# Sibling

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A sibling is a gender neutral word for a relative that shares at least one parent with the subject. 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. Sometimes, they may grow up apart in foster care. 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]

## Contents

Medically a full sibling is a first-degree relative and a half sibling is a second-degree relative as they are related by 50% and 25% respectively [2] while legally both are classifies as second-degree relatives. [3] Siblings are in conflict of interest, with regard to inheritance.

## Definitions

Siblings and half siblings
 Adam Agatha Anthony Bryan Betty Cyrus
Bryan and Betty are full siblings while Cyrus is their half brother

Siblings or full siblings ([full] sisters or brothers) share the same biological parents. Full siblings are also the most common type of siblings.[ citation needed ] Twins are siblings that are born at the same time.[ citation needed ] 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] Twins generally share a greater bond due to growing up together and being the same age.

Half-siblings (half-sisters or half-brothers) 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-siblings), or they may have the same father but different mothers (in which case, they are known as agnate siblings or paternal half-siblings. In law, the term consanguine is used in place of agnate).[ citation needed ] 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, [5] but still exists in the U.S. state of Florida. [6]

Diblings, a portmanteau of donor sibling, or donor-conceived sibling, or donor-sperm sibling, are biologically connected through donated eggs or sperm. [7] [8] Diblings are biologically siblings though not legally for the purposes of family rights and inheritance. The anonymity of donation is seen to add complication to the process of courtship.

### Non blood relations

Related through affinity

• Stepsibling (stepbrothers or stepsisters) are the children of one's stepparent from a previous relationship.
• Adoptive siblings are raised by a person who is the adoptive parent of one and the adoptive or biological parent of the other.
• Sibling-in-laws are the siblings of one's spouse or the spouse of one's sibling.

Not related

• Foster siblings are children who are raised in the same foster home, foster children of the person's parents, or foster parents' biological children.[ citation needed ]
• God siblings are the children of the godfather or godmother or the godchildren of the father or mother.[ citation needed ]
• Milk siblings are children who have been nursed by the same woman. This relationship exists in cultures with milk kinship and in Islamic law.[ citation needed ]
• 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. [9] [ dubious ]

## Consanguinity and genetics

Consanguinity is the measure of how closely people are related.[ citation needed ] Genetic relatedness measures how many genes a person shares. As humans share many of the same genes [10] , people can be related in more than one way, and inheritance of genes has a random element to it, [10] these two concepts are different. [11] Consanguinity decreases by half for every generation of reproductive separation through their most recent common ancestor. Siblings are 50% related by consanguinity as they are separated from each other by two generation (sibling to parent to sibling) and they share two parents as common ancestors (${\displaystyle \left({\tfrac {1}{2}}\right)^{2}+\left({\tfrac {1}{2}}\right)^{2}}$).

A fraternal twin is a sibling and therefore is related by 50% consanguinity.[ citation needed ] Fraternal twins are no more genetically similar than regular siblings. As identical twins come from the same zygote, their most recent common ancestor is each other. They are genetically identical and 100% consanguineous as they are separated by zero generations (${\displaystyle \left({\tfrac {1}{2}}\right)^{0}}$). [10] Twin studys have been conducted by scientists to examine the roles that genetics and environment play in the development of various traits. Such studies examine how often identical twins possess the same behavioral trait and compare it to how often fraternal twins possess the same trait.[ citation needed ] In other studies twins are raised in separate families, and studies compare the passing on of a behavioral trait by the family environment and the possession of a common trait between identical twins. This kind of study has revealed that for personality traits which are known to be heritable, genetics play a substantial role throughout life and an even larger role during early years. [12]

Half siblings are 25% related by consanguinity as they share one parent and separated from each other by two generations (${\displaystyle \left({\tfrac {1}{2}}\right)^{2}}$). There is a very small chance that two half-siblings might not share any genes, if they didn't inherit any of the same chromosomes from their shared parent. This is possible for full siblings as well, though even more unlikely. But because of how homologous chromosomes swap genes (due to chromosomal crossover during meiosis) during the development of an egg or sperm cell, however, the odds of this ever actually occurring are practically non-existent. [11]

