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A sibling is one of two or more individuals having one or both parents in common. A female sibling is a sister and a male sibling is a brother. In most societies throughout the world, siblings often grow up together, thereby facilitating the development of strong emotional bonds. 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.
Identical twins share 100% of their DNA. Full siblings are first-degree relatives and, on average, share 50% of their genes out of those that vary among humans. Half-siblings are second-degree relatives and have, on average, a 25% overlap in their human genetic variation.
Full siblings (full brothers or full sisters) share the same biological parents and are 50% related (full siblings share 50% of their genes out of those that vary among humans). Identical twins by definition are 100% related.
Half-siblings only share one parent instead of two as full siblings do and are on average 25% related, i.e. the children that the parent and stepparent (i.e. mother and stepfather, or father and stepmother) have together. Theoretically, there is a chance that they might not be related at all though this is very rare and is due to there being a smaller possibility of inheriting the same chromosomes from the shared parent. Half-siblings can have a wide variety of interpersonal relationships, from a bond as close as any full siblings, to total strangers. Some consider their paternal siblings their half-sibling or sibling on their father's side and some consider their maternal sibling still just their full sibling or sibling because they give more weight to a shared mother and womb, and because children are usually raised by and/or have more interaction with their mother.
There are specific terms for referring to half-siblings based on the sex of the shared parent:
In law (and especially inheritance law), half-siblings were often accorded unequal treatment. 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 other siblings of full-blood. Unequal treatment of this type has been wholly abolished in England and throughout the United States.
Three-quarter siblings have one common parent, while their unshared parents have a mean consanguanuity of 50%. This includes full siblings and parent/child. (Similar terminology is used in horse breeding, where it occurs more frequently). Three-quarter siblings share more genes than half siblings, but fewer than full siblings. There are two genetic scenarios for 3/4 siblings:
In this case the unshared parents are full siblings. Furthermore, the three-quarter siblings are also first cousins. An example of this is that of Charles Lindbergh's children with his mistress Brigitte Hesshaimer, and his children with her sister, Marietta Hesshaimer. Another recent example relates to Jermaine and Randy Jackson, of the Jackson 5, who have both fathered children with Alejandra Genevieve Oaziaza.
In the case where the unshared parents are identical twins, the children share as much genetic material as full siblings do.
In this case a woman has children with two men who are father and son, or a man has children with two women who are mother and daughter. These children will be three-quarter siblings.
"Stepsiblings" (stepbrothers or stepsisters) are the children of one's stepparent from a previous relationship. They are not, however, related by blood.
"Milk siblings" are children breastfed by either a woman who is the mother of one of the two babies, or by someone other than their biological mother, the latter a practice known as wetnursing and once widespread in the developed world, as it still is in parts of the developing world.
In Islam those who are fed in this way become siblings to the biological children of their wetnurse, provided that they are less than 2 years old. Islamic law (shariah) codifies the relationship between these people, and certain specified relatives, as rada'a; given that a child is breastfed five fulfilling (satisfactory to him) times, once they are adult, they are mahram, meaning that they are not allowed to marry each other, and the rules of modesty known as purdah are relaxed, as with other family members. But, laws of inheritance do not apply in the case of milk siblings.
"Foster siblings" are children who are raised in the same foster home, foster children of the person's parents, or foster parents' biological children.
"Adoptive siblings" are when two children are legally related, but are not related by blood. Adopted siblings are not biologically related but may consider each other siblings because they act like they are.
"Cross siblings" are two unrelated people who share one or more half-siblings (not to be confused with stepsiblings, as described earlier). For example, in The Young and the Restless, Michael is the maternal half sibling of Kevin, and the paternal half sibling of Eden. In this case, Kevin and Eden are cross siblings - they share no blood. In Desperate Housewives, M.J. Delfino and Julie Mayer share a mother, and Julie and Evan Mayer share a father, so M.J. and Evan are cross siblings.
