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One's sibling is one's brother or sister, respectively meaning a male or female with whom one shares at least one parent. This is usually taken to mean that the two people are genetically very close, though it is not always necessarily the case, for example one or more siblings may have been adopted by their parents.
In most societies throughout the world, siblings will usually grow up together and spend a good deal of their time during childhood together. They may have conflicts during their childhood years, but usually resolve them later in life. This closeness may be marked with the development of strong emotional associations such as love and enmity. The sibling bond is often complicated and is influenced by factors such as parental treatment, birth order, personality, and people and experiences outside the family. 
Types of siblings
A full sibling (full brother or full sister), is a sibling that shares both biological or adoptive parents. In law, the terms brother German and sister German are used[How to reference and link to summary or text].
A half sibling (half brother or half sister) is a sibling with one shared biological or adoptive parent. A half sibling that shares the same mother (but different fathers) is known as a uterine sibling, whereas one that shares the same father is known as an agnate sibling. In law, the term consanguine is used in place of agnate. Half siblings can have a wide variety of interpersonal relationships, from a bond as close as any full siblings, to total strangers. For half siblings in twins, see semi-identical twin.
At 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 almost wholly abolished in England and throughout much of the United States.
A stepsibling (stepbrother or stepsister) is a sibling with whom an individual bears no blood or equivalent adoptive relation, and is related by the marriage or relationship of one parent of the individual to one parent of the sibling.
- See also: stepfamily
Siblings through breast feeding
Milk brothers or milk sisters are children breastfed by a woman other than their biological mother, a practice known as wetnursing and once widespread.
In Islam those who are fed in this way become siblings to the biological children of their wetnurse, provided that they are less than two years old and have been breastfed five times or more by her. According to shariah (Islamic law) these siblings are mahram, meaning that they are not allowed to marry each other.
- Main article: 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 “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 to be 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.
Regressive behavior at the birth of a new sibling
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. Parents can deal with these behaviors by explaining to the older child their new sibling role, making this role sound exciting, answering questions about the baby and the process of birth (as appropriate), or reserving time each day just for the parent and older child.
The AAP (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. For the younger child if the older child hurts them its worse then the other way around. There are a few exceptions such as the older child is a female while the younger is a male.
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 and 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 one year to differences in parental treatment and by three 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 to 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 percent of siblings over age 60 enjoy close ties. 
According to researchers, each child in a family competes to define who they are as individuals 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 fathers.  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. 
Westermarck effect and its opposite
Anthropologist Edvard Westermarck found that children who are brought up together as siblings are desensitized to form sexual attraction 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.
- Mersky Leder, Jane. Adult Sibling Rivalry. Psychology Today, Jan/Feb 1993. URL accessed on November 28, 2006.
- Ernst, C. & Angst, J. (1983). Birth order: Its influence on personality. Springer.
- Jefferson, T., Herbst, J. H., & McCrae, R. R. (1998). Associations between birth order and personality traits: Evidence from self-reports and observer ratings. Journal of Research in Personality, 32, 498-509.
- Harris, J. R. (1998). The nurture assumption: Why children turn out the way they do. New York: Free Press.
- Carey, Benedict. Family dynamics, not biology, behind higher IQ. International Herald Tribune, June 21, 2007. URL accessed on July 15, 2007.
- Rodgers, J. L., Cleveland, H. H., van den Oord, E. and Rowe, D. (2000). Resolving the Debate Over Birth Order, Family Size and Intelligence. American Psychologist, Vol. 55.
- The Effects of Sibling Competition Syliva B. Rimm, Educational Assessment Service, 2002.
- New Baby Sibling University of Michigan Health System, June 2006
- Sibling Rivalry University of Michigan Health System, October 2006
- Sibling Rivalry in Degree and Dimensions Across the Lifespan Annie McNerney and Joy Usner, 30 April 2001.
- Freud Lecture: Juliet Mitchell, 2003
- Westermarck, E. A. (1921). The history of human marriage, 5th edn. London: Macmillan, 1921.
- Arthur P. Wolf. Childhood Association and Sexual Attraction: A Further Test of the Westermarck Hypothesis. American Anthropologist, New Series, Vol. 72, No. 3 (Jun., 1970), pp. 503-515. URL accessed on November 29, 2006.
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