite error: The <ref> tag has too many names (see the help page). This is more effective in other mammals whose fur fluffs up to create air pockets between hairs that insulate the body from the cold. The opposite actions occur when the body is too warm; The arrector muscles make the hair lay flat on the skin which allows heat to leave.

Human hair may not compete with the painful spines of the porcupine, but much of the hair on the human body is suited to protect it. This natural armor cannot directly protect humans from potential predators, but it does help to keep the sense organs, such as the eyes, working properly.

Movements of hair shafts are detected by nerve receptors within the skin and by hair follicle receptors through displacement and vibration of hair shafts. Hairs can sense movements of air as well as touch by physical objects and are especially sensitive to the presence of insects. Some hairs, such as eyelashes, are especially sensitive to the presence of potentially harmful matter.^[10][11][12]

The eyebrows are situated above the eyelids on the forehead. In many other mammals they contain much longer hairs that are upright, functioning as tactile sensors. While they have little significance to the survival of humans, they remain a part of the body's physiological makeup because of the role they play in protecting the eyes from dirt, sweat, and rain, as well as non-verbal communication.^[13] People have developed other means, such as hats, umbrellas, and goggles to serve the purpose of eyebrows more effectively, but the presence of eyebrows may suggest that they were once important to earlier species' survival.^[13]

The eyelash grows at the edges of the eyelid and protects the eye from dirt entering the eye. Camels have particularly long and thick eyelashes. The eyelash is to humans, camels, horses, ostriches etc., what whiskers are to cats; they are used to sense when dirt, dust, or any other potentially harmful object is too close to the eye.^[14] The eye reflexively closes as a result of this sensation.

A 2008 study by the Medical University of Vienna traced the origins of hair to the common ancestor of mammals, birds, and lizards that lived 310 million years ago. The study found chickens, lizards, and humans all possessed a similar set of genes that was involved in the production of alpha keratin. In chickens and lizards, the α-keratin produced was found in their claws, but in mammals it was used to produce hair. The scientists involved continued searching for the mechanisms that allowed mammals to use the keratins of animal claws to produce hair.^[15][16]

This article or section possibly contains synthesis of material which does not verifiably mention or relate to the main topic. Relevant discussion may be found on the talk page. (November 2010) (Learn how and when to remove this message)

Human body hair is barely visible as it is thinner, shorter, and more translucent than the hair of other mammals. Humans are part of a trend toward sparser hair in larger animals (fewer follicles per square inch of exposed skin). There are several African mammals that have very sparse fur, including the elephant and the hippopotamus, both at the upper end of this trend. The density of human hair follicles on the skin is about the average for an animal of equivalent size.^[17] It still is not clear why so much of human hair is short, under-pigmented vellus hair, rather than terminal hair, and the effect of testosterone on the hair follicles in both human and other mammalian species. Because most human hair is less apparent vellus hair, this gives the appearance of being hairless, especially in females.

Most mammals have light skin that is covered by fur, and biologists believe that early human ancestors started out this way also. Dark skin probably evolved after humans lost their body fur, because the naked skin was vulnerable to the strong UV radiation as would be experienced in Africa. Therefore, evidence of when human skin darkened has been used to date the loss of human body hair, assuming that the dark skin was needed after the fur was gone.

Dr. Alan R. Rogers, an evolutionary geneticist at the University of Utah, used mutations in the MC1R gene to estimate when human skin darkened. He said early humans may have gone through several genetic "clean sweeps" with light-skinned individuals dying off and dark-skinned individuals surviving. He estimates the last of these clean sweeps took place 1.2 million years ago.^[18] Therefore, apparent bareness in humans, likely has existed at least since that time. Later when humans began to live in more northerly latitudes, however, light skin became an advantage for the absorption of vitamin D and the recessive gene began to become more prevalent in those populations.

Balding, where terminal hair switches to vellus hair, usually occurs at around thirty to forty years of age. In prehistoric times, most individuals were not as likely to live past 30, with few reaching their fourth decade,^[19] and thus it wasn't likely that this trait was subject to selection.

