Debra Lobato, Ph.D. is the Director of Child Psychology and Director of Integrated Care at Rhode Island Hospital and Hasbro Children’s Hospitals. She is the founder and Director of SibLink, a clinical and research program for siblings of children with medical, developmental, and behavioral disabilities. She is a Professor of Psychiatry and Human Behavior (Clinical) at Brown Medical School.

On April 15, 2013, two homemade pressure-cooker bombs exploded near the finish line of the Boston Marathon. Three people, including an 8-year-old boy, were killed. Hundreds of others were seriously injured. The city of Boston was paralyzed by a gripping manhunt for two suspects — two brothers. Days later, 26-year-old Tamerlan Tsarnaev was killed during a shoot-out with police following the death of an MIT police officer and carjacking. Tamerlan’s younger brother, 19-year-old Dzhokhar Tsarnaev, was injured and subsequently captured. Dzhokhar was charged with using and conspiring to use a weapon of mass destruction resulting in death and malicious destruction of property resulting in death. Dzhokhar now awaits trial and is facing the death penalty in Massachusetts. At the time of the manhunt, the brothers allegedly were plotting together to detonate more explosives in Times Square in New York City.

Meanwhile in Boston, after decades on the FBI’s “Ten Most Wanted List,” James “Whitey” Bulger Jr. was about to stand trial on 32 counts of racketeering, money laundering, extortion, weapons, and murder charges. In the summer of 2013, Whitey was found guilty on 31 of the charges and was sentenced to two consecutive life terms. During Whitey’s trial, his younger brother, William Bulger, was in active retirement after 18 years as the president of the Massachusetts Senate as well as president of the University of Massachusetts, a position he resigned in the wake of controversy surrounding his refusal to testify about his brother. In contrast to Whitey, William was an army veteran and lawyer whose life was characterized by education and public service.

What do these two divergent tales of brothers teach us about sibling relationships? How is it that one pair of siblings bonds together to walk the same course while other siblings seek opposite pursuits?

Characteristics of sibling relationships

Children in the United States are more likely to grow up with their siblings than with their fathers. Over the course of their lifetimes, most children will spend more time with their siblings than they spend with anyone else, including their parents. Despite this reality, research and interventions for children focus on the parent-child relationship as the primary source of influence on child outcomes; the effects of siblings on child behavior and health are often underestimated. However, recent research reveals what brothers and sisters have recognized all along — that siblings play a key role in child development and behavior.

The differential power and roles between siblings are related to their broader cultural context. Cultures define who is considered a sibling, the meaning and importance of the relationship, and the obligations siblings have within the family and to one another. These cultural proscriptions are often dependent on immutable structural characteristics of the sibling relationship, such as birth order, gender, and age spacing. Prince William and Prince Henry (“Harry”) of Great Britain’s royal family provide a vivid public example of how the structural features of a sibling relationship can be institutionalized and shape expectations and life trajectories.

Sibling relationships are not elective; children pick their friends but not their brothers and sisters. Emotions between siblings are characteristically intense and can cycle rapidly between love and hostility. During early childhood, siblings are often primary companions as well as competitors. This is believed to provide a fertile training ground for the development of social skills and future relationships. The impact of a warm and supportive sibling relationship or a conflictual, unsupportive one is lifelong. Warmth in the sibling relationship is associated with significant social and emotional advantages in later life. For example, compared to only children, children who have at least one sibling at home display greater social competence and peer acceptance from kindergarten through their early school years. Adolescents who report positive sibling relationships have better peer relationships and fewer depressive symptoms later in life. Warmth and closeness in the sibling relationship are also associated with greater ease and intimacy in romantic relationships. Finally, during middle age and old age, mood, health, loneliness, and depression are related to how people feel about their sibling relationships. Having a close, positive relationship with one’s siblings is not only a source of life satisfaction, but can also provide a buffer from stressful events, such as parental absence, marital conflict, and illness. Younger siblings tend to imitate and “look up to” their older siblings. If the older sibling responds in an attentive and caring manner, the seeds of similarity are planted. If the older sibling is rejecting of the younger child, their paths are more likely to separate.

Conflict and aggression between siblings is very common. It has been suggested that one way siblings learn to manage competition and conflict is by differentiating themselves, carving out different identities or roles within the family. Thus, one child becomes known as the reckless, defiant one, while the other sibling becomes the easygoing, conservative one. Even when siblings pursue very distinct identities and life courses, their bond can be very enduring. Think of the case of the Bulger brothers. One brother led a life of crime and the other a life of public service. William was not able or willing to overcome the sibling bond to testify about his contact with his brother, thereby forfeiting his position as president of UMass. Physical aggression occurs among siblings in the majority of families (i.e., 70% of families). Sibling conflict has been measured up to eight times in a single hour and is the number-one reason for discord between parents and children. However, parents will also dismiss levels of violence between siblings that they would never conceive of tolerating if they occurred outside the context of a sibling relationship. Violence between siblings is the most common form of child abuse and is significantly related to later substance use and delinquency.

Sibling – strong effects within the family

Sibling relationships do not occur in a vacuum. Siblings share parents, relatives, and school and other social environments, as well as their genes. Siblings raised in a home with authoritarian, harsh parents are more likely to be at odds with one another. How parents handle sibling conflict has a significant impact on the sibling relationship. When parents intercede in sibling conflict to determine which child is most at fault or “who started it,” the sibling relationship sours. Siblings often compare themselves to one another and compare how they are treated by others, particularly parents. When children perceive favoritism or unfair differential parental treatment, the quality of the parent-child and sibling relationships suffers. For the less favored sibling, differential parental treatment is associated with lower self-esteem, more depressive symptoms, and more antisocial and delinquent behavior, as well as more substance use.

One area of particularly strong sibling influence, even stronger than the influence of parents, is in the development of antisocial attitudes and conduct, and health risk behavior. The process of “sibling deviancy training” refers to the situation in which (generally older) siblings model, encourage, and reinforce antisocial behavior in their younger siblings. Younger siblings who shadow and hang out with their older siblings are introduced to their antisocial peers and behavior and begin to display negative behaviors more than siblings who do not hang out with their antisocial siblings. Eventually, the sibling relationship transitions from deviancy training to partners in crime; older and younger siblings begin to conspire together in more antisocial behavior and substance use. Even after controlling for the effects of parent substance use, siblings are four times more likely to smoke if their older sibling smokes, and are twice as likely to drink alcohol if an older sibling does. Finally, younger sisters are five times more likely to become pregnant if they have an older sister who was pregnant. The power of the antisocial or behaviorally risky older sibling is magnified when parents are unstable or absent. Think back to the Tsarnaev brothers, whose parents, after years of struggling in the United States, returned to Russia, leaving the younger brother under the influence of his elder, radicalizing brother.

Conclusions and implications

Siblings hold the power to inspire as well as corrupt. Given the intensity and longevity of their bond, how can we engage this power for positive change? It is time to move beyond family interventions that are directed solely at parents (i.e., mothers). When working with children and adolescents, inquire about siblings — their relationships, strengths, and challenges. How much time do they spend together? How are they similar and where do their interests overlap? Be attentive and responsive to reports of sibling violence; do not dismiss these as “normal” and, therefore, acceptable. Consider how recommendations regarding one child might affect other children in the family. Is the heightened attention expected from parent-directed intervention likely to plant negative feelings of differential parental treatment among the other children? Actively help families understand that the investment made early in the sibling relationship has value that lasts a lifetime.