Whether the behaviour is seen as problematic depends on culture. Externalizing disorders are characterised by more outward-directed behaviours. Internalizing disorders are characterised by more inward-focused experiences and behaviours.

There are three sub-types of ADHD:

1. Predominantly inattentive
2. Predominantly hyperactive-impulsive
3. Combined

People with ADHD tend to have problems getting along with peers and establishing friendships. They show poor social skills (1), aggressive behaviour (2) and overestimate their social skills (3). The combined type is the most likely to develop conduct problems.

The heritability of ADHD is ±70-80%. They have smaller dopaminergic areas in the brain, there are differences in brain structure, connectivity and function and they exhibit less activation in frontal areas. Frontal areas are important for the inhibition of behaviour. Low birth-weight (1), maternal smoking (2), interaction with family (3) and family characteristics (parental psychopathology) (4) are factors that can impact the development and can maintain or exacerbate the symptoms of ADHD. Food additives may help maintain or exacerbate the symptoms.

There are two paths of conduct disorder:

1. Life-course persistent conduct disorder (age 3 – life)
   Problematic behaviour from age three, which lasts throughout life.
2. Adolescence-limited conduct disorder (adolescence)
   A relatively normal childhood, problematic adolescence and normal adulthood.

The adolescence-limited conduct disorder could result from a difference in physical maturation and the opportunity to assume adult responsibilities. Deficits in brain regions that support emotion (empathy) (1), deficiency in moral awareness (2), social information processing deficits (3), autonomic nervous system abnormalities (4) and peer rejection (5) are risk factors for conduct disorder. Ambiguous cues are often misinterpreted as evidence of hostile intent. Neighbourhood and family factors play a role in whether children associate with deviant peers.

The prevalence of depression in school-age children under 13 is ±2-3% and rises to ±6-16% in adolescence. Genetics (1), significant interpersonal stressful live events (2)¸ rejection by parents, cognitive distortions and a negative attributional style are risk factors for depression in children. The attributional style becomes stable by adolescence. The benefits of CBT may not last long for children and the side-effects of medication may be extreme for children.

Separation anxiety disorder is characterized by constant worry that some harm will befall their parents or themselves when they are away from their parents. For children, in order to be diagnosed with anxiety disorders, the fear does not need to be considered irrational as children are unable to make this judgement.

The heritability of anxiety disorders is ±29-50%. Parental control (1), insecure attachment style in infancy (2), overprotectiveness of parents (3), emotion regulation problems (4) and high levels of behavioural inhibition (5) are risk factors for the development of anxiety disorders in children.

A specific learning disorder is a condition in which a person shows a problem in a specific area of academic, speech, language or motor skills that is not due to intellectual disability or deficient educational opportunities. It includes dyslexia and dyscalculia.

Children with dyslexia have a problem with phonological awareness and children with dyscalculia have less activation in areas of the parietal lobe during mathematics. Genes play a bigger role in dyslexia among children whose parents have more education.

The severity of the intellectual disability is assessed in three domains: practical (1), conceptual (2) and social (3). An intellectual disability can be caused by recessive-gene diseases, infectious diseases, environmental pollutants and chromosomal abnormalities.

The fragile X syndrome is caused by a mutation of the fMRI1 gene on the X chromosome and causes large underdeveloped ears, a long thin face, difficulties on neuropsychological tests, mood swings and intellectual disability or specific learning disorder. Chromosomal abnormality can also cause Down syndrome.

Phenylketonuria (PKU) is a disease in which a person begins to suffer from a deficiency of a liver enzyme. A diet low in phenylalanine can reduce the effects to a minimum.

Children with ASD (autism spectrum disorder) have profound difficulties with the social world (1), have a problem in joint attention (2), spend less time looking at other people’s faces (3), show echolalia (repeating the thing that was just said) (4), have communication deficits (5), show pronoun reversal (referring to themselves as the wrong pronoun) (6).

The heritability of ASD is ±80%. The brain of people with ASD is larger than that of people without ASD, brain growth slows abnormally in later childhood and they have an enlarged cerebellum. They also might have isolated skills that reflect great talent.