Why are neurons and the brain relevant?

The early development of the brain and nervous system is largely determined by biological factors, but the influence that experience has is also fairly large. The nervous system begins to develop shortly after conception, as the neural plate (a group of cells) thickens, folds in and forms the neural tube. The cells start to migrate to fixed locations. The brain contains millions of multifunctional cells, glial cells , and neurons . Neurons carry messages within the nervous system and to and from other body parts. The extensions of these neurons, called nerves, get a layer of myelin, a white substance that promotes the efficiency of communication in the brain. An excess of neurons and connections is produced both before and after birth to ensure the flexibility of the brain. Some parts of the brain develop faster than others. For example, the development of brain parts for vision and hearing is faster than the development of the frontal brain area, which is involved in complex thinking.

There are many developments in the brain during adolescence. In this way the connections between brain areas increase. Also, the amount of grey matter in the frontal brain area decreases, while the white matter shows an increase, which is a reflection of constant myelination. These changes have implications for psychological and behavioural functioning.

The development of the brain depends on the interaction between biological predisposition and experiences (activity-dependent processes). There is pruning occurring both before and after birth, which means unnecessary cells and connections between cells are broken down. This process is probably the cause of the decline in grey matter in adolescence.

Structure

The brain and backbone together form the central nervous system . The peripheral nervous system is formed by the nerves outside the central nervous system, which carry signals to and from the central nervous system. The peripheral nervous system has two subsystems:

• Somatic system: contains the senses and muscles and is involved in sensory experiences and voluntary movements.
• Autonomous system: is involved in the involuntary regulation of alertness (arousal) and emotions. The autonomous system causes
  
  • An increase in alertness (sympathetic system)
  • a decrease in alertness and the maintenance of body functioning (parasympathetic system ).

The entire nervous system interacts with the endocrine system : a collection of glands that exert influence on the body through the release of hormones.

The brain consists of three important parts, each of which consists of different components:

• The hind brain :
  • Pons: provides information
  • Medulla: regulates heart rate and breathing
  • Cerebellum: is responsible for movement and cognitive processing
• The middle brain : connects the rear brain with the upper brain parts. The midbrain also has an influence on sleeping and waking up, which is exercised through the reticular activation system. The midbrain and the back brain are sometimes referred to together as the brainstem.
• The fore brain: consist of two cerebral hemispheres. The surface is called the cerebral cortex. The two hemispheres (brain parts) are connected by the corpus callosum. Each part of the brain has four lobes and is responsible for various activities, such as sensory processing, motor control and higher mental functioning, such as information processing and memory. Among the cerebral hemispheres are the subcortical structures:
  
  • Thalamus: processes information and transports information between the two halves of the brain and other parts of the central nervous system.
  • Hypothalamus : regulates basic needs such as hunger, thirst and sexual activity.
  • Limbic system: includes the hippocampus and amygdala and plays an important role in memory and emotion.

Neurotransmitters and hazards

Neurons differ in size and shape, but all consist of (1) a cell body, (2) dendrites and (3) an axon. Neurons can communicate with each other via synapses, small endings the leave openings that are located between the cells (synaptic gap). The dendrites of a neuron receive messages from other neurons. This results in an electrical signal that is sent to the axon. When the signal reaches the end of the axon, substances (called neurotransmitters) are excreted. Neurotransmitters cross the synaptic gap and are received by the dendrites of the receiving neuron. The receiving neuron then generates new electrical impulses. Examples of neurotransmitters are dopamine, serotonin and GABA. Neurotransmitters can inhibit neurons or make them more active.

What is the importance of the nervous system?  

A child's nervous system can be damaged as a result of heredity or early abnormalities in genetic processes. However, the nervous system can also be damaged by events during pregnancy (prenatal), during birth (perinatal) or after birth (postnatal).

Prenatal influences

There are several prenatal influences that can damage the development of the nervous system, including poor nutrition and stress in the mother. Teratogens are substances that are dangerous for the development of a foetus. Examples are alcohol, drugs, radiation and illnesses in mothers.

Teratogens are associated with malformations, low birth weight, miscarriage and functional and behavioural limitations. They interfere with various development processes, including the formation and migration of brain cells. The outcome depends on the extent and especially the timing of teratogenic exposure. Specific structures and systems are most susceptible to damage in periods in which they develop rapidly. The prevalence of the genes of the developing foetus can serve as a risk or protective factor for the effects of teratogens or maternal stress.

The adverse effects of prenatal exposure are illustrated by foetal alcohol syndrome, which is characterized among other things by abnormal brain development, facial abnormalities, growth problems, impaired motor skills and psychological problems. The effects of alcohol consumption by the mother are related to various factors, such as the degree of exposure, the timing of exposure, the age and health of the mother and the vulnerability of the child.

Because teratogens often go together, it is difficult to distinguish between the influence of different teratogens. For example, prenatal drug use is often associated with alcohol and tobacco use. In addition, drug use is often associated with poverty, which has both a prenatal and a postnatal impact on the child. This makes it difficult to determine the timing of the influence.

Perinatal and postnatal influences

Risk factors during birth include giving too much medication to the mother, oxygen deprivation, premature birth and a low birth weight. Examples of postnatal influences are malnutrition, accidents, diseases and poisoning. If young people suffer from brain damage, the question is to what extent recovery is possible. In specific, it is to what extent the plasticity (flexibility) of the brain can recover. There is evidence that the young nervous system is relatively well able to recover or to transfer functions to undamaged brain areas. On the other hand, injury to the immature brain can cause negative effects on later brain development, making deficits visible over time. The timing and severity of the injury, but also treatment and support from the environment are factors that influence recovery.

What are genes?

The basic genetic material is found in all body cells. It consists of chromosomes that contain DNA . DNA is another part of genes. Millions of chromosome combinations are possible for one individual. Chromosomes can exchange genes, disassemble them and attach them to each other. These are spontaneous changes to the DNA molecule. In some cases, early genetic processes result in structural defects in the chromosomes or a lack or excess of the 23 pairs of chromosomes that most people have. These 'errors' can be inherited, but usually arise spontaneously, and can lead to death or medical syndromes with physical, intellectual and psychological disorders.

Behavioural genetics investigates the genetic influences on individual differences in behaviour. Research is being conducted into the extent of genetic influences on traits, the genes involved, the way genes work and the way genes influence traits. Many traits and psychological disorders have a genetic component.

The term genetic code refers to the order in which four nucleotides (adenine, thymine, guanine, and cytosine) occur in certain regions of a gene. This order is the basis for transcription, or synthesis, of messenger RNA: a molecule that transports information to other parts of the cell, where it plays a role in the translation of the code into the production of proteins.

There is increasing evidence of the influence of both the internal and external environment on the processes of transcription, translation and gene expression. The path from the genotype (individual genetic predisposition) to the phenotype (observable traits of the person) is indirect and complex.

What is the importance of heredity?

Heredity through a single gene

Mendel stated that certain properties are ones that are influenced by a single gene. He claimed that each parent has two heredity factors (genes), but only transfers one to their offspring. A gene can be dominant or recessive. A dominant gene is manifesting when at least one of the parents transfers this gene to the child. A recessive gene only manifests itself if both parents transfer this gene. Dominant and recessive genes are involved in inheriting many human traits and disorders.

In general, the effects of individual genes are fairly predictable and cause the individual to have the phenotype or not. One way to investigate 'single-gene' influence on a specific disorder is to identify a person with the disorder (the index case or proband ) and see if there is a known pattern of 'single-gene' heredity in his / her family occurs.

Multigenetic heredity

Complex traits, such as intelligence, are determined by multiple genes. These genes are also called quantitative trait loci (QTL) and each individually have little effect, but in combination with each other, they have a lot of effect. A separate gene may therefore not be sufficient or necessary for a disorder and can be carried by a person without the disorder. Multigenetic influences are less predictable than influences by a single gene.

