Social, Genetic and Developmental Psychiatry Centre, Institute of Psychiatry
Division of Psychological Medicine, Institute of Psychiatry, London, UK
Dunedin Multidisciplinary Health and Development Research Unit, University of Otago, New Zealand
Social, Genetic and Developmental Psychiatry Centre, Institute of Psychiatry, London, UK
Correspondence: Louise Arseneault, Institute of Psychiatry, PO Box 80, De Crespigny Park, London SE5 8AF, UK. E-mail: l.arseneault{at}iop.kcl.ac.uk
Declaration of interest None. Funding detailed in Acknowledgements.
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Aims To examine whether violent behaviour in adults with psychosis can be accounted for by psychotic symptoms or physical aggression in childhood.
Method We used data from a prospective longitudinal study of a complete birth cohort born in New Zealand. When cohort members were 26 years old, information was obtained on past-year psychiatric diagnosis of schizophreniform disorder and on violent behaviour. Childhood psychotic symptoms were measured at age 11 years using a diagnostic interview, and childhood physical aggression was assessed by teachers when cohort members were aged 7, 9 and 11 years.
Results Participants with schizophreniform disorder were more likely to be violent than participants without, even after controlling for sociodemographic variables and concurrent substance dependence disorders. Childhood psychotic symptoms were a strong risk factor for violence in adults with schizophreniform disorder, as was childhood physical aggression, although to a lesser extent.
Conclusions Violence by individuals with schizophreniform disorder could be prevented by monitoring early signs of psychotic symptoms and by controlling childhood physical aggression.
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One important question yet to be answered is whether violent behaviour is a direct consequence of psychosis, or whether violent tendencies are already present from an early age in individuals who will develop psychotic disorders in adulthood. The longitudinal Dunedin study allows us to carry out such an analysis. This birth cohort has prospective measures of physical aggression throughout childhood, a self-report measure of psychotic symptoms at age 11 years, as well as measures of violent behaviour and psychiatric disorders at age 26 years. A previous analysis from the Dunedin study showed a strong concurrent relationship between violent behaviour and schizophreniform disorder when study participants were aged 21 years, the peak age for committing violent crimes (Arseneault et al, 2000). In this report we examine the association between violence and schizophreniform disorder in this group at age 26 years, controlling for comorbid substance dependence disorders, and investigate whether violent behaviour in adults with schizophreniform disorder can be accounted for by childhood psychotic symptoms or childhood physical aggression.
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Adult schizophreniform disorder
Data on schizophreniform disorder at age 26 years were collected in a
private interview using the Diagnostic Interview Schedule for DSMIV
(Robins et al, 1995).
The reporting period covered the 12 months prior to the interview. At this
assessment the diagnosis of schizophreniform disorder required:
Our interview protocol ruled out symptoms occurring under the influence of alcohol or drugs. Following this protocol, 3.7% of the cohort assessed at age 26 years met diagnostic criteria for schizophreniform disorder, including 1% of the cohort who met full criteria for schizophrenia. Further details on the procedures for ascertaining schizophreniform disorder are explained elsewhere (Poulton et al, 2000).
Adult substance dependence disorders
Seventeen per cent of the total sample met DSMIV diagnostic criteria
for alcohol dependence (12-month prevalence), 9.3% met diagnostic criteria for
cannabis dependence and 3.6% met criteria for dependence on other drugs
(amphetamines, sedatives, cocaine, crack cocaine, opiates, hallucinogens or
inhalants). Because cannabis and alcohol dependence disorders were both
associated with an increased risk of violence in this cohort
(Arseneault et al,
2000), we created a categorical variable grouping participants who
met diagnostic criteria for at least one of these three substance dependence
disorders (23.3% of the cohort).
Adult violent behaviour
For this report we used two different measures of violent behaviour: court
convictions and self-reports. Court convictions for violence in all New
Zealand and Australian courts were obtained by searching the central computer
system of the New Zealand police. In the Dunedin sample, violent convictions
included inciting or threatening violence, using an attack dog on a person,
presenting an offensive weapon, threatening a police officer, rape, manual
assault, assault on a police officer, assault with a deadly weapon, aggravated
robbery, and homