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.

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.

Alfred Adler saw siblings as "striving for significance" within the family and felt that birth order was an important aspect of personality development.

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."