Most species evolved as the climate in Africa changed, to adjust their thermoregulation to the intense UV and sunlight at the equator, mostly by panting. Early hominids likely possessed fur similar to other large apes, but about 2.5 million years ago they developed a greater distribution of sweat glands {{citation}}: Empty citation (help) that enabled them to perspire over most of the body. It is not clear whether the change in body hair appearance occurred before or after the development of sweat glands. Humans have eccrine sweat glands all over our body.^[20] Aside from the mammary glands that produce a specialized sweat called, milk, most mammals just have apocrine sweat glands on their armpits and loin. The rest of their body is covered in eccrine glands. There is a trend in primates to have increased eccrine sweat glands over the general surface of the body.^[20] It is unclear to what degree other primates sweat in response to heat, however.

The sweat glands in humans could have evolved to spread from the hands and feet as the body hair changed, or the hair change could have occurred to facilitate sweating. Horses and humans are two of the few animals capable of sweating on most of their body, yet horses are larger and still have fully developed fur. In humans, the skin hairs lie flat in hot conditions, as the erector pili muscles relax, preventing heat from being trapped by a layer of still air between the hairs, and increasing heat loss by convection.

Historically, some ideas have been advanced to explain the apparent hairlessness of humans, as compared to other species.

The thermoregulatory hypothesis (developed by Dr. Peter Wheeler, 1984, 1985) suggests that when human ancestors started living on the savanna, humans began sweating more to stay cool.^[21] It is posited that thick hair got in the way of the sweat evaporating, so humans evolved a sparser coat of fur. Although hair provides protection against harmful UV radiation, since our hominin ancestors were bipedal, only our heads were exposed to the noonday sun.^[22] Humans kept the hair on our head which reflects harmful UV rays, but our body hair was reduced. Opponents of the thermoregulatory hypothesis would say that losing hair added an extreme weakness to cold, but, seeing as how humans figured out cutlery around 2.6 million years ago, our ancestors easily could have found clothing within the 1.4 million years between cutting up their kills and losing their hair.

The aquatic ape hypothesis posits that sparsity of hair is an adaptation to a semi-aquatic environment, but the theory has yet to gain support among scientists.^[23]

Another hypothesis for the thick body hair on humans proposes that Fisherian runaway sexual selection played a role (as well as in the selection of long head hair), (see types of hair and vellus hair), as well as a much larger role of testosterone in men. Sexual selection is the only theory thus far that explains the sexual dimorphism seen in the hair patterns of men and women. On average, men have more body hair than women. Males have more terminal hair, especially on the face, chest, abdomen, and back, and females have more vellus hair, which is less visible. The halting of hair development at a juvenile stage, vellus hair, would also be consistent with the neoteny evident in humans, especially in females, and thus they could have occurred at the same time.^[24]

A final hypothesis is that human hair was reduced in response to ectoparasites.^[25][26] Getting rid of our hair might have reduced the risk of fleas, ticks, lice, and other biting insects.

Evolutionary biologists suggest that the genus Homo arose in East Africa approximately 2.5 million years ago.^[27] They devised new hunting techniques.^[27] The higher protein diet led to the evolution of larger body and brain sizes.^[27] Jablonski^[27] postulates that increasing body size, in conjunction with intensified hunting during the day at the equator, gave rise to a greater need to rapidly expel heat. As a result, humans evolved the ability to sweat: a process which was facilitated by the loss of body hair.^[27] A major problem with this theory, however, is that it does not explain why males are larger, hairier, and were more active in hunting than females. The female-male size differential among other closely associated primates is much greater than among humans, however, so it might have been reduced during evolution.

