Editing Human variability (section)

==Classification and evaluation of traits==
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. 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 [[Genetic carrier|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.<ref>{{cite journal|last1=Luzzatto|first1=Lucio|title=Sickle Cell Anaemia and Malaria|journal=Mediterranean Journal of Hematology and Infectious Diseases|date=3 October 2012|volume=4|issue=1|pmc=3499995|pmid=23170194|doi=10.4084/MJHID.2012.065|pages=e2012065}}</ref>

Each trait has its own advantages and disadvantages, but sometimes a trait that is found desirable may not be favorable in terms of certain biological factors such as reproductive fitness, and traits that are not highly valued by the majority of people may be favorable in terms of biological factors. For example, women tend to have fewer pregnancies on average than before and therefore net worldwide fertility rates are dropping.<ref>{{cite news|url=http://www.cbsnews.com/news/dropping-birth-rates-threaten-global-economic-growth/|title=Dropping birth rates threaten global economic growth|date=2014-05-07|agency=CBS|access-date=2016-11-13}}</ref> Moreover, this leads to the fact that multiple births tend to be favorable in terms of number of children and therefore offspring count; when the average number of pregnancies and the average number of children was higher, multiple births made only a slight relative difference in number of children. However, with fewer pregnancies, multiple births can make the difference in number of children relatively large. A hypothetical scenario would be that couple 1 has ten children and couple 2 has eight children, but in both couples, the woman undergoes eight pregnancies. This is not a large difference in ratio of fertility. However, another hypothetical scenario can be that couple 1 has three children and couple 2 has one child but in both couples the woman undergoes one pregnancy (in this case couple 2 has triplets). When the proportion of offspring count in the latter hypothetical scenario is compared, the difference in proportion of offspring count becomes higher. A trait in women known to greatly increase the chance of multiple births is being a tall woman (presumably the chance is further increased when the woman is very tall among both women and men).<ref>{{cite web|title=Tall women more likely to have twins|url=http://www.livescience.com/1019-tall-women-twins.html|website=livescience|date=24 September 2006}}</ref><ref>{{cite web|title=Taller Women Are More Likely To Have Twins, Obstetrician's Study Confirms|url=https://www.sciencedaily.com/releases/2006/09/060923104930.htm}}</ref> Yet very tall women are not viewed as a desirable phenotype by the majority of people, and the phenotype of very tall women has not been highly favored in the past. Nevertheless, values placed on traits can change over time.

Such an example is homosexuality. 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.<ref>{{cite web|title=Homosexuality|url=https://www.pbs.org/empires/thegreeks/background/19a_p1.html|website=pbs.org}}</ref> However, homosexuality became more condemned. Attitudes towards homosexuality alleviated in modern times.