A person may share more than the standard consanguinity with their sibling if their parents are related (the coefficient of inbreeding is greater than zero).[ citation needed ]Interestingly half siblings can be related by as "three quarters siblings" (related by 3/8) if their unshared parents have a consanguinity of 50%. This means the unshared parents are either siblings, making the half siblings cousins, or parent and child, making them half aunt-uncle and niece-nephew. [13]

## 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. Despite its lasting presence in the public domain, studies have failed to consistently produce clear, valid, and compelling findings. Therefore, it has earned the title of a pseudo-psychology amongst the scientific psychological community. [14]

### History

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.[ citation needed ]

#### Galton

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. [15] He theorized three main reasons as to why first-borns are generally more eminent:

1. Primogeniture laws: first-borns have access to their parents' financial resources to continue their education. [15]
2. First-borns are given more responsibility than their younger siblings and are treated more as companions by their parents. [15]
3. First-borns are given more attention and nourishment in families with limited financial resources. [15]

• First Borns: Fulfilling family roles of leadership and authority, obedient of protocol and hierarchy. Seek out and prefer order, structure and adherence to norms and rules. They partake in goal-striving behaviour as their lives are centred around achievement and accomplishment themes. They fear the loss of their position in the top of the hierarchy. [16]
• Middle Children: Feel like outcasts of families as they lack primacy of the first child and the "attention garnering recency" of the youngest. These children often go to great lengths to de-identify themselves with their siblings, in an attempt to make a different and individualized identity for themselves as they feel like they were "squeezed out" of their families. [16]
• Youngest Children: Feel disadvantaged compared to older siblings, are often perceived as less capable or experienced and are therefore indulged and spoiled. Because of this, they are skilled in coaxing/charming others to do things for them or provide. This contributes to the image of them being popular and outgoing, as they engage in attention-seeking behaviour to meet their needs. [17]

### Contemporary findings

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. [16] 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. [18]

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. [19] 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. [20] However, other research finds no such effect. [21] 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. [14] However, it is impossible to generalize birth order characteristics and apply them universally to all individuals in that subgroup.

#### Contemporary explanations for IQ findings

##### Resource dilution model

(Blake, 1981) provide three potential reasons for the higher scoring of older siblings on IQ tests: [15]

1. Parental resources are finite, first-born children get full and primary access to these resources. [15]
2. As the number of a children in a family goes up, the more resources must be shared. [15]
3. These parental resources have an important impact on a child's educational success. [15]
##### Confluence model

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: [15]

1. Firstborns do not need to share parental attention and have their parents' complete absorption. More siblings in the family limit the attention devoted to each of them. [15]
2. Firstborns are exposed to more adult language. Later-borns are exposed to the less-mature speech of their older siblings. [15]
3. Firstborns and older siblings must answer questions and explain things to younger siblings, acting as tutors. This advances their cognitive processing of information and language skills. [15]

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. [22] Literature reviews that have examined many studies and attempted to control for confounding variables tend to find minimal effects for birth order on personality. [23] [24] 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. [25]

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.

## Regressive behavior at birth

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.[ citation needed ] 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[ citation needed ] 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[ citation needed ] 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.

## Rivalry

"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. [26] 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. [27] 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. [28] One study found that the age group 10–15 reported the highest level of competition between siblings. [29] 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. [28] 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. [30] 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. [29]

## Relationships

### Jealousy

Jealousy is not a single emotion. The basic emotions expressed in jealous interactions are fear, anger, relief, sadness, and anxiety. [31] 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. [31]

#### Newborn

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. [31] These children closely watch their parents interact with their newborn sibling, approach them positively and sometimes join the interaction. [31] They show fewer behaviour problems in the months following the new birth and do not display problematic behaviours during the parent-infant interaction. [31] 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. [31] Approach-Avoidant Children: 30% of children fall into this category. [31] 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. [31] 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. [31] Disruptive Children: 2.7% of children fall into this category. [31] 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. [31]