Other primates have sweat gland in their armpits that function as those of humans, and thus it is probable that human sweat glands evolved from a similar distribution, spreading to more areas of the body, rather than occurring through evolution of a new trait. It is not known whether the increased distribution of sweat glands occurred before, during, or after, the change in body hair, or even whether the two are related developments. Horses also sweat, and they are larger, hairier, and expend more energy running than human males, so there may not be any connection between the ability to sweat and the apparent hairlessness of humans.

Another factor in human evolution that also occurred in the prehistoric past was a preferential selection for neoteny, particularly in females. The idea that adult humans exhibit certain neotenous (juvenile) features, not evinced in the great apes, is about a century old. Louis Bolk made a long list of such traits,^[28] and Stephen Jay Gould published a short list in Ontogeny and Phylogeny.^[29] In addition, paedomorphic characteristics in women are widely acknowledged as desirable by men. For instance, vellus hair is a juvenile characteristic. However, while men develop longer, coarser, thicker, and darker terminal hair through sexual differentiation, women do not, leaving their vellus hair visible.

Jablonski^[27] agrees that it was evolutionarily advantageous for pre-humans to retain the hair on their heads in order to protect the scalp as they walked upright in the intense African (equatorial) UV light. While some might argue that, by this logic, humans should also express hairy shoulders given that these body parts would putatively be exposed to similar conditions, the protection of the head, the seat of the brain that enabled humanity to become one of the most successful species on the planet (and which also is very vulnerable at birth), was arguably a more urgent issue (axillary hair in the underarms and groin were also retained as signs of sexual maturity). During the gradual process by which Homo erectus made a transition from furry to naked skin, their hair texture putatively changed gradually from straight ^{[citation needed]} (the condition of most mammals, including humanity's closest cousins—chimpanzees), to Afro-like or 'kinky' (i.e. tightly coiled). In this sense, during the period in which humans were gradually losing their straight body hair and thereby exposing initially the pale skin underneath their fur to the sun, straight hair would have been an adaptive liability. Hence, tightly coiled or 'kinky' Afro-hair may have evolved to prevent the entry of UV light into the body during the transition toward dark, UV-protected skin.

Alternatively, some intuit that tightly coiled hair that grows into a typical Afro-like formation would have greatly reduced the ability of the head and brain to cool. They reason that although hair density in African peoples is much less than their European counterparts, in the intense sun the effective 'woolly hat' produced would have been a disadvantage. However, anthropologists such as Nina Jablonski make the opposite argument with regards to this hair texture. Specifically, Jablonski's assertions ^[27] suggest that the adjective "woolly" in reference to Afro-hair is a misnomer to the extent that it connotes the high heat insulation derivable from the true wool of sheep. Instead, the relatively sparse density of Afro-hair, combined with its springy coils actually results in an airy, almost sponge-like effect. This, in turn, Jablonski argues,^[27] more likely facilitates an increase in the circulation of cool air onto the scalp. Further, Afro-hair does not respond so easily to moisture and sweat as straight hair. Thus it does not stick to the neck and scalp when wet. Rather, unless totally drenched, it tends to retain its basic springy puffiness. In this sense, the trait may enhance comfort levels in intense equatorial climates compared to straight hair (which, on the other hand, tends to naturally fall over the ears and neck to a degree that provides slightly enhanced comfort levels in cold climates relative to tightly coiled hair).

Further, some interpret the ideas of Charles Darwin as suggesting that some traits, such as hair texture, were so arbitrary to human survival that the role natural selection played was trivial. They argue that Darwin's explanation which was that sexual selection may be responsible for such traits.^[30] However, the concept of "triviality" is a human value judgment, and hair texture still may have played a role. In fact, while the sexual selection hypothesis cannot be ruled out, the asymmetrical distribution of this trait vouches for environmental influence. Specifically, if hair texture were simply the result of adaptively arbitrary human aesthetic preferences, one would expect that the global distribution of the various hair textures would be fairly random. ^{[dubious – discuss]} Instead, the distribution of Afro-hair is strongly skewed toward the equator. Further, it is notable that the most pervasive expression of this hair texture can be found in sub-Saharan Africa; a region of the world that abundant genetic and paleo-anthropological evidence suggests, was the relatively recent (~200,000 year old) point of origin for modern humanity. In fact, although genetic findings (Tishkoff, 2009) suggest that sub-Saharan Africans are the most genetically diverse continental group on Earth, Afro-textured hair (along with a small cluster of other physical features) approaches ubiquity is this region. ^{[citation needed]} This points to a strong, long-term selective pressure that, in stark contrast to most other regions of the genomes of sub-Saharan groups, left little room for genetic variation at the determining loci. Such a pattern is, again, not indicative of human sexual aesthetics.

