There is a bi-directional influence between peer relationships and children’s disorders. Children’s problems with peers may contribute to the genesis of the disorder (e.g. anxiety) and the disorders may make peer contact more problematic.

Matching of negative emotion with another infant (e.g. making another infant cry in the nursery) is the first form of peer interaction. Variation in responsiveness to young peers may relate to general individual differences in emotionality and behavioural inhibition.

Infants start to smile at, reach towards and touch other infants by the second half of the first year of life. This behaviours develops simultaneously with peers and adults. Topic-related interactions between infant peers is a characteristic of the second year of life (e.g. direct peers’ attention to toys).

Contingent peer interactions (i.e. contingent responsiveness) refers to an interaction which resembles turn-taking and can be observed at 6 months. Infants are better at this when a toy is not present. One year olds are able to engage in cooperative games with their peers. This makes use of sustained interactions with mutual engagement (1), repetition of actions (2), alternating of turns (3) and playful quality (4). Other forms of prosocial behaviour (e.g. helping; sharing; comforting) emerge around the first birthday.

One year olds also have conflict with peers and use force to pursue their goals, next to being cooperative and prosocial. Most conflict regards the possession of toys or violation of personal space. Conflicts between young peers are characterized by communicative gestures and signs of social influence. Toddlers use hitting and grabbing. Grabbing tends to decrease over time while hitting remains stable. This may be a precursor for later aggression. Younger toddlers show more frequent, brief aggression and older toddlers show fewer but more sustained aggressive behaviours. This peaks around 30 months of age. Adult behaviour affects toddlers’ responses to conflict.

Infants are more likely to respond positively to unfamiliar peers than to unfamiliar adults. They also interact differently with unfamiliar peers compared to familiar peers. Preferences for playmates develop in toddlerhood as well as preferences for same sex peers.

There are individual differences in the quality of play with peers during the first few years of life. It is predicted by the social environment (1), experience with siblings (2), attachment figures (3) and other caregivers (4).

Stable individual differences in competence with peers emerge in the first years of life. Individual differences in aggression and externalizing problems emerge during toddlerhood. Socially inhibited behaviour in preschool settings is predicted by earlier patterns of inhibition with peers.

An infant’s ability to engage successfully in interaction with peers requires several skills:

1. Joint attention
   This refers to the ability to coordinate attention with another person and develops with adult partners between 6-12 months of age. Joint attention in the second year predicts theory of mind skills. Successful peer interaction requires being able to read and produce joint attention signals (e.g. pointing).
2. Emotion regulation
   This refers to the ability to regulate one’s emotion and this is required for peer interaction. Self-regulation is essential in peer interaction and is more successful in reducing anger than in reducing fear. Differences in this ability can be seen by excessive crying very early in infancy and attachment security.
3. Inhibitory control
   This is required for successful social interaction and is part of executive function. Impulses to explore their peers as objects need to be inhibited when meeting new peers as this may provoke conflict. This exploratory behaviour typically decreases with experience with that peer. Infants need to suppress their tendencies to seize toys held by peers.
4. Imitation
   This is required to successfully harmonize with their peers. Matching the peer’s behaviour is an important strategy for interaction amongst young toddlers. Later in development the interactions become more complementary rather than imitative.
5. Causal understanding
   Successful social interaction depends on a mutual understanding of each participant’s status as an active and intentional agent. This can be an important step towards the development of theory of mind. Children need to realize that things can happen by choice (e.g. causal) or by accident. This tends to develop over infancy. Causal understanding is associated with intentional communication and social referencing, which is essential for social interaction. Causal understanding is associated with aggression and prosocial behaviour.
6. Language
   A child’s language use is essential in their social interaction.

Deficits in either of these skills could hinder adjustment to peers in group settings and impair peer relationships. There are different types of interactions in early childhood:

1. Conversation
   This refers to simply talking to a peer. Conversational competence is greatest in familiar situations where children can rely on script knowledge. This influences the degree to which they are accepted by peers.
2. Cooperation
   This refers to cooperating activities with those of peers. Children can be either cooperative or competitive and this is affected by gender (1), culture (2) and attachment status (3). Children cooperate more often with friends. Preschool children start to learn to balance cooperative and competitive behaviour whilst dealing with their peers but this is regulated by sex.
3. Helpfulness, sympathy and kindness
   These capacities increase over the years of childhood. Helping and sharing are more common forms of prosocial behaviour in preschool groups. Gender differences in prosocial behaviour emerge during the preschool years but this partially depends on the social context.
4. Conflict and aggression
   Conflict over resources (e.g. toys) decreases over the preschool years but conflict over social issues (e.g. violation of gender-role stereotype; inclusion of children in group activities) increase. The overall frequency of conflict does not change. Gender differences in aggression and conflict become more clear between three and four years of age.
5. Pretend play
   This is important in the consolidation of early friendships and the persistence of those friendships into later childhood. The ability to engage in pretend play emerges in the second year of life and increases in complexity afterwards, partially as a function of language development.

In the preschool years, the ability to relate to peers in groups as opposed to one-to-one relationships is developed. More formal group activity develops as young children spend more time in groups. There are several patterns in group activity:

1. Friendships
   Children develop early preferences for particular peers as early as the second year of life. Young children judge children’s transgressions differently depending on whether or not they are friends.
2. Gender segregation
   Children gradually start to prefer same-sex peers. Increasing gender segregation of preschool children’s groups impact on their friendships. Girls’ early friendships are more stable than boys’. It may result from differential responses to peer requests (e.g. 3-year old girls are more likely to share with girls than boys).
3. Dominance hierarchies
   There are dominance hierarchies in early children’s groups. The tendency to yield to an individual depends on their identity. It tends to start in toddlerhood and dominant peers tend to engage conflict with other dominant children. Dominance affects both play and conflict.
4. Peer acceptance and rejection
   A peer’s acceptance is influenced by their social environment (e.g. the mother’s social network) and their styles of interaction. The frequency of positive and negative interactions with peers is important.