Research into multigenetic influences is done with various quantitative genetic methods. Family, twin and adoption studies are important in determining genetic influence on a trait. Quantitative genetic methods make the assessment of heredity possible: the extent to which genetic influence is explanatory for variance in behaviour among the individuals in the population studied. Research shows that heredity for psychological disorders or dimensions is rarely more than 50%. This means that a considerable part of the variation in traits has a basis in other biological factors, the environment or an interaction between genes and other influences.

Quantitative genetic research also provides insight into the contribution of environmental influences and how genes work together. Both shared and non-shared environmental factors are important. Shared environmental factors represent factors that affect all members of a family. Consider, for example, parents who neglect all children because of substance addiction. Non-shared environmental factors are factors that are different for all children in a family. For example, children within a family all have different friends and thus different influences exerted upon them.

While quantitative research methods can study a single trait, multivariate designs can focus on multiple traits. This research design makes it possible to estimate the extent to which genetic and environmental factors that influence one trait also influence another trait. For example, twin research has shown that there is a genetic overlap in a language and reading disorder. These disorders have a common genetic basis. Non-shared genes and / or environmental influences make the disorder manifest differently from each other.

Molecular genetics

Molecular genetics focuses on discovering genes related to a disorder. Research among people uses two research methods: linkage analysis and association analysis.

By using linkage analysis, one can sort out what the precise location of a defective gene is. With linkage analysis, it can be determined whether a specific disorder (which occurs in several family members) is the result of defective genes or not.

Association analysis is another way to study genes. This method can be used to study whether a certain form of a gene is associated with a trait or disorder in the population. For this a comparison is made between the genetic predisposition of persons with a specific disorder and the genetic predisposition of persons from a control group. Often based on previous research, a specific gene (including a ‘candidate gene’) is looked into. Compared to linkage analysis, association analysis is a better method to discover multiple genes that each have a relatively small effect on a trait or disorder.

Advanced genetic technologies have improved the search for genes. 'Genome-wide' linkage analysis and 'genome-wide' association analysis make it possible to scan the genomes of individuals. These analyses are able to investigate millions of DNA sequences. In addition, these methods can investigate variations in the number of copies and deletions of segments of DNA. Variations herein that occur more often in individuals with a disorder indicate a possible causal role.

What does gene environment collaboration mean?

Gene-environment interaction refers to differences in sensitivity to certain experiences due to differences in genotype. For example, children with two recessive genes for PKU disease (phenylketonuria) tend to be intellectually impaired when they ingest certain foods.

Gene-environment correlation stands for genetic differences in exposure to environments. There are three types of gene-environment correlations:

• Passive: the genes of the parents are transferred to the offspring. In addition, the genes of the parents influence the environment in which the child is raised. (Such as, the parents love reading, this is passed on to the child, so the child with a predisposition to loving reading grows up in an environment with a lot of books.)
• Reactive: the child expresses certain environmental reactions due to his / her genetic predisposition. (Such as, the child shies from people because they are prone to social anxiety.)
• Active: based on genetic predisposition, the child selects gene-related experiences. (Such as, the child is prone to aggressive, violent outbursts, so it takes up qa violent martial arts sport to control their anger.)

As children get older, the influence of passive gene-environment correlations decreases, and the influence of reactive and active gene-environment correlations increases. Gene-environment correlations make it clear that the experiences of an individual are not independent of genetic influences. Genetic influences therefore play an important role in the risk and protective factors to which the child is exposed.

What are the concepts learning and cognition?

Classic conditioning

Pavlov focused on the process of classical conditioning. He showed that dogs that normally only drool when they see food, also will start drooling as a stimulus (such as a bell) repeatedly precedes getting food. This way of learning also occurs in people. With classical conditioning, an individual therefore learns to respond to a stimulus that the reaction did not elicit before.