"Sibling cousins" are those who have the same mother with their fathers being brothers or cousins, or who share the same father with their mothers being sisters or cousins. This is a broader category than, but inclusive of, the horizontal 3/4 sibling above.
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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 "firstborn" 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. In his book Born to Rebel, Frank Sulloway argues that firstborns are more conscientious, more socially dominant, less agreeable, and less open to new ideas compared to laterborns. Literature reviews that have examined many studies and attempted to control for confounding variables tend to find minimal effects for birth order on personality. In her review of the scientific literature, Judith Rich Harris suggests that birth order effects may exist within the context of the family of origin, but that they are not enduring aspects of personality.
In practice, systematic birth order research is a challenge because it is difficult to control for all of the variables that are statistically related to birth order. For example, large families are generally lower in socioeconomic status than small families, so third born children are more likely than firstborn children to come from poorer families. Spacing of children, parenting style, and gender are additional variables to consider.
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The arrival of a new baby is especially stressful for firstborns and for siblings between 3 and 5 years old. Regressive behavior and aggressive behavior, such as handling the baby roughly, can also occur. All of these symptoms are considered to be typical and developmentally appropriate for children between the ages of 3–5. While some can be prevented, the remainder can be improved within a few months. Regressive behavior may include demand for a bottle, thumb sucking, requests to wear diapers (even if toilet-trained), or requests to carry a security blanket.
Regressive behaviors are the child's way of demanding the parents' love and attention.
The American Academy of Pediatrics suggests that instead of protesting or telling children to act their age, parents should simply grant their requests without becoming upset. The affected children will soon return to their normal routine when they realize that they now have just as important a place in the family as the new sibling. Most of the behaviors can be improved within a few months.
The University of Michigan Health System advises that most occurrences of regressive behavior are mild and to be expected; however, it recommends parents to contact a pediatrician or child psychologist if the older child tries to hurt the baby, if regressive behavior does not improve within 2 or 3 months, or if the parents have other questions or concerns.
"Sibling rivalry" is a type of competition or animosity among brothers and sisters. It appears to be particularly intense when children are very close in age or of the same gender. Sibling rivalry can involve aggression; however, it is not the same as sibling abuse where one child victimizes another.
Sibling rivalry usually starts right after, or before, the arrival of the second child. While siblings will still love each other, it is not uncommon for them to bicker and be malicious to each other. Children are sensitive from the age of 1 year to differences in parental treatment and by 3 years they have a sophisticated grasp of family rules and can evaluate themselves in relation to their siblings. Sibling rivalry often continues throughout childhood and can be very frustrating and stressful to parents. One study found that the age group 10–15 reported the highest level of competition between siblings. Sibling rivalry can continue into adulthood and sibling relationships can change dramatically over the years. Approximately one-third of adults describe their relationship with siblings as rivalrous or distant. However, rivalry often lessens over time and at least 80% of siblings over age 60 enjoy close ties.
Each child in a family competes to define who they are as persons and want to show that they are separate from their siblings. Sibling rivalry increases when children feel they are getting unequal amounts of their parents' attention, where there is stress in the parents' and children's lives, and where fighting is accepted by the family as a way to resolve conflicts. Sigmund Freud saw the sibling relationship as an extension of the Oedipus complex, where brothers were in competition for their mother's attention and sisters for their father's. Evolutionary psychologists explain sibling rivalry in terms of parental investment and kin selection: a parent is inclined to spread resources equally among all children in the family, but a child wants most of the resources for him or herself.
Anthropologist Edvard Westermarck found that children who are brought up together as siblings are desensitized to form sexual attraction to one another later in life. This is known as the Westermarck Effect. It can be seen in biological and adoptive families, but also in other situations where children are brought up in close contact, such as the Israeli kibbutz system and the Chinese Shim-pua marriage.
The opposite phenomenon, when relatives do fall in love, is known as genetic sexual attraction. This can occur between siblings brought up apart from each other, for example, adoptees who are re-united in adulthood.
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