According to the recent single origin hypothesis, anatomically modern humans arose in East Africa approximately 200,000 years ago (Tishkoff, 1996). Then, ~150,000 years later, modern humans began to expand their range to regions outside of (and within) this continent (Tishkoff, 1996). The skin of those in the group that left the African continent had developed the ability to manufacture vitamin D (which was essential for bone development) upon exposure to UV light.^[27] However, the UV light of northern regions was too weak to penetrate the highly pigmented skin of the initial migrants in order to provide enough vitamin D for healthy bone development.^[27] Malformed bones in the pelvic area were especially deadly for women because they interfered with the successful delivery of babies; leading to the death of both the mother and the infant during labor. Hence, those with lighter skin survived and had children at higher rates because their skin allowed more UV light for the production of vitamin D.^[27]

In this sense, the evidence with regard to the evolution of straight hair texture seems to support Jablonski's suggestions ^[27] that the need for vitamin D triggered the transition back from dark to light skin. Specifically, the distribution of this trait suggests that this need may have grown so intense at certain points that Northerners with mutations for straighter hair survived and had children at higher rates. This argument is made based on the principle that straight fibers better facilitate the passage of UV light into the body relative to curly hair. It is substantiated by Iyengar's (1998) findings that UV light can pass through straight human hair roots in a manner similar to the way that light passes through fiber optic tubes (Iyengar, 1998).

Nonetheless, some argue against this stance because straighter hair ends tend to point downward while fiber optics requires that light be transmitted at a high angle to the normal of the inner reflective surface. In light of this, they suggest that only light reflected from the ground could successfully enter the hair follicle and be transmitted down the shaft. Even this process, they argue, is hindered by the curvature at the base of the hair. Therefore, coupled with the amount of skin covered by long head hair, these factors seem to militate against the adaptive usefulness of straight hair at northern latitudes. They further argue that UV light also is poorly reflected from soil and dull surfaces. These ideas can be countered by the fact that during the winter, the time of year in which UV light is most scarce at northern latitudes, the ground is often covered with white snow. Given that white is the most effective color in terms of facilitating the reflection of ground light, the hypothesis that straight hair could have been adaptively favorable, cannot be fully discounted in this regard. In addition, as mentioned in the previous section, straight hair also may have contributed to enhanced comfort levels in the north. This is evident in the extent to which, relative to curly hair, it tends to provide a layer of protection for ears and necks against the cold.

The latter hypothesis seems the more plausible evolution determinant as the surface area of the head is minute compared to the remainder of the body, thus the energy required in producing long hair for the express purpose of "optical" amplification of UV light reflected from the snow seems counterproductive. Scientists point to the fact that straight hair found in many ethnic groups is denser as well and has a greater ability to "show" as it does not coil, hence providing more warmth as the likely deterministic factor for the evolution of straight long hair. Some scientists argue that since the head and appendages are the greatest areas for heat loss from the body, the ability to grow long hair on the crown of the head as well as the face provides a distinct advantage in a cold climate. Since the main sensory organs are anatomically located on the head, long hair provides the necessary warmth and protection in a cold climate that allows the use of these organs by exposing them to the elements to "sense", in for example a hunt, yet still providing necessary warmth and protection to sustain prolonged exposure. It may be argued, therefore, that the ability to grow long, straight, densely packed hair provides a distinct evolutionary advantage in cold climate; however, it would be a distinct disadvantage in a hot climate, when compared to loosely packed, spongy, closely cropped hair.