Operant conditioning

Skinner focused on the process of operant conditioning . This type of learning is based on Thorndike's law of effect , which states that a positive effect leads to an increase in behaviour and that a negative effect leads to a decrease thereof. With operant conditioning the emphasis is on the consequences of behaviour. Skinner argued that behaviours acquired, strengthened, weakened, maintained, eliminated and adjusted through empowerment, punishment and other learning processes. There are several important processes in operant conditioning:

• Positive reinforcement: After a certain behaviour, a pleasant stimulus is offered, with which the frequency of the behaviour increases. Example: rewarding good behaviour increases the chance that this behaviour will be shown.
• Negative reinforcement : After a certain behaviour, an unpleasant stimulus is withdrawn, which increases the frequency of the behaviour. Example: If a mother withdraws her demands in response to a tantrum of the child, the chance of tantrums increases.
• Extinction: If the confirmation of a certain behaviour no longer takes place, the learned behaviour will weaken. Example: If parents ignore behaviour meant to draw attention, it will lessen over time.
• Punishment: A certain behaviour is followed by the offering of an unpleasant stimulus or the withdrawal of a pleasant stimulus, which reduces the frequency of the behaviour.
  
  • Positive: a lower frequency of the behaviour is rewarded, thus making the behaviour less likely to occur
  • Negative: The behaviour is met with something unpleasant, thus making the behaviour less likely to occur
• Generalization: A certain behaviour is displayed in response to a new stimulus that is different from, but similar to, the stimulus that was present during learning. Example: A child has a strict uncle with a moustache and develops fear of all men with a moustache.
• Distinction : A stimulus works as a cue that a certain behaviour is probably followed by a certain consequence. Example: The smile of an adult indicates that a child's request is likely to be granted.
• Forms: A desired behaviour is taught by behaviours that increasingly resemble ('successive approaches to') the desired behaviour. Example: A child who does not speak learns to speak by being first empowered for every sound it makes, then for a sound that is close to a word, and so on.

Operant conditioning ensures that behaviour is learned and formed in a specific way. The principles of operant learning are applied in the treatment of various behavioural problems.

Observational learning

Observational learning is when people learn something by observing others (duh). Observational learning can lead to both the acquisition and the extinction of specific behaviour. Bandura has demonstrated how problem behaviour can be acquired through the observation of an example. Children are more likely to show modelled behaviour if they see that the example is validated for the behaviour. Conversely, if the example is punished for behaviour, the chances are lower that children will imitate the behaviour. Similar to other learning processes, observational learning can generalize over time. A child who sees another child getting a punishment because he / she is screaming can become silent (inhibition). Watching fights on television can cause the child to show other forms of aggression, such as verbal abuse (disinhibition). In these cases, the exact behaviour of the example is not imitated, but rather a type of behaviour that is similar to it or exaggerated.

Cognitive behavioural perspective

The cognitive behavioural perspective focuses on the connection between cognition, behaviour, emotion and social factors. Behaviour would be learned and sustained through the interaction of internal cognitions and emotions with external events. Cognitive factors influence whether an individual pays attention to environmental events, how the individual perceives the events and whether these events will influence future behaviour. A basic assumption of the cognitive behavioural perspective is that maladaptive cognitions are related to maladaptive behaviour. For example, children with a phobia often have a negative self-image.

Kendall has suggested a way to distinguish between the complex cognitive functions that contribute to the development, maintenance and treatment of psychopathology:

• Cognitive structures: mental representations of information stored in memory.
• Cognitive content: the content of cognitive structures that are stored in memory.
• Cognitive processes: influence the way in which experiences are perceived and interpreted.

The combination of cognitive structures, content and processes results in interaction with external events in cognitive products.

Kendall also makes a distinction between cognitive deficits and cognitive distortions. Cognitive deficiencies refer to an absence of thought. An impulsive child who does first and then thinks has a cognitive deficiency. Cognitive distortions are incorrect and dysfunctional thinking processes. For example, depressed children feel that they are less competent than other children, while normal kids do not. The purpose of cognitive behavioural therapy is to adjust maladaptive cognitive structures, defects and distortions.

What does socio-cultural context entail?

