82% of people in Western countries have at least one sibling, and siblings generally spend more time together during childhood than they do with parents. 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.  According to child psychologist HO Sylvia Rimm, sibling rivalry is particularly intense when children are very close in age and of the same gender, or where one child is intellectually gifted.
Sibling rivalry is not unique to Western culture. For example, there is an Arabic saying: "I against my brother; my brother and I against my cousin; I, my brother, and my cousin against the stranger". Sibling rivalry can involve aggression; however, it is not the same as sibling abuse where one child victimizes another.
Throughout the lifespan
According to observational studies by Judy Dunn, children are sensitive from the age of one year to differences in parental treatment. From 18 months on siblings can understand family rules and know how to comfort and hurt each other. By three years old, children have a sophisticated grasp of social rules, can evaluate themselves in relation to their siblings, and know how to adapt to circumstances within the family. 
Sibling rivalry often continues throughout childhood and can be very frustrating and stressful to parents.  Adolescents fight for the same reasons younger children fight, but they are better equipped physically and intellectually to hurt and be hurt by each other. Physical and emotional changes cause pressures in the teenage years, as do changing relationships with parents and friends. Fighting with siblings as a way to get parental attention may increase in adolescence.  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. Events such as a parent’s illness may bring siblings closer together, whereas marriage may drive them apart, particularly if the in-law relationship is strained. Approximately one-third of adults describe their relationship with siblings as rivalrous or distant. However, rivalry often lessens over time. Sibling rivalry during youth will more often than not, result in siblings having a closer bond to each other in adulthood than siblings who experience no rivalry. At least 80 percent of siblings over age 60 enjoy close ties. 
According to Kyla Boyse from the University of Michigan, each child in a family competes to define who they are as individuals and want to show that they are separate from their siblings. Children may feel they are getting unequal amounts of their parents’ attention, discipline, and responsiveness. Children fight more in families where there is no understanding that fighting is not an acceptable way to resolve conflicts, and no alternative ways of handling such conflicts. Stress in the parents’ and children’s lives can create more conflict and increase sibling rivalry. 
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.  For example, in the case of Little Hans, Freud postulated that the young boy's fear of horses was related to jealousy of his baby sister, as well as the boy's desire to replace his father as his mother's mate.
David Levy introduced the term "sibling rivalry" in 1941, claiming that for an older sibling "the aggressive response to the new baby is so typical that it is safe to say it is a common feature of family life."
Evolutionary psychology view
Evolutionary psychologists such as Robert Trivers and biologists such as W. D. Hamilton explain sibling rivalry in terms of parental investment and kin selection. A parent shares 50% of her genes with each child, and is inclined to spread resources equally among all children in the family. However, a child shares 50% of his genes with a sibling but 100% with himself; so if the relationship follows Hamilton's rule, he should only share resources if the benefit to the sibling is greater than twice the benefit to himself (this is not a conscious calculation, but a genetic coding that unconsciously guides the behavior). So parents try to encourage their children to share, but often meet resistance. Children have motivation to feel both positively and negatively towards brothers and sisters, which may explain the mixed feelings that siblings sometimes have towards each other.
Parents can reduce the opportunity for rivalry by refusing to compare or typecast their children, teaching the children positive ways to get attention from each other and from the parent, planning fun family activities together, and making sure each child has enough time and space of their own.  They can also give each child individual attention, encourage teamwork, refuse to hold up one child as a role model for the others, and avoid favoritism. 
However, according to Sylvia Rimm, although sibling rivalry can be reduced it is unlikely to be entirely eliminated. In moderate doses, rivalry may be a healthy indication that each child is assertive enough to express his or her differences with other siblings. 
Sibling rivalry is common among various animal species, in the form of competition for food and parental attention. An extreme type of sibling rivalry occurs when young animals kill their siblings. For example, a black eagle mother lays two eggs, and the first-hatched chick pecks the younger one to death within the first few days. Among spotted hyenas, sibling competition begins as soon as the second pup is born, and 25% of pups are killed by their siblings.  (see: Siblicide)
Famous sibling rivalry instances
A number of Shakespeare's plays display incidences of sibling rivalry. King Lear provokes rivalry among his three daughters by asking them to describe their love for him; in the same play, Edmund contrives to force his half-brother Edgar into exile. In The Taming of the Shrew, sisters Kate and Bianca are shown fighting bitterly. In Richard III, the title character is at least partially motivated by rivalry with his brother, King Edward. In As You Like It, there is obvious sibling rivalry and antagonism between Orlando and Oliver, and also between Duke Frederick and Duke Senior.
In the Bible the story of Cain and Abel tells of one brother's jealousy after God appeared to favour his sibling, and the jealousy ultimately leads to murder. Jacob tricks his brother Esau out of his inheritance; sisters Leah and Rachel compete for the love of Jacob; while Joseph's brothers are so jealous that they effectively sell him into slavery.