A group of studies have recently shown that genetic patterns at the EDAR locus, a region of the modern human genome that contributes to hair texture variation among most individuals of East Asian descent, support the hypothesis that (East Asian) straight hair likely developed in this branch of the modern human lineage subsequent to the original expression of tightly coiled natural afro-hair.^[31][32][33] Specifically, the relevant findings indicate that the EDAR mutation coding for the predominant East Asian 'coarse' or thick, straight hair texture arose within the past ~65,000 years, which is a time frame that covers from the earliest of the 'Out of Africa' migrations up to now.

Though growing hair is an inevitable part of being human, many believe that it is unsightly and should be removed. Hair removal is almost always motivated by cosmetic reasons.

Depilation is the removal of hair from the surface of the skin. This can be achieved through methods such as shaving. Epilation is the removal of the entire hair strand, including the part of the hair that has not yet left the follicle. A popular way to epilate hair is through waxing it.

Shaving is accomplished with bladed instruments, such as razors. The blade is brought close to the skin and stroked over the hair in the desired area to cut the terminal hairs and leave the skin feeling smooth. Depending upon the rate of growth, one can begin to feel the hair growing back within hours of shaving. This is especially evident in men who develop a five o'clock shadow after having shaved their faces. This new growth is called stubble. Stubble typically appears to grow back thicker than the original hair in the area because the very ends of terminal hairs are the thinnest parts of the hair shaft.

Waxing involves using a sticky wax and strip of paper or cloth to pull hair from the root. Waxing is the ideal hair removal technique to keep an area hair-free for long periods of time. It can take five to nine weeks for waxed hair to begin to resurface again. Hair in areas that have previously been waxed also is known to grow back finer and thinner, especially compared to hair that has been shaved with a razor.

Because the hair on the head is normally longer than other types of body hair, it is cut with scissors or clippers. People with longer hair will most often use scissors to cut their hair, whereas shorter hair is maintained using a trimmer. Depending on the desired length and overall health of the hair, periods without cutting or trimming the hair can vary.

Many people will confuse what a haircut is versus what a trim is. A haircut is usually performed in order to change one's hairstyle, while a trim helps to keep away split ends and keep the hair well-groomed. Cutting hair tends to take off more hair than trimming hair does. When hair is trimmed, only the first few centimeters need to be removed, whereas haircuts can sometimes result in the loss of many inches of hair.

Hair has great social significance for human beings. It can grow on most external areas of the human body, except on the palms of the hands and the soles of the feet (among other areas). Hair is most noticeable on most people in a small number of areas, which are also the ones that are most commonly trimmed, plucked, or shaved. These include the face, ears, head, eyebrows, legs, and armpits, as well as the pubic region. The highly visible differences between male and female body and facial hair are a notable secondary sex characteristic.

Healthy hair indicates health and youth (important in evolutionary biology). Hair colour and texture can be a sign of ethnic ancestry. Facial hair is a sign of puberty in men. White hair is a sign of age or genetics, which may be concealed with hair dye. Male pattern baldness is a sign of age, which may be concealed with a toupee, hats, or religious and cultural adornments. Although drugs and medical procedures exist for the treatment of baldness, many balding men simply shave their heads. Hair whorls have been discovered to be associated with brain development.

Hairstyle may be an indicator of group membership. During the English Civil War, the followers of Oliver Cromwell decided to crop their hair close to their head, as an act of defiance to the curls and ringlets of the king's men.^[34] This led to the Parliamentary faction being nicknamed Roundheads. Having bobbed hair was popular among the flappers in the 1920s as a sign of rebellion against traditional roles for women. Female art students known as the "cropheads" also adopted the style, notably at the Slade School in London, England. Regional variations in hirsutism cause practices regarding hair on the arms and legs to differ. Some religious groups may follow certain rules regarding hair as part of religious observance. The rules often differ for men and women.