The development takes place within and is influenced by a socio-cultural context. Children find themselves in and interact with three transactional systems: (1) family, (2) the community and (3) society / culture. Every system consists of structures, institutions, values, rules, relationships and other aspects that influence the development of the child. The arrows in the model emphasize the possible interactions between the systems. For example, there may be a reciprocal interaction between the child and peers and a reciprocal interaction between the child and the child's parents. It is generally assumed that the proximal contexts (the inner circles) have a relatively more direct influence on the child than more distal contexts. The family has the greatest influence, followed by the community and lastly society. In addition, the influence of each domain is expected to vary based on the developmental level of the child. Peers in adolescence thus become more important.

Family context - The role of parents

The reciprocal interaction between parents and children is of great importance. In the past, much more attention has been paid to the role of mothers than that of the father, because mothers are considered the primary caretaker. These days, however, much more attention is paid to the role of the father in addition to the mother. There may be differences between fathers and mothers in the way they interact with the child, but a sensitive upbringing by father has just as much influence on the development of the child as a sensitive upbringing by mother. The influence of father on the child can be direct or can work indirectly, for example through the interaction between father and mother.

What are parenting styles?

Parenting styles refer to the characteristic way in which parents treat their child. A parenting style includes attitudes, goals, and patterns of parenting practices that affect the child's development. Parenting styles can be categorized on the basis of two dimensions: the degree of control (discipline) and the degree of warmth and acceptance.

• Authoritative parenting style : The parents apply clear rules and expect the child to adhere to the rules. At the same time, parents are warm, accepting and responsive.
• Authoritarian parenting style : The parents apply strict rules and show little warmth.
• Permissive parenting style : The parents have few rules and leave the child very free.
• Neglecting parenting style : The parents are not involved with the child.

An authoritative parenting style is associated with the most positive child traits, such as independence, prosocial and self-assured behaviour. Children of authoritarian, permissive or neglecting parents, on the other hand, have a bigger chance on antisocial behaviour, aggression, low self-confidence and issues at school. A number of issues need to be considered when studying parenting practices:

• Effective upbringing takes into account the needs and level of development of the child.
• Parenting practices can in part be a response to the characteristics of the child and other relationships and circumstances within the family.
• The question is to what extent the analysis of parenting practices applies to all cultures and situations. For example, an authoritative upbringing may be less suitable if local cultural values ​​deviate from the values ​​of the dominant culture. In addition, an authoritarian upbringing can offer protection against children who grow up in an adverse environment.

Psychopathology of parents

Psychopathology in parents is a risk factor for the development of children. Both genetic and environmental factors underlie this connection. Children can inherit a genetic predisposition that increases the risk of a disorder and / or increases the risk of being affected by an adverse environment. In addition, psychopathology can have a negative influence on the upbringing of parents, making it a threat to the child's development.

What qualifies as mistreatment?

The 'Keeping Children and Families Safe' law from 2003 defines child abuse as a (lack of) action by parents / guardians, which leads to death, serious physical or emotional damage, sexual abuse or the risk of serious injury. The term abuse therefore refers to both abuse and neglect. There are four general forms of abuse:

• Physical abuse: an act of a parent that may result in physical damage to the child, including death. For example, biting, hitting, stabbing and shaking. Beating can be seen as a disciplinary measure but is classified as abuse when the child has bruises or injuries.
• Sexual abuse: an act such as penetrating or violating private regions, damaging genitals, or other forms of sexual acts whereby the child is used for the sexual satisfaction of the perpetrator. Sexual exploitation and child pornography are also included.
• Neglect: negligence by a parent or guardian, whereby that person refuses to take care of the child or postpones this care. Not providing basic needs such as food, clothing, shelter, affection and attention. This applies to both physical and emotional abuse.
• Emotional abuse: acts such as ignoring, rejecting, isolating or terrorizing a child. Examples of this are: locking up; depriving of sleep, food or shelter; domestic violence; involve the child in substance use or criminal activities; refusing to provide psychological help; and other carelessness where damage or potential damage to the child may result. The behaviour must be constant and repeated.