There are several individual differences that affect peer acceptance:

1. Prosocial behaviour
   This emerges in the preschool years and there tends to be long-term stability. It is a key factor in gaining acceptance.
2. Aggressiveness
   This emerges in toddlerhood and is consolidated during the preschool years and there is long-term stability. Aggressive children are liked less by peers. Peer rejection is influenced by the beliefs about the stability of aggressive behaviours in particular peers.
3. Shyness
   General behavioural inhibition in infancy predicts shyness with peers in the preschool years. There tends to be stability across the childhood years. It has been linked to insecure attachment relationships although it may depend on the infant’s temperament. There may be a transmission of the mother’s social phobias (i.e. genetic transmission). Solitary play shows groups of shy (1), non-shy (2), isolated (3) and depressed (4) children. Only the isolated and depressed children are reason for concern.

There are several underlying processes which are important for peer relations in the preschool years:

1. Emotionality and emotion regulation
   This is linked to the quality of their friendships with their peers. The higher the level of emotion regulation, the weaker the relationship between negative emotional responding and social competence. The experience of negative emotions and problems in regulating those have long-term consequences for peer relations. Negative emotionality and problems with emotion regulation interfere with social competence. This is associated with aggression (1), lack of prosocial skills (2) and shyness (3).
2. Social understanding
   A child’s degree of social understanding predicts their competence at dyadic interaction with peers. The peer’s degree of social understanding is also important. Social understanding may influence which peers get rejected and peer rejection may interfere with the development of social understanding. However, children who are rejected by peers are not necessarily deficient in social understanding. Problems in social understanding can be biases or deficits (e.g. deficit in theory of mind). It is associated with aggression and lack of prosocial skills.
3. Executive function
   This is associated with a lack of prosocial skills and aggressiveness.

In middle childhood, a tendency to play alone is linked to loneliness (1), social isolation (2) and depressive symptoms (3). For adolescents, solitude has a positive effect on emotional state. This means that whether a child is alone or not should be assessed according to their developmental stage.

In childhood, loneliness is associated with lower levels of peer competence. This may be the result of social conditions and emotional competence. Lonely children are rejected and victimized by their peers leading to them getting rejected more often over time. Anxious solitude refers to being alone due to anxiety and it is not the same as being alone due to peer exclusion. Anxious solitude, however, does predict peer exclusion. These children are at greater risk for depressive symptoms if they experience peer exclusion.

Children who reported more loneliness in early childhood more often had an insecure-ambivalent attachment in infancy. There may be intergenerational transmission of loneliness. Family conflict heightens feelings of loneliness and loneliness is related to the way in which parents socialize their children to resolve difficulties that arise during interactions with their peers. A controlling interaction style is positively related to loneliness and depressive symptoms.

Bully-victim relationships begin to emerge in the preschool years. There may be a decrease in bullying with age though this may be the result of self-report. A decrease in bullying may exist because younger children do not yet have the social and assertiveness skills necessary to combat bullying incidents and discourage further incidents. Next, younger children are in a more vulnerable position. Boys are victimized more often than girls and the victim is more likely to be the same sex as the bully.

Early family experiences predict which boys become bullies, victims and victimised aggressors. Children with a more punitive, hostile and abusive family treatment during preschool years were aggressive and victimised by others. Children who were aggressive but were not victimised by their peers had received greater exposure to adult aggression and conflict. However, this is not the same as victimization by adults. Victims who were not aggressive did not differ from children who were neither victims or bullies.

Deficits in emotion regulation and social understanding influence acceptance by peers and the subsequent roles in the bully-victim relationship. This, in turn, is influenced by family factors. Bullies do not necessarily have poor theory of mind or social cognitive skills as they may need this to successful bully someone.

Self-control processes and the ability to regulate one’s attention influence bullying and victimization. Lower levels of self-control are associated with being the bully. Poor social skills and inappropriate behaviour is associated with being bullied.

More problematic peer relationships in childhood is associated with emotional disorders (e.g. depression). However, problematic peer relationships do not always lead to emotional disorders.

It is possible that children who are more aggressive are more often rejected by peers and this leads them to develop a friendship with more aggressive peers which increases the risk of conduct disorder. Peer rejection and early aggression could also independently increase the risk of conduct disorder.

The presence of sociocognitive problems increases the likelihood that aggressive children are rejected. The base rate of aggression within a peer group also influences whether aggressive children are rejected. Peers may evaluate aggressive behaviour in light of the social norms of the peer group.  

Peer rejection is predictive of antisocial behaviour. It may be especially detrimental to children who are already showing maladaptive behaviour. Girls and reactive-aggressive children are most vulnerable to the consequences of peer rejection. Boys who were victims of peer aggression did not become more aggressive while girls did.

Peer groups of aggressive children have more conflict than peer groups of non-aggressive children. This increased conflict could lead to more aggressive episodes, meaning that aggressive children’s friendships facilitate aggression. The establishment of social norms in aggressive peer groups may facilitate further aggression. Aggressive children tend to make each other more aggressive over time. Reducing the time spent with the deviant peer group may reduce aggression.

Association with deviant peers around 14 or 15 years of age has the biggest impact on criminal activity and substance use. Peer acceptance and friendships are a protective factor against the development of externalizing problems.