A sibling is one of two or more individuals having one or both parents in common. A full sibling is a first-degree relative. A male sibling is a brother, and a female sibling is a sister. 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.^[1]

Identical twins share 100% of their DNA.^[2] Full siblings are first-degree relatives and, on average, share 50% of their genes out of those that vary among humans, assuming that the parents share none of those genes.^[2] Half-siblings are genetically second-degree relatives and have, on average, a 25% overlap in their human genetic variation.^[3]

Full siblings (full brothers or full sisters; or brother and sister) have the same biological parents and are 50% related (full siblings share 50% of their genes out of those that vary among humans).^[2][3] Identical twins by definition are 100% related.^[2] Full siblings are also the most common type of siblings.

There are two types of twins: identical and fraternal. Identical twins have exactly the same genes; fraternal twins are no more similar than regular siblings. Often, twins with a close relationship will develop a twin language from infanthood, a language only shared and understood between the two. Studies suggest that identical twins appear to display more twin talk than fraternal twins. At about 3 years of age, twin talk usually ends.^[4]

Researchers were interested in subjects who were in the later years of life.

They knew that past studies suggested that genetics played a larger role in one's personality in the earlier years of their life.

However, they were curious about whether or not this was true later on in life.

They gathered subjects with a mean age of 59, who included 99 pairs of identical twins, and 229 pairs of fraternal twins who were all reared apart.

They also gathered twins who were reared together: 160 pairs of identical twins, and 212 pairs of fraternal twins.

They studied the most heritable traits in regard to personality, which are emotionality, activity level and sociability; also known as EAS.

This study found that identical twins resembled each other twice as much as fraternal twins, due to genetic factors.

Furthermore, environment influences personality substantially; however, it has little to do with whether they are reared together or apart.

This study also suggests that heritability is substantial, but not as substantial as for younger subjects; it has less significance later on in life.^[5]

Half-siblings are people who share one parent. They may share the same mother but different fathers (in which case they are known as uterine siblings or maternal half-brothers/half-sisters), or they may have the same father but different mothers (in which case, they are known as agnate siblings or paternal half-brothers/half-sisters. In law, the term consanguine is used in place of agnate). They share only one parent instead of two as full siblings do and are on average 25% related.^[3]

Theoretically, there is a chance that they might not share genes.

This is very rare and is due to there being a smaller possibility of inheriting the same chromosomes from the shared parent.

However, the same is also theoretically possible for full siblings, albeit (comparatively) much less likely.

Because of the formation of Chiasma in late prophase II (cross-over events), both previous statements are generally impossible. ^[3]

In law (and especially inheritance law), half-siblings have often been accorded treatment unequal to that of full siblings. 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 siblings of full-blood. Unequal treatment of this type has been wholly abolished in England,^[6] but still exists in the U.S. state of Florida.^[7]

Three-quarter siblings have one common parent, while their unshared parents have a mean consanguinity of 50%. This means the unshared parents are either siblings or parent and child (similar terminology is used in horse breeding, where it occurs more frequently). Three-quarter siblings are likely to share more genes than half siblings, but fewer than full siblings.

In this case the unshared parents are full siblings.

Furthermore, the three-quarter siblings are also first cousins.

In the case where the unshared parents are identical twins, the children share as much genetic material as full siblings do.

Real:

Fictional:

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.

Furthermore, the two offspring will have an aunt/uncle-nephew/niece relation.

An historical example of this is actress Gloria Grahame. She bore children with her second husband Nicholas Ray, and her fourth husband Anthony Ray, who was Nicholas Ray's son by another marriage. ^[12]

Fictional:

A sibling who shares a parent and the unshared parents are related, not siblings though as this would be a Three-quarter sibling

"Stepsiblings" (stepbrothers or stepsisters) are the children of one's stepparent from a previous relationship. They are unrelated by blood.

"Foster siblings" are children who are raised in the same foster home, foster children of the person's parents, or foster parents' biological children.