It is probably easier to detect physical abuse than other forms of abuse. However, the nature and severity of injury can vary considerably. Moreover, physical abuse is often accompanied by other forms of abuse, such as emotional abuse and neglect. Sexual abuse is more common among girls than among boys. Neglect is the most common form of abuse. Neglect can take various forms, such as a failure to meet physical needs (for example, the need for healthcare), inadequate supervision, or a failure to meet educational needs. Emotional abuse is the hardest to define. Emotional abuse is defined as extreme acts or neglect, that pose a threat to the child's emotional needs and to the child's behavioural, cognitive, affective, or physical functioning. It can be considered as a separate form of abuse, but also as part of all forms of abuse.

There are several factors that contribute to abuse: characteristics of the abuser, characteristics of the child, parenting practices, parent-child interaction processes and socio-cultural factors. In about 80% of the cases, the parents are the abusers. Parents who have had children at a very young age are more likely to abuse. This is partly explained by defects in parenting skills. They exhibit fewer (positive) interactions with their child and use more coercive and negative discipline techniques. Other characteristics of parents who are abusing their child are; difficulty coping with stress, difficulty impeding impulsive behaviour, social isolation, emotional symptoms, physical health problems, drug abuse and domestic violence. In addition, parents who abuse their child were also often abused themselves. However, research has shown that the majority of abused children do not abuse themselves later.

Young children and children with disabilities, such as a physical or intellectual disability, have a higher risk of being abused. Children with emotional or behavioural problems also have a high chance of being mistreated.

The larger socio-cultural context also has an impact on abuse. Socio-cultural factors include both the immediate social environment, such as the family, and larger socio-cultural contexts, such as poverty. For example, there is a connection between a low socio-economic status and abuse (in particular neglect). However, this connection can be explained by other factors. For example, poor interpersonal and problem-solving skills of parents can lead to both socio-economic disadvantage and problematic upbringing.

Research shows that abuse at a young age can influence neurobiological outcomes, such as a dysregulation of the stress regulation system, changes in neurotransmitter systems and changes in structural and functional brain areas. Neurobiological outcomes can in turn contribute to cognitive and psychosocial problems. However, it should be noted that some abused children are resilient and develop into competent individuals. The outcomes are influenced by various factors, such as the type and severity of the abuse, the timing and characteristics of the child, the family, peer relationships and the neighbourhood.

What are other risk factors?

Separation

In recent decades there has been a significant increase in the percentage of children living in families with one parent. Children often experience stress prior to divorce. Children and adolescents of divorced (or remarried) parents have an increased chance of problems of adaptation in the emotional, behavioural, social and academic field. However, most young people living in a non-traditional family (a family with a different composition than with two biological parents) are developing well and some are even experiencing positive outcomes because they are no longer in an environment with many conflicts.

Hetherington and colleagues have developed a transactional model to explain the links between divorce (or second marriage) and the adaptation of the child. Various factors play a role, such as parental characteristics, family composition, child characteristics, social support and stress from the parents. An important aspect of the divorce process is the interaction between family members and especially the interaction between parents. The extent to which parents are dissatisfied with their marriage (marital discord) before they get divorced has a lot of influence on the child's adjustment. Much dissatisfaction and violence within the marriage can have a major negative impact on a child. The child is also influenced by communication between parents after they are divorced.

Individual characteristics of parents (such as antisocial behaviour) can increase the chance of a divorce and influence parenting skills. The characteristics of a child also play a role. Children with an easy temperament are better able to adapt to a divorce than children with a difficult temperament. In addition, the relationship between divorce and child adaptation becomes smaller if one takes into account the level of behavioural problems before the divorce. It is difficult to indicate what role gender plays in the relationship; connections were found in earlier studies that were not found to the same extent later.

However, the relationship between parent characteristics, child characteristics and the divorce process is complex. For example, behavioural problems of the child can contribute to conflicts between parents, leading to a divorce. In addition, the parental characteristics that play a role in the divorce and behavioural problems of the child can be influenced by common genetic influences. For example, common genes can contribute to the risk of antisocial behaviour in both parents (risk of divorce) and the child.

