Human variability

Frequently debated areas of variability include cognitive ability, personality, physical appearance (body shape, skin color, etc.)

Evolution has caused production of folate (folic acid) from UV radiation, thus giving them darker skin tone with more melanin to make sure child development is smooth and successful.

Nutrition and diet play a substantial role in determining phenotype because they are arguably the most controllable forms of environmental factors that create epigenetic changes.

For example, Bisphenol A (BPA) is a known endocrine disruptor that mimics the hormone estradiol and can be found in various plastic products.

A recent study by researchers at the MRC International Nutrition Group shows that "methylation machinery can be disrupted by nutrient deficiencies and that this can lead to disease" susceptibility in newborn babies.

[7] Genetic variation in humans may mean any variance in phenotype which results from heritable allele expression, mutations, and epigenetic changes.

Measurement of human variation can fall under the purview of several scholarly disciplines, many of which lie at the intersection of biology and statistics.

[23] Common measurements include height, weight, organ size (brain, stomach, penis, vagina), and other bodily metrics such as waist–hip ratio.

The first human population studies based on mitochondrial DNA were performed by restriction enzyme analyses (RFLPs) and revealed differences between the four ethnic groups (Caucasian, Amerindian, African, and Asian).

After grinding tissue to release the cytoplasm, wicks are used to absorb the resulting extract and placed in a slit cut into a starch gel.

[40] Commercial motivation for understanding variation in human populations arises from the competitive advantage of tailoring products and services for a specific target market.

A business may undertake some form of market research in order to collect data on customer preference and behavior and implement changes which align with the results.

Other characteristics (e.g., skin color) vary continuously in a population, but the continuum may be socially divided into a small number of distinct categories.

When an inherited difference of body structure or function is severe enough to cause a significant hindrance in certain perceived abilities, it is termed a genetic disease, but even this categorization has fuzzy edges.

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.

In Ancient Greece, what in present terms would be called homosexuality, primarily between a man and a young boy, was not uncommon and was not outlawed.

Some of the traits that societies try to measure by perception are intellectual aptitude in the form of ability to learn, artistic prowess, strength, endurance, agility, and resilience.

Examples of practices designed to minimize differences include tanning, hair straightening, skin bleaching, plastic surgery, orthodontia, and growth hormone treatment for extreme shortness.

In some societies, such as the United States, circumcision is practiced on a majority of males, as well as sex reassignment on intersex infants, with substantial emphasis on cultural and religious norms.

Circumcision is highly controversial because although it offers health benefits, such as less chance of urinary tract infections, STDs, and penile cancer, it is considered a drastic procedure that is not medically mandatory and argued as a decision that should be taken when the child is old enough to decide for himself.

[47] Similarly, sex reassignment surgery offers psychiatric health benefits to transgender people but is seen as unethical by some Christians, especially when performed on children.

For example, there has long been a debate among sex researchers on whether sexual orientation is due to evolution and biology (the "essentialist" position), or a result of mutually reinforcing social perceptions and behavioral choices (the "constructivist" perspective).

Biological theories for same gender sexual orientation include genetic influences, neuroanatomical factors, and hormone differences but research so far has not provided any conclusive results.

In contrast, the social constructivist position argues that sexuality is a result of culture and has originated from language or dialogue about sex.

In some cultures, differences in physical appearance, mental ability, and even sex can exclude one from traditions, ceremonies, or other important events, such as religious service.

Differences between humans can lead to an individual's "nonhuman" status, in the sense of withholding identification, charity, and social participation.

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 biological characteristics.

Hitler and Nazi leaders wanted to create a "master race" consisting of only Aryans, or blue-eyed, blonde-haired, and tall individuals, thus discriminating and attempting to exterminate those who didn't fit into this ideal.

On one end are people who would argue that Down syndrome is not a disability but a mere "difference," and on the other those who consider it such a calamity as to assume that such a child is better off "not born".

[52] Turner syndrome Female Male Albinism Martin scale Hair coloring Hirsutism Supernumerary body part Amniotic band constriction Earlobe Visual impairment Hearing loss Selective Mutism Trisomy 21 Natural fertility Sterility Birth rate Plastic surgery Anorexia nervosa Puberty Childhood Werner syndrome Big Five personality traits Creativity