Two "adoptive siblings" are raised by a person who is the adoptive parent of one and the adoptive or biological parent of the other. Adoptive siblings are legally related but need not be blood-related or biologically related.

One's sibling-in-law is the sibling of one's spouse or the spouse of one's sibling.

They are the children of the godfather or godmother or the godchildren of the father or mother.

They are unrelated by blood.

In cultures with milk kinship, a milk sibling is a person who is not one's biological sibling but was nursed by the same woman as oneself. The concept exists in Islamic law and Jewish law.

Cross-siblings are individuals who share one or more half-siblings; if one person has at least one maternal half-sibling and at least one paternal half-sibling, the maternal and paternal half-siblings are cross-siblings to each other.

Stepsiblings are not cross-siblings unless their married parents have a child together.

Alternatively, cross-siblings may not be stepsiblings at all, in the event that the respective parents have a child without marrying, or the cross-siblings are born after the parents of the mutual half-sibling have separated.

Cross-siblings are not biologically related, unless the parents have a biological relationship irrespective of their children's cross-sibling status.^[13]

A pseudosibling is someone that is not a sibling but has a relationship like a sibling, or that one considers to be like a sibling.

Pseudosiblings are often not related by blood (consanguinity) and are typically very close friends.

An ex-stepsibling is someone who was once one's stepsibling but no longer is.

Ex-stepsiblings are neither related by blood nor by law, so they fall under the category of "pseudosiblings".

Another definition of an ex-stepsibling is "someone who was previously the stepchild of one's parent".

This definition is broader because it includes people who were never stepsiblings.

In this scenario, the two children are ex-stepsiblings by definition, even though they were never stepsiblings.

The term "ex-stepsibling" is less commonly used in situations like these, but is still accurate nonetheless.

Diblings, a portmanteau of donor sibling, or donor-conceived sibling, or donor-sperm sibling, are biologically connected through donated eggs or sperm.^[14][15]

A blood sibling is someone you have a very strong connection with and have done the blood brother ritual with, where you both cut yourselves and intermingle your blood.

A spirit sibling is a sort of extreme pseudosibling, in that one person has asked the other to be their sibling, and in some case sign a document stating this, (documents are not legal and only exist as a sort of symbolism).

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. Despite its lasting presence in the public domain, studies have failed to consistently produce clear, valid, and compelling findings. Therefore, it has honed the title of a pseudo-psychology amongst the scientific psychological community.^[16]

The theorizing and study of birth order can be traced back to Francis Galton's (1822–1911) theory of birth order and eminence and Alfred Adler's (1870–1937) theory of birth order and personality characteristics.

In his book English Men of Science: Their Nature and Nurture (1874), Galton noted that prominent composers and scientists are over-represented as first-borns.^[17] He theorized three main reasons as to why first-borns are generally more eminent:

Today, the flaws and inconsistencies in birth order research eliminate its validity.

It is very difficult to control solely for factors related to birth order, and therefore most studies produce ambiguous results.^[18] Embedded into theories of birth order is a debate of nature versus nurture.

It has been disproved that there is something innate in the position one is born into, and therefore creating a preset role.

Birth order has no genetic basis.^[20]

The social interaction that occurs as a result of birth order however is the most notable.

Older siblings often become role models of behaviour, and younger siblings become learners and supervisees.

Older siblings are at a developmental advantage both cognitively and socially.

The role of birth order also depends greatly and varies greatly on family context.

Family size, sibling identification, age gap, modeling, parenting techniques, gender, class, race, and temperament are all confounding variables that can influence behaviour and therefore perceived behaviour of specific birth categories.^[21] The research on birth order does have stronger correlations, however, in areas such as intelligence and physical features, but are likely caused by other factors other than the actual position of birth.