Finally, the divorce process causes changes in family circumstances that affect family adjustment. For example, single mothers more often experience money problems after a divorce, which increases the risk of problems with the child.

Dealing with peers

Interactions with peers play an important role in the development of empathy, cooperation, morality, negotiation of conflicts, competition, aggression control and socialization of gender roles. Relationships with peers can promote the development of social competence in spite of negative circumstances, thereby reducing the chance of a disorder. However, peer relationships can also be related to the presence of a disorder.

The development of peer relationships is influenced by child characteristics, parent characteristics, characteristics of teachers and characteristics of the neighbourhood. For example, an authoritarian parenting style is associated with aggression in the child and rejection by peers. Close friendships can serve as protection against risk factors. Children tend to choose friends who look like themselves. While this can reinforce positive behaviour, it can also lead to a strengthening of psychological problems and deviant behaviour.

Peer relationships are related to the later adaptation of children. Children who are rejected by peers or have deviant friends, for example, have a greater risk of problems. There is a reciprocal link between peers problems and the child's adaptation problems.

Education and socio-economic status

In addition to learning intellectual skills and knowledge, education also has a socializing function. At school, children learn to behave according to certain norms and values and motivation, mental health and socio-emotional development are promoted. School has influence through distal factors, such as education policy, and proximal factors, such as the atmosphere in the classroom, instructions and social relationships. The relationship between a child and a teacher can also serve as a risk or protective factor.

Risk factors for dropping out of school (early) are a general socio-economic status, poor academic performance, behavioural problems and a lack of family support. The social economic status (SES) is determined by factors such as income, education level and professional level. Research shows that children from families with a low SES have a higher risk of negative outcomes, such as psychological problems, learning disabilities and developmental delays. Duncan and Brooks-Gunn state that the influence of poverty depends on (1) persistence, (2) intensity and (3) timing. A child who has been growing up in constant and severe poverty from an early age has the highest risk of negative outcomes.

Exposure to accumulated risks is also related to development outcomes. Children who grow up in poverty often live in less hygienic conditions. In addition, they are less likely to come into contact with learning materials, such as books and suitable toys. Parenting styles in poorer families are often harsher and less sensitive or responsive. Poverty causes stress, which leads to more conflicts between parents and children, more dissatisfaction with marriage and a greater chance of divorce.

Poverty can influence development in various ways. Research shows that there is a direct link between poverty and the volume of grey matter in the hippocampus: a brain area that is influenced by stress and that is linked to cognition and behavioural regulation.

Living environment

Poor families often live in specific neighbourhoods, because the houses there are cheaper. Minority groups, such as migrants, are over-represented in these neighbourhoods. There is a connection between poor neighbourhoods and aggressive behaviour. Living in a good neighbourhood increases the chance of successful school achievements.

The living environment influences the child's development by a variety of sources;

• Through sources of society (community resources) , such as libraries, museums, good day care centres, medical services and job opportunities.
• Via relationships , such as family relationships, social support networks for parents and characteristics of the house, such as hygiene, safety and routines.
• Via community norms / collective efficacy: the extent to which communities are organized to maintain order and norms of conduct.

Culture and ethnicity

Cultural factors can greatly influence the development of children. Children with a different ethnic or racial background than the dominant group in society have a higher risk when it comes to troubles or problems. Minority groups have to deal with acculturation : changes in own culture as a result of interactions with other cultures. In many countries, children from minority groups exhibit more psychopathology. Among Native & Indian Americans, for example, addiction, suicide and unemployment are more common. In addition, immigrants often have to deal with prejudice, discrimination and poverty. In African American youth, discrimination goes hand in hand with aggression, antisocial behaviour and depression. Prejudices have a negative influence on academic achievement, health, behaviour and emotions. Finally, minority groups often experience disadvantages when seeking and receiving mental and other health services. For example, professionals sometimes have a lack of awareness of cultural differences.