#### Parental effect

Children are more jealous of the interactions between newborns and their mothers than they are with newborns and their fathers. [31] 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. [31]

Children that have parents with a better marital relationship are better at regulating their jealous emotions. [31] 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. [31] 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. [31]

#### Implicit theories

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. [32] These implicit beliefs determine both the intensity of their jealous feelings, and how long those jealous feelings last. [32] Malleable Theorists display engaging behaviours, like interacting with the parent or sibling in an attempt to improve the situation. [32] 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. [32] 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. [32] They tend to have less intense and shorter lasting feelings of jealousy than malleable theorists. [32]

#### Different ages

Older children tend to be less jealous than their younger sibling. [31] 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. [31] 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. [31] 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. [31] Younger siblings' feelings of jealousy are overpowered by feelings of anger. [31] 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. [31]

### Conflict

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. [33] 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. [33]

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. [33]

### Warmth

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. [34]

### Negative effects of conflict

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. [34] 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. [35] 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. [33]

### Parental management techniques of conflict

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. [36]

### Long-term effects of presence

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. [37]

## Gender roles among children and parents

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. 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. [38] 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. [38] 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. [38] In contrast, in homes where the father did not hold traditional values, the house chores were divided more equally among his kids. [38] 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. However, education may be a confounder affecting both the father's attitude and the siblings' behavior, and the mother's attitudes did not have a noticeable impact. [38]

## Westermarck effect

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. [39] [40]

## Related Research Articles

Incest is human sexual activity between family members or close relatives. This typically includes sexual activity between people in consanguinity, and sometimes those related by affinity, adoption, clan, or lineage.

Jealousy generally refers to the thoughts or feelings of insecurity, fear, and concern over a relative lack of possessions or safety.

Pathological jealousy, also known as morbid jealousy, Othello syndrome or delusional jealousy, is a psychological disorder in which a person is preoccupied with the thought that their spouse or sexual partner is being unfaithful without having any real proof, along with socially unacceptable or abnormal behaviour related to these thoughts. The most common cited forms of psychopathology in morbid jealousy are delusions and obsessions. It is considered a subtype of delusional disorder.

Anti-social behaviours are actions that harm or lack consideration for the well-being of others. It has also been defined as any type of conduct that violates the basic rights of another person and any behaviour that is considered to be disruptive to others in society. This can be carried out in various ways, which includes, but is not limited to, intentional aggression, as well as covert and overt hostility. Anti-social behaviour also develops through social interaction within the family and community. It continuously affects a child's temperament, cognitive ability and their involvement with negative peers, dramatically affecting children's cooperative problem-solving skills. Many people also label behaviour which is deemed contrary to prevailing norms for social conduct as anti-social behaviour. However, researchers have stated that it is a difficult term to define, particularly in the United Kingdom where there are an infinite number of acts that fall into its category. The term is especially used in British English.

Birth order refers to the order a child is born in their family; first-born and second-born are examples. Birth order is often believed to have a profound and lasting effect on psychological development. This assertion has been repeatedly challenged. Recent research has consistently found that earlier born children score slightly higher on average on measures of intelligence, but has found zero, or almost zero, robust effect of birth order on personality. Nevertheless, the notion that birth-order significantly influences personality continues to have a strong presence in pop psychology and popular culture.

In Freudian psychology, psychosexual development is a central element of the psychoanalytic sexual drive theory. Freud believed that personality developed through a series of childhood stages in which pleasure seeking energies from the id became focused on certain erogenous areas. An erogenous zone is characterized as an area of the body that is particularly sensitive to stimulation. The five psychosexual stages are the oral, the anal, the phallic, the latent, and the genital. The erogenous zone associated with each stage serves as a source of pleasure. Being unsatisfied at any particular stage can result in fixation. On the other hand, being satisfied can result in a healthy personality. Sigmund Freud proposed that if the child experienced frustration at any of the psychosexual developmental stages, they would experience anxiety that would persist into adulthood as a neurosis, a functional mental disorder.

