The type of behaviour seen will depend on the age and gender of the individual.

Younger children, say from 3 to 7 years of age, usually present with general defiance of adults wishes, disobedience of instructions, angry outbursts with temper tantrums, physical aggression to people especially siblings and peers, destruction of property, arguing, blaming others for things that have gone wrong, and a tendency to annoy and provoke others.

In middle childhood, say from 8 to 11, the above features are often present but as the child grows older, stronger, and spends more time out of the home, other behaviours are seen. They include: swearing, lying about what they have been doing, stealing of others belongings outside the home, persistent breaking of rules, physical fights, bullying of other children, cruelty to animals, and setting of fires.

In adolescence, say from 12 to 17, more antisocial behaviours are often added: cruelty and hurting of other people, assault, robbery using force, vandalism, breaking and entering houses, stealing from cars, driving and taking away cars without permission, running away from home, truanting from school, extensive use of narcotic drugs.

Not all children who start with the type of behaviours listed in early childhood progress on to the later, more severe forms. Only about half continue from those in early childhood to those in middle childhood⁽²⁾; likewise only about a further half of those with the behaviours in middle childhood progress to show the behaviours listed for adolescence. However, the early onset group are important as they are far more likely to display the most severe symptoms in adolescence, and to persist in their antisocial tendencies into adulthood. Indeed over 90 per cent of severe, recurrent adolescent offenders showed marked antisocial behaviour in early childhood. In contrast, there is a large group who only start to be antisocial in adolescence, but whose behaviours are less extreme and who tend to desist by the time they are adults.

Severe antisocial behaviour is less common in girls who are less likely to be physically aggressive and engage in criminal behaviour, but more likely to show spitefulness, emotional bullying (such as excluding children from groups, spreading rumours so others are rejected by their peers), frequent unprotected sex leading to sexually transmitted diseases and pregnancy, drug abuse, and running away from home.

Prognosis is determined by the frequency and intensity of antisocial behaviours, the variety of types, the number of settings in which they occur (e.g. home, school, and in public), and their persistence. For general populations of children, the correlation between parent and teacher ratings on the same measures is only 0.2 to 0.3, so that there are many children who are perceived to be mildly or moderately antisocial at home but well behaved at school, and vice versa. However, for more severe antisocial behaviour, there are usually manifestations both at home and at school.

At home the child often is subject to high levels of criticism and hostility, and sometimes made a scapegoat for a catalogue of family misfortunes. Frequent punishments and physical abuse are not uncommon. The whole family atmosphere is often soured and siblings also affected. Maternal depression is often present, and families who are unable to cope may, as a last resort, give up the child to be cared for by the local authority. At school, teachers may
take a range of measures to attempt to control the child and protect the other pupils, including sending the child out of the class, sometimes culminating in permanent exclusion from the school. This may lead to reduced opportunity to learn subjects on the curriculum and poor examination results. The child typically has few if any friends, who get fed up with their aggressive behaviour. This often leads to exclusion from many group activities, games, and trips, so restricting the child’s quality of life and experiences. On leaving school the lack of social skills, low level of qualifications, and presence of a police record make it harder to gain employment.

To make a diagnosis, three symptoms from this list have to be present, one for at least 6 months. There is no impairment criterion. There are three subtypes: conduct disorder confined to the family context (F91.0), unsocialized conduct disorder (F91.1, where the young person has no friends and is rejected by peers), and socialized conduct disorder (F91.2, where peer relationships are normal). It is recommended that age of onset be specified, with childhood onset type manifesting before age 10, and adolescent onset type after. Severity should be categorized as mild, moderate, or severe according to number of symptoms or impact on others, e.g. causing severe physical injury, vandalism, theft.

For younger children, say up to 9 or 10 years old, there is a list of eight symptoms for the subtype known as Oppositional Defiant Disorder (F91.3):

To make a diagnosis of the oppositional defiant type of conduct disorder, four symptoms from either this list or the main conduct disorder 15 symptom list have to be present, but no more than two from the latter. Unlike the main variant, there is an impairment criterion: the symptoms must be amaladaptive and inconsistent with the developmental level (p. 161).

Where there are sufficient symptoms of a comorbid disorder to meet diagnostic criteria, the ICD-10 system discourages the application of a second diagnosis, and instead offers a single, combined category. There are two major kinds: mixed disorders of conduct
and emotions, of which Depressive Conduct Disorder (F92.0) is the best researched; and Hyperkinetic Conduct Disorder (F90.1). There is modest evidence to suggest these combined conditions may differ somewhat from their constituent elements.

The DSM IV-R system⁽⁵⁾ follows the ICD-10 research criteria very closely and does not have separate clinical guidelines. The same 15 behaviours are given for the diagnosis of conduct disorder 312.8, with almost identical wording. As for ICD-10, three symptoms need to be present for diagnosis. Severity and childhood or adolescent onset are specified in the same way. However, unlike ICD-10, there is no division into socialized/unsocialized, or family context only types, and there is a requirement for the behaviour to cause a clinically significant impairment in social, academic, or social functioning. Comorbidity in DSM IV-R is handled by giving as many separate diagnoses as necessary, rather than by having single, combined categories.

In DSM IV-R, oppositional defiant disorder is classified as a separate disorder on its own, and not as a subtype of conduct disorder. Diagnosis requires four symptoms from a list of eight behaviours which are the same as for ICD-10, but unlike ICD-10, all four have to be from the oppositional list, and none may come from the main conduct disorder list. It is doubtful whether oppositional defiant disorder differs substantially from conduct disorder in older children in any associated characteristics, and the value of designating it as a separate disorder is arguable. In this article, the term conduct disorder will henceforth be used as it is in ICD-10, to refer to all variant including oppositional defiant disorder.

The sex ratio is approximately 2:5 males for each female overall, with males further exceeding females in the frequency and severity of behaviours. On balance, research suggests that the causes of conduct problems are the same for the sexes, but males have more conduct disorder because they experience more of its individual-level risk factors (e.g. hyperactivity, neurodevelopmental delays). However, recent years have seen increasing concern amongst clinicians about treating antisocial behaviour amongst girls.⁽¹¹⁾

There has been considerable attention paid to the distinction between conduct problems that are first seen in early childhood versus those that start in adolescence⁽²⁾ and these two subtypes are encoded in the DSM-IV. Early onset is a strong predictor of persistence through childhood, and early onset delinquency is more likely to persist into adult life. Those with early onset differ from those with later onset in that they have lower IQ, more attentional and impulsivity problems, poorer scores on neuropsychological tests, greater peer difficulties and they are more likely to come from adverse family circumstances.⁽²⁾ Those with later onset become delinquent predominantly as a result of social influences such as association with other delinquent youths. Findings from the follow-up of the Dunedin cohort support relatively poorer adult outcomes for the early onset group in domains of violence, mental health, substance abuse, work, and family life.⁽²⁾ However the ‘adolescence-limited’ group were not without adult difficulties. As adults they still engaged in self-reported offending, and they also had problems with alcohol and drugs. Thus, the age-of-onset subtype distinction has strong predictive validity, but adolescent onset antisocial behaviours may have more long-lasting consequences than previously supposed, and so both conduct problems warrant clinical attention.