Many subcultures have hairstyles which may indicate an unofficial membership. Many hippies, metalheads, and Indian sadhus have long hair. Many punks wear a hairstyle known as a Mohawk or other spiked and dyed hairstyles; skinheads have short-cropped or completely shaved heads. Mullet hairstyles stereotypically have been associated with rednecks.^{[citation needed]}

Heads were shaved in concentration camps, and head-shaving has been used as punishment, especially for women with long hair. The shaven head is common in military haircuts, while Western monks are known for the tonsure. By contrast, among some Indian holy men, the hair is worn extremely long.^{[citation needed]}

The ethnically Manchu Qing Dynasty, beginning in the late seventeenth century China, ordered all Chinese citizens to adopt Manchurian hairstyles by shaving the front of their head and adopting a queue.^{[citation needed]}

Regular hairdressing in some cultures is considered a sign of wealth or status. The dreadlocks of the Rastafari movement were despised early in the movement's history. In some cultures, having one's hair cut can symbolize a liberation from one's past, usually after a trying time in one's life. Cutting the hair also may be a sign of mourning.

Tightly coiled hair in its natural state may be worn in an Afro. This hairstyle was once worn among African Americans as a symbol of racial pride. Given that the coiled texture is the natural state of some African Americans' hair, or perceived as being more "African", this simple style is now often seen as a sign of self-acceptance and an affirmation that the beauty norms of dominant (northern/European) culture are not absolute. It is important to note that African Americans as a whole have a variety of hair textures, as they are not an ethnically homogeneous mixture, but an Ad-Hoc of many ethnicities, one component being African. It is also to note that "Afro Hair" is not universal in Africa as well, although is common among many "West African" ethnicities.^{[citation needed]}

The film Easy Rider (1969) includes the assumption that the two main characters could have their long hairs forcibly shaved with a rusty razor when jailed, symbolizing the intolerance of some conservative groups toward members of the counterculture.

At the conclusion of the Oz obscenity trials in the UK, the defendants had their heads shaved by the police, causing public outcry. During the appeal trial, they appeared in the dock wearing wigs.^{[citation needed]}

Women's hair may be hidden using headscarves, a common part of the hijab in Islam and a symbol of modesty required for certain religious rituals in Orthodox Christianity. Orthodox Judaism endorses the use of scarves and other head coverings for women for modesty reasons as in Islam. Sikhs generally keep their hair uncut and men keep it tied in a bun on the head, which is then covered appropriately using a turban. Multiple religions, both ancient and contemporary, require or advise one to allow their hair to become dreadlocks, though people also wear them for fashion.

• Hypotrichosis, the state of having a less than normal amount of hair on the head or body
• Hypertrichosis, the state of having an excess of hair on the head or body
• Natural afro-hair
• Hair follicle
• Cowlick

This article needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. Find sources: "Hair" – news · newspapers · books · scholar · JSTOR (December 2007) (Learn how and when to remove this message)

• Iyengar, B. (1998). The hair follicle is a specialized UV receptor in human skin? Bio Signals Recep, 7(3), 188–194.
• Jablonski, N.G. (2006). Skin: a natural history. Berkley, CA: University of Califiornia Press.
• Rogers, Alan R.; Iltis, David & Wooding, Stephen (2004), “Genetic variation at the MC1R locus and the time since loss of human body hair”, Current Anthropology 45 (1): 105–108.
• Tishkoff, S.A. (1996). Global patterns of linkage disequilibrium at the CD4 locus and modern human origins. Science. 271(5254), 1380–1387.

Wikimedia Commons has media related to Hair.

Look up hair in Wiktionary, the free dictionary.

• Discussion about shaving and cultures
• Answers to several questions related to hair from curious kids
• How to measure the diameter of your own hair using a laser pointer
• Instant insight outlining the chemistry of hair from the Royal Society of Chemistry