Commonly "cousin" refers to a "first cousin", a relative whose most recent common ancestor with the subject is a grandparent. More generally, in the lineal kinship system used in the English-speaking world, a cousin is a type of familial relationship in which two relatives are two or more familial generations away from their most recent common ancestor.

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Parental investment, in evolutionary biology and evolutionary psychology, is any parental expenditure that benefits offspring. Parental investment may be performed by both males and females, females alone or males alone. Care can be provided at any stage of the offspring's life, from pre-natal to post-natal.

An only child is a person with no siblings, biological nor adopted.

Child psychopathology refers to the scientific study of mental disorders in children and adolescents. Oppositional defiant disorder, attention-deficit hyperactivity disorder, and autism spectrum disorder are examples of psychopathology that are typically first diagnosed during childhood. Mental health providers who work with children and adolescents are informed by research in developmental psychology, clinical child psychology, and family systems. Lists of child and adult mental disorders can be found in the International Statistical Classification of Diseases and Related Health Problems, 10th Edition (ICD-10), published by the World Health Organization (WHO) and in the Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition (DSM-5), published by the American Psychiatric Association (APA). In addition, the Diagnostic Classification of Mental Health and Developmental Disorders of Infancy and Early Childhood is used in assessing mental health and developmental disorders in children up to age five.

Sibling rivalry is a type of competition or animosity among siblings, whether blood-related or not.

A sister is a woman or girl who shares one or more parents with another individual. The male counterpart is a brother. Although the term typically refers to a familial relationship, it is sometimes used endearingly to refer to non-familial relationships. A full sister is a first degree relative.

Sibling abuse includes the physical, psychological, or sexual abuse of one sibling by another. A fourth category that brought attention from researchers during the first decade of the 21st century is sibling relational abuse, which derives from relational aggression.

Gene–environment correlation is said to occur when exposure to environmental conditions depends on an individual's genotype.

Siblings play a unique role in one another's lives that simulates the companionship of parents as well as the influence and assistance of friends. Because siblings often grow up in the same household, they have a large amount of exposure to one another, like other members of the immediate family. However, though a sibling relationship can have both hierarchical and reciprocal elements, this relationship tends to be more egalitarian and symmetrical than with family members of other generations. Furthermore, sibling relationships often reflect the overall condition of cohesiveness within a family.

Niche picking is a psychological theory that people choose environments that complement their heredity. For example, extroverts may deliberately engage with others like themselves. Niche picking is a component of gene-environment correlation.

Animals, including siblings, compete for resources such as food, territory, and potential mating partners. In animal sibling rivalry, individuals compete for parental care or limited resources, which can sometimes result in siblicide. Sibling rivalry occurs in many different forms. Siblings may compete for resources in a prenatal and/or post-birth environment. The degree of rivalry varies, ranging from a low level of violence in non-aggressive to the killing of kin in siblicide.

A firstborn is the first child born to in the birth order of a couple through childbirth. Historically, the role of the firstborn child has been socially significant, particularly for a firstborn son in patriarchal societies. In law, many systems have incorporated the concept of primogeniture, wherein the firstborn child inherits their parent's property. The firstborn in Judaism, the bechor, is also accorded a special position.

Adoption studies are one of the classic tools of behavioral genetics. These studies are used to estimate the degree to which variation in a trait is due to environmental and genetic influences. Adoption studies are typically used together with twin studies when estimating heritability. There are two types of adoption study designs; the Adoptee's method and the Familial method. The most powerful form of adoption studies compare pairs of genetically-identical monozygotic identical twins who are adopted into different families (MZA); however, historical changes in adoption practices to try to keep twin pairs together make these now the rarest kind of adoption studies. Adoption studies have pinpointed that some traits are linked to genetics, for example, Schizophrenia, IQ and criminality, however other compound factors can have an impact too.

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