Sibling


A sibling is a relative that shares at least one parent with the other person. A male sibling is a brother, and a female sibling is a sister. A person with no siblings is an only child.
File:Me and my other brother.jpg|thumb|A sister carrying her brother.
While some circumstances can cause siblings to be raised separately, most societies have siblings grow up together. This causes the development of strong emotional bonds, with siblinghood considered a unique type of relationship. The emotional bond between siblings is often complicated and is influenced by factors such as parental treatment, birth order, personality, and personal experiences outside the family.
Medically, a full-sibling is a first-degree relative and a half-sibling is a second-degree relative as they are related by 50% and 25%, respectively.

Definitions

The word sibling was reintroduced in 1903 in an article in Biometrika, as a translation for the German Geschwister, having not been used since Middle English, specifically 1425.
Siblings or full-siblings share the same biological parents. Full-siblings are also the most common type of siblings. Twins are siblings that are born from the same pregnancy. Often, twins with a close relationship will develop a twin language from infanthood, a language only shared and understood between the two. Studies corroborate that identical twins appear to display more twin talk than fraternal twins. At about 3, twin talk usually ends. Twins generally share a greater bond due to growing up together and being the same age.
Half-siblings are people who share one parent. They may share the same mother but different fathers, or they may have the same father but different mothers. In law, half-siblings have often been accorded treatment unequal to that of full-siblings. Old English common law at one time incorporated inequalities into the laws of intestate succession, with half-siblings taking only half as much property of their intestate siblings' estates as siblings of full-blood. Unequal treatment of this type has been wholly abolished in England, but still exists in Florida.
Three-quarter siblings share one parent, while the unshared parents are first-degree relatives to each other, for example, if a man has children with two women who are sisters, or a woman has children with a man and his son. In the first case, the children are half-siblings as well as first cousins; in the second, the children are half-siblings as well as a half-avuncular pair. They are genetically closer than half-siblings but less genetically close than full-siblings, a degree of genetic relationship that is rare in humans and little-studied. One notable example of three-quarter siblings is the family of American aviator Charles Lindbergh, who fathered children with two German sisters, Brigitte and Marietta Hesshaimer.
Diblings, a portmanteau of donor sibling, or donor-conceived sibling, or donor-sperm sibling, are biologically connected through donated eggs or sperm. Diblings are biologically siblings though not legally for the purposes of family rights and inheritance. The anonymity of donation is seen to add complication to the process of courtship.

Non-blood relations

Related through affinity:
  • Stepsiblings are the children of one's stepparent from a previous relationship.
  • Adoptive siblings are raised by a person who is the adoptive parent of one and the adoptive or biological parent of the other.
  • Siblings-in-law are the siblings of one's spouse, the spouse of one's sibling, or the spouse of one's spouse's sibling. The spouse of one's spouse's sibling may also be called a co-sibling.
Not related:
  • siblings are children who are raised in the same foster home: foster children of one's parent, or the children or foster children of one's foster parent.
  • God siblings are the children of the godfather or godmother or the godchildren of the father or mother.
  • Milk siblings are children who have been nursed by the same woman. This relationship exists in cultures with milk kinship and in Islamic law.
  • Cross-siblings are individuals who share one or more half-siblings; if one person has at least one maternal half-sibling and at least one paternal half-sibling, the maternal and paternal half-siblings are cross-siblings to each other.
Siblings and half-siblings

Bryan and Betty are full siblings while Cyrus is their half brother; their relation percentage of consanguinity is 50%.

Siblings, half siblings, three-quarter siblings, and cross siblings

Erika and Efram are full siblings; to them, Abram, Aserna, and Agnia are their half-siblings, and Bein is their cross sibling.
Julia and Janine were born to one father and two full-sibling mothers, and are thus three-quarter siblings. Jaden is their cousin, while Jrake and Jakob are their half-cousins.
Jrake and Jaden were born to one mother and two half-sibling fathers, and are thus three-quarter siblings, however, their actual percentage of genetic relation is 31.25% instead of 37.5%.
Jrake and Jakob are unrelated to each other, although they are both half-cousins of Julia and Janine.

Consanguinity and genetics

is the measure of how closely people are related. Genetic relatedness measures how many genes a person shares. As all humans share over 99% of the same genes, consanguinity only matters for the small fraction of genes which vary between different people. Inheritance of genes has a random element to it, and these two concepts are different. Consanguinity decreases by half for every generation of reproductive separation through their most recent common ancestor. Siblings are 50% related by consanguinity as they are separated from each other by two generation, and they share two parents as common ancestors.
A fraternal twin is a sibling and, therefore, is related by 50% consanguinity. Fraternal twins are no more genetically similar than regular siblings. As identical twins come from the same zygote, their most recent common ancestor is each other. They’re genetically identical and 100% consanguineous as they’re separated by zero generations. Twin studies have been conducted by scientists to examine the roles that genetics and environment play in the development of various traits. Such studies examine how often identical twins possess the same behavioral trait and compare it to how often fraternal twins possess the same trait. In other studies twins are raised in separate families, and studies compare the passing on of a behavioral trait by the family environment and the possession of a common trait between identical twins. This kind of study has revealed that for personality traits which are known to be heritable, genetics play a substantial role throughout life and an even larger role during early years.
Half-siblings are 25% related by consanguinity as they share one parent and separated from each other by two generations.
A person may share more than the standard consanguinity with their sibling if their parents are closely biologically related. Interestingly, half-siblings can be related by as "three-quarters siblings" if their unshared parents have a consanguinity of 50%. This means the unshared parents are either siblings, making the half-siblings cousins, or parent and child, making them half- aunt-uncle and niece-nephew.

Percentage distribution

In practice, full siblings do not share exactly 50% of their DNA, as chromosomal crossover only occurs a limited number of times and, therefore, large chunks of a chromosome are shared or not shared at one time. In fact, the mean DNA fraction shared is 50.28% with a standard deviation of 3.68%, meaning approximately 1/4 of sibling pairs share more than 52.76% of their DNA, while 1/4 share less than 47.8%.
There is a very small chance that two half-siblings might not share any genes if they didn't inherit any of the same chromosomes from their shared parent. This is possible for full-siblings as well, though even more unlikely. But because of how homologous chromosomes swap genes during the development of an egg or sperm cell, however, the odds of this ever actually occurring are practically non-existent.

Birth order

Birth order is a person's rank by age among his or her siblings. Typically, researchers classify siblings as "eldest", "middle child", and "youngest" or simply distinguish between "first-born" and "later-born" children.
Birth order is commonly believed in pop psychology and popular culture to have a profound and lasting effect on psychological development and personality. For example, firstborns are seen as conservative and high-achieving, middle children as natural mediators, and youngest children as charming and outgoing. Despite its lasting presence in the public domain, studies have failed to consistently produce clear, valid, compelling findings; therefore, it has earned the title of a pseudo-psychology amongst the scientific psychological community.

History

The theorizing and study of birth order can be traced back to Francis Galton's theory of birth order and eminence and Alfred Adler's theory of birth order and personality characteristics.

Galton

In his book English Men of Science: Their Nature and Nurture, Galton noted that prominent composers and scientists are over-represented as first-borns. He theorized three main reasons as to why first-borns are generally more eminent:
  1. Primogeniture laws: first-borns have access to their parents' financial resources to continue their education.
  2. First-borns are given more responsibility than their younger siblings and are treated more as companions by their parents.
  3. First-borns are given more attention and nourishment in families with limited financial resources.