Some research has found that firstborn children have slightly higher IQs on average than later born children.^[22] However, other research finds no such effect.^[23] It has been found that first-borns score three points higher compared to second borns and that children born earlier in a family are on average, taller and weigh more than those born later.^[16] However, it is impossible to generalize birth order characteristics and apply them universally to all individuals in that subgroup.

(Blake, 1981) provide three potential reasons for the higher scoring of older siblings on IQ tests:^[17]

Robert Zajonc proposed that the intellectual environment within a family is ever-changing due to three factors, and therefore more permissive of first-born children's intellectual advancement:^[17]

In 1996, interest in the science behind birth order was re-sparked when Frank Sulloway’s book Born To Rebel was published. In this book, Sulloway argues that firstborns are more conscientious, more socially dominant, less agreeable, and less open to new ideas compared to later-borns. While being seemingly empirical and academic, as many studies are cited throughout the book, it is still often criticized as a biased and incomplete account of the whole picture of siblings and birth order. Because it is a novel, the research and theories proposed throughout were not criticized and peer-reviewed by other academics before its release.^[24] Literature reviews that have examined many studies and attempted to control for confounding variables tend to find minimal effects for birth order on personality.^[25][26] 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.^[27]

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 first-born children to come from poorer families.

Spacing of children, parenting style, and gender are additional variables to consider.

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.^[28] 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.^[29] 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.^[1] Sibling rivalry often continues throughout childhood and can be very frustrating and stressful to parents.^[30] One study found that the age group 10–15 reported the highest level of competition between siblings.^[31] 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.^[1]

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.^[30] 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.^[32] 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.^[31]

Jealousy is not a single emotion. The basic emotions expressed in jealous interactions are fear, anger, relief, sadness, and anxiety.^[33] Jealousy occurs in a social triangle of relationships which do not require a third person. The social triangle involves the relationships between the jealous individual and the parent, the relationship between the parent and the rival, and the relationship between jealous individual and the rival.^[33]

First-borns' attachment to their parents is directly related to their jealous behaviour.

In a study by Volling, four classes of children were identified based on their different responses of jealousy to new infant siblings and parent interactions.

Regulated Exploration Children: 60% of children fall into this category.^[33] These children closely watch their parents interact with their newborn sibling, approach them positively and sometimes join the interaction.^[33] They show fewer behaviour problems in the months following the new birth and do not display problematic behaviours during the parent-infant interaction.^[33] These children are considered secure as they act how a child would be expected to act in a familiar home setting with their parents present as secure bases to explore the environment.^[33] Approach-Avoidant Children: 30% of children fall into this category.^[33] These children observe parent-infant interaction closely and are less likely to approach the infant and the parent. They are anxious to explore the new environment as they tend to seek little comfort from their parents.^[33] Anxious-Clingy Children: 6% of children fell into this category. These children have an intense interest in parent-infant interaction and a strong desire to seek proximity and contact with the parent, and sometimes intrude on parent-child interaction.^[33] Disruptive Children: 2.7% of children fall into this category.^[33] These children are emotionally reactive and aggressive. They have difficulty regulating their negative emotions and may be likely to externalize it as negative behaviour around the newborn.^[33]

Children are more jealous of the interactions between newborns and their mothers than they are with newborns and their fathers.^[33] This is logical as up until the birth of the infant, the first-born child had the mother as his or her primary care-giver all to his or herself.

Some research has suggested that children display less jealous reactions over father-newborn interactions because fathers tend to punish negative emotion and are less tolerant than mothers of clinginess and visible distress, although this is hard to generalize.^[33]

Children that have parents with a better marital relationship are better at regulating their jealous emotions.^[33] Children are more likely to express jealousy when their parents are directing their attention to the sibling as opposed to when the parents are solely interacting with them.^[33] Parents who are involved in good marital communication help their children cope adaptively with jealousy. They do this by modelling problem-solving and conflict resolution for their children. Children are also less likely to have jealous feelings when they live in a home in which everyone in the family shares and expresses love and happiness.^[33]

Implicit theories about relationships are associated with the ways children think of strategies to deal with a new situation.

