Human variability

Human variability, or human variation, refers to the range of possible values for any measurable characteristic, physical or mental, of human beings. Differences can be trivial or important, transient or permanent, voluntary or involuntary, congenital or acquired, genetic or environmental. This article discusses variabilities that characterize a person for all or much of his or her lifetime, and are perceived as not purely learned or readily changed (such as religion, language, customs, or tastes). Each person being different is so essential a part of human experience that it is difficult to even imagine a human existence in which other people are identical. Furthermore, the social value put on these differences by the society in which one lives affects every aspect of a person's life.

Sources of human variability

 * biological inheritance, shaped by
 * mutations, allelic differences
 * genetic drift
 * natural selection
 * prenatal environment and fetal "programming"
 * disease
 * artificial or cultural selection
 * nutrition and malnutrition
 * quality of life and health care
 * pollution and toxin exposure and other stressors
 * education
 * cultural environment
 * social environment
 * family environment and upbringing (especially before age 5)
 * child abuse and neglect
 * accidental or intentional injury, mutilation, or change of the body

While nearly all of the variables listed above are at least partially determined or affected by genetic factors, few of them are controlled by simple Mendelian inheritance. Most are polygenic or are determined by a complex combination of genes and early environment. Essentially, genes provide proclivities and potentialities continuously involving feedback mechanisms with the environment throughout life, but especially during prenatal and early childhood.

Many genetic differences (polymorphisms) have little effect on health or reproductive success, but serve to statistically distinguish one population from another. Researchers in the field of population genetics have been using these to elucidate ancient migrations and relationships between population groups.

Another purely genetic set of individual differences are the blood types and immune types we all carry. While some may carry mild advantages or disadvantages in terms of risks of particular diseases, the primary life-or-death significance comes when we attempt to transfer blood or organs from one person to another. Our immune system is designed to recognize these human differences with great sensitivity and enforce our individuality.

Social significance and valuation of human variability
Human beings rarely give all possible values for a given parameter the same value, though not all people agree on the values or relative rankings. Examples of differences which may be given different values in different societies include darker/lighter skin color or thinness/fatness. Local valuation may affect social standing, reproductive opportunities, or even survival.

Possession of above average amounts of some abilities is valued by most societies: ability to learn; musical aptitude; strength, endurance, agility; resilience.

Each individual's distinctive differences, even the negatively valued ones, are usually considered an essential part of self-identity. Membership or status in a social group may depend on having specific values for certain attributes. It is not unusual for people to deliberately try to amplify or exaggerate differences, or to conceal or minimize them, for a variety of reasons. Examples of practices designed to minimize differences include hair straightening or skin bleaching, plastic surgery, orthodontia, and growth hormone treatment for extreme shortness. Conversely, male-female differences are enhanced and exaggerated in most societies.

These differences may vary or be distributed in various ways. Some, like height for a given sex, vary in close to a "normal" or Gaussian distribution. Some characteristics (e.g., skin color) vary continuously in a population, but the continuum may be socially divided into a small number of distinct categories. Some characteristics vary bimodally (sexual orientation, handedness), with fewer people in intermediate categories.

Different human societies may assign different values to various differences. The obvious examples are race and sex, while handedness has a much weaker value difference, but nearly all human differences will have social value dimension. In some societies, such as the United States, circumcision is practiced on a majority of males, as well as sex reassignment of intersex infants, with substantial emphasis on cultural norms.

Much social controversy surrounds the assigning or distinguishing of some categories, with variation between groups in a society or between societies as to the degree to which a difference is part of a person's "essential" nature or is partly a socially constructed attribution. For example, in the United States and Europe there has been a centuries-long debate over whether sexual orientation is an essential part of one's nature (the "essentialist" position), or a result of mutually reinforcing social perceptions and behavioral choices (the "constructivist" perspective). Other cultures may not even understand the controversy.

Controversy also surrounds the boundaries of "wellness", "wholeness," or "normality." In some cultures, physical imperfections can exclude one from religious service. In western culture there has been large-scale renegotiation of the social significance of variations which reduce the ability of a person to do one or more functions. Laws have been passed to alleviate the reduction of social opportunity available to those with disabilities. The concept of "differently abled" has been pushed by those persuading society to see limited incapacities as a human difference of less negative value.

When an inherited difference of body structure or function is severe enough, it is termed a genetic disease, but even this classification has fuzzy edges. There are many instances in which the degree of negative value of a human difference depends completely on the social or physical environment. For example, in a society with a large proportion of deaf people (as Martha's Vineyard in the 19th century), it was possible to deny that deafness is a disability. Another example of social renegotiation of the value assigned to a difference is reflected in the controversy over management of ambiguous genitalia, especially whether abnormal genital structure has enough negative consequences to warrant surgical correction.

Furthermore, many genetic traits may be advantageous in certain circumstances and disadvantageous in others. Being a heterozygote or asymptomatic carrier of the sickle-cell disease gene confers some protection against malaria, apparently enough to maintain the gene in populations of malarial areas. In a homozygous dose it is a significant disability.

The extreme exercise of social valuation of human difference is in the definition of "human." What difference is great enough to assign an individual "nonhuman" status, in the sense of withholding our identification, charity, and social participation? This can change enormously between cultures and over time. For example, nineteenth century European and American ideas of race and eugenics culminated in the attempts of the Nazi-led German society of the 1930s to deny not just reproduction, but life itself to a variety of people with "differences" attributed in part to biologic characteristics. Western society's revulsion to this contributed to a considerable readjustment of valuation of differences.

Contemporary controversy continues over "what kind of human" is a fetus or child with a significant disability. On one end are people who would argue that Down's syndrome is not a disability but a mere "difference," and on the other those who consider such a calamity as to assume that such a child is better off "not born". In India and China, being female is widely considered such a negatively valued human difference that similar decisions are made by the hundreds of thousands.

Acknowledgement and study of human differences does have a wide range of uses, such as tailoring the size and shape of manufactured items. See Ergonomics.

Common human variations

 * Sex
 * Male, female, see also intersex
 * those identifying with neither or other than their physical sex, see transgender
 * Sexual orientation, heterosexual, homosexual, bisexual, pansexual, asexual
 * Monogamy, serial monogamy, polygamy, polyamory, "swinging"
 * Race
 * Body shape and size
 * Height
 * Shortness, dwarfism, Little people
 * Tallness, gigantism
 * Body type/Somatotype, thinness, obesity
 * Motor skills, handedness, dexterity
 * Physical disabilities
 * Amputation, loss of limbs or limb function
 * Blindness, color blindness
 * Deafness, tone deafness
 * Diseases and defects of other organ systems
 * Other aspects of human physical appearance, attractiveness (highly subjective, variable, and impermanent)
 * Skin or eye coloring, complexion
 * Hair color, baldness, hirsutism
 * Supernumerary body part (such as Polydactylism, Supernumerary nipples, Hyperdontia) or missing body parts (such as Hypogenesis)
 * Cleft lip and Cleft palate
 * Reproductive attributes and experience
 * Genital modification and mutilation
 * Virginity, fertility, parity, parenthood
 * Psychological and personality traits
 * Intelligence, spatial aptitude
 * Temperament, introversion, extroversion, impulsiveness, risk-taking
 * Developmental disability, cognitive disability, social disability
 * Emotional stability, mental illness
 * Musical ability