In film and television
Sibling rivalry is a common theme in media that features child characters, reflecting the importance of this issue in early life. These issues can include jealousy on the birth of a new baby, different sibling roles such as troublemaker versus nerd (Bart and Lisa Simpson in The Simpsons), frequent arguments (Malcolm and Reese Wilkerson in Malcolm in the Middle; Doug and Judy Funnie in Doug) (Linus and Lucy van Pelt in Peanuts), competitiveness for mother's affection (Phil and Lil in Rugrats), and tensions between step-siblings (The Brady Bunch, Drake & Josh, Life with Derek), even in competition over territory (Ares and Athena from Disney Hercules the animated series)
Adult siblings can also be portrayed with a rivalrous relationship, often a continuation of childhood conflicts. Situation comedies exploit this to comic effect (for example, Ross and Monica Geller in Friends, Raymond, Robert Barone in Everybody Loves Raymond, Lizzie and Matt McGuire in Lizzie McGuire). Sibling relationships may be shown as alternately loving and argumentative (Rose and Maggie in In Her Shoes, Sam and Dean Winchester in Supernatural). Brothers or sisters in a similar line of work may display professional rivalry (Frasier and Niles Crane in Frasier). In serious drama, conflict between siblings can be fatal (Michael and Fredo Corleone in The Godfather).
Real-life siblings in the media
Occasionally real-life instances of sibling rivalry are publicized in the mass media. Siblings who play the same sport will often be compared with each other; for example, American football players Peyton and Eli Manning, or tennis players Venus and Serena Williams. Musicians Liam and Noel Gallagher of Oasis are portrayed as having a turbulent relationship, similar to that of Ray and Dave Davies of The Kinks.
Actresses Olivia de Havilland and Joan Fontaine had an uneasy relationship from childhood and in 1975 the sisters stopped speaking to each other completely.  The rivalry between singers Lata Mangeshkar and Asha Bhosle is often talked about in the Indian media, in spite of their insistence that these are just tales. Twin sisters and advice columnists Ann Landers and Abigail Van Buren had a relationship that was alternately very close and publicly antagonistic.  Journalists Christopher and Peter Hitchens have had many public disagreements and at least one protracted falling-out due to their differing political and religious views.  Singing siblings Michael and Janet Jackson, are often compared as well as the rest of the Jackson family. Australian pop star sisters Dannii Minogue and Kylie Minogue are often compared in the British and Australian press and public.
- Adult Sibling Rivalry Jane Mersky Leder, Psychology Today, Publication Date: Jan/Feb 93, Last Reviewed: 30 Aug 2004
- The Effects of Sibling Competition Syliva B. Rimm, Educational Assessment Service, 2002.
- Sibling Rivalry University of Michigan Health System, June 2009
- Living With Your Teenager: Dealing With Sibling Rivalry Donna Rae Jacobson, North Dakota State University, July 1995
- Sibling Rivalry in Degree and Dimensions Across the Lifespan Annie McNerney and Joy Usner, 30 April 2001.
- Freud Lecture: Juliet Mitchell, 2003
- The Hostile Act David M. Levy (1941) First published in Psychological Review, 48, 356-361.
- Center for Effective Parenting Arkansas State Parent Information & Resource Center
- Birth Order, Sibling Competition, and Human Behavior Frank J Sulloway
- Mothers and Others Sarah Blaffer Hrdy, Natural History Magazine, May 2001.
- Higham, Charles. Sisters: The Story of Olivia De Haviland and Joan Fontaine. Coward McCann, May 1984, 257 pages.
- Jitesh Pillaai (July 31, 2005). "Notes to Myself (An interview with Asha Bhosle)". Times Life, The Times of India, Mumbai (The Times Group): p. 43. http://epaperdaily.timesofindia.com/Repository/ml.asp?Ref=VE9JTS8yMDA1LzA3LzMxI0FyMDQzMDA=&Mode=HTML&Locale=english-skin-custom. Retrieved 2007-09-08.
- Ann Landers (1918-2002) by Robin Judd, Jewish Virtual Library. Accessed online June 21, 2007.
- James Macintyre, The Hitchens brothers: Anatomy of a row, The Independent, 2007-06-11, accessed 2007-06-11
- YourChild: Sibling Rivalry University of Michigan Health System
- YourChild: New Baby Sibling: Helping Your Older Child (or Children) Adjust University of Michigan Health System
- How to stop sibling rivalry by Virginia K. Molgaard
- Sibling rivalry: You vs. Them
- Living with your teenager: Dealing with Sibling rivalry
- Sibling Rivalry Raising Children Network
- Siblings in Conflict Film and Text (in German)