Children can fall into two categories of implicit theorizing.

They may be malleable theorists and believe that they can affect change on situations and people.

Alternatively, they may be fixed theorists, believing situations and people are not changeable.^[34] These implicit beliefs determine both the intensity of their jealous feelings, and how long those jealous feelings last.^[34] Malleable Theorists display engaging behaviours, like interacting with the parent or sibling in an attempt to improve the situation.^[34] They tend to have more intense and longer-lasting feelings of jealousy because they spend more time ruminating on the situation and constructing ways to make it better.^[34] Fixed Theorists display non-engaging behaviours, for example retreating to their room because they believe none of their actions will affect or improve the situation.^[34] They tend to have less intense and shorter lasting feelings of jealousy than malleable theorists.^[34]

Older children tend to be less jealous than their younger sibling.^[33] This is due to their ability to mentally process the social situation in a way that gives them more positive, empathetic feelings toward their younger sibling.^[33] Older children are better able to cope with their jealous feelings toward their younger sibling due to their understanding of the necessary relationship between the parent and younger sibling.^[33] Older children are also better at self-regulating their emotions and are less dependent on their caregivers for external regulation as opposed to their younger siblings.^[33] Younger siblings' feelings of jealousy are overpowered by feelings of anger.^[33] The quality of the relationship between the younger child and the older child is also a factor in jealousy, as the better the relationship the less jealous feelings occurred and vice versa.^[33]

Sibling conflict is pervasive, and often shrugged off as an accepted part of sibling dynamics.

In spite of the broad variety of conflict that siblings are often involved in, sibling conflicts can be grouped into two broader categories.^[35] The first category is conflict about equality or fairness.

It is not uncommon to see siblings who think that their sibling is favored by their teachers, peers, or especially their parents.

In fact it is not uncommon to see siblings who both think that their parents favor the other sibling.

Perceived inequalities in the division of resources such as who got a larger dessert also fall into this category of conflict.

This form of conflict seems to be more prevalent in the younger sibling.^[35]

The second category of conflict involves an invasion of a child’s perceived personal domain by their sibling.

An example of this type of conflict is when a child enters their sibling’s room when they are not welcome, or when a child crosses over into their sibling’s side of the car in a long road trip.

These types of fights seem to be more important to older siblings due to their larger desire for independence.^[35]

Sibling warmth is a term for the degree of affection and companionship shared by siblings.

Sibling warmth seems to have an effect on siblings.

Higher sibling warmth is related to better social skill and higher perceived social competence. Even in cases where there is a high level of sibling conflict if there is also a high level of sibling warmth then social skills and competence remain unaffected.^[36]

The saying that people "fight like siblings" shows just how charged sibling conflict can be and how well recognized sibling squabbles are.

In spite of how widely acknowledged these squabbles can be, sibling conflict can have several impacts on the sibling pair.

It has been shown that increased levels of sibling conflict are related to higher levels of anxiety and depression in siblings, along with lower levels of self-worth and lower levels of academic competence.

In addition, sibling warmth is not a protective factor for the negative effects of anxiety, depression, lack of self-worth and lower levels of academic competence. This means that sibling warmth does not counteract these negative effects.^[36] Sibling conflict is also linked to an increase in more risky behavior including: smoking cigarettes, skipping days of school, contact with the police, and other behaviors in Caucasian sibling pairs with the exception of firstborns with younger brothers. Except for the elder brother in this pair sibling conflict is positively correlated with risky behavior, thus sibling conflict may be a risk factor for behavioral problems.^[37] A study on what the topic of the fight was (invasion of personal domain or inequality) also shows that the topic of the fight may have a result on the effects of the conflict. This study showed that sibling conflict over personal domain were related to lower levels of self-esteem, and sibling conflict over perceived inequalities seem to be more related to depressive symptoms. However, the study also showed that greater depressive and anxious symptoms were also related to more frequent sibling conflict and more intense sibling conflict.^[35]

Techniques used by parents to manage their children’s conflicts include parental non-intervention, child-centered parental intervention strategies, and more rarely the encouragement of physical conflict between siblings.

Parental non-intervention included techniques in which the parent ignores the siblings' conflict and lets them work it out between themselves without outside guidance.

In some cases this technique is chosen to avoid situations in which the parent decides which sibling is in the right and may favor one sibling over the other, however, by following this technique the parent may sacrifice the opportunity to instruct their children on how to deal with conflict.

Child-centered parental interventions include techniques in which the parent mediates the argument between the two children and helps them come to an agreement.

Using this technique, parents may help model how the children can deal with conflicts in the future; however, parents should avoid dictating the outcome to the children, and make sure that they are mediating the argument making suggestions, allowing the children to decide the outcome.

Techniques in which parents encourage physical aggression between siblings may be chosen by the parents to help children deal with aggression in the future, however, this technique does not appear to be effective as it is linked to greater conflict levels between children.

Parental non-intervention is also linked to higher levels of sibling conflict, and lower levels of sibling warmth.

It appears that child-centered parental interventions have the best effect on sibling's relationship with a link to greater levels of sibling warmth and lower levels of sibling conflict.^[38]

Studies on social skill and personality differences between only children and children with siblings suggest that overall the presence of a sibling does not have any effect on the child as an adult.^[39]

There have always been some differences between siblings, especially different sex siblings.

Often, different sex sibling may consider things to be unfair because their brother or sister is allowed to do certain things because of their gender, while they get to do something less fun or just different.

McHale and her colleague conducted a longitudinal study using middle-childhood aged children and observed the way in which the parents contributed to stereotypical attitudes in their kids.^[40] In their study the experimenters analysed two different types of families, one with the same sex siblings, and the other with different sex siblings, as well as the children’s birth order.^[41] The experiment was conducted using phone interviews, in which the experimenters would ask the children about the activities they performed throughout their day outside of school.^[42] Surprisingly, the experimenters found that in the homes where there were mixed gender kids, and the father held traditional values, the kids also held traditional values and therefore also played gender based roles in the home.^[42] In contrast, in homes where the father did not hold traditional values, the house chores were divided more equally among his kids.^[42] However, if fathers had two male children, the younger male tended to help more with household chores, but as he reached his teenage years the younger child stopped being as helpful around the house.^[41] There are two important factors that need to be taken into account from this study. First, in cases where the father figure had more traditional values it was found that he may have also had less education than the other dads who participated in the study. Secondly, the mother’s attitudes did not have a noticeable impact on her children’s gender role values.^[41] Altogether this experiment is a good example of the way in which environment and kinship help develop certain perspectives on gender role association in children.

In a similar study, Croft and colleagues observed the mother and father's gender roles and examined whether their attitudes would have a long-term effect on the future occupation of their children.^[47] In this study mothers and fathers were asked a series of questions regarding their work hours and their chores at home, including who looked after the children more often.^[47] The study demonstrated that mothers felt like they were performing more household duties and they tended to look more after the children.^[47] Something that is quite remarkable in this experiment is the way in which the kids perceive their parents' gender roles. When the kids were asked which parent they would be like when they grew up, some kids did not associate themselves with either male or female occupations, but rather remained neutral, while others leaned towards being more like their same gender parent. However, similar to the study conducted by McHale and her colleagues, this often depended on the father’s traditional values.^[47][51] It was also found that girls who observed their parents playing out gendered roles envisioned themselves playing a role similar to their mothers', while daughters who lived in a home where parents did not have sex oriented tasks viewed themselves as working women and family oriented females in the future.^[47] Altogether Croft and her team agreed that in order to create more equality regarding work occupation, the house work also needs to be divided equally.^[47]

Anthropologist Edvard Westermarck found that children who are brought up together as siblings are desensitized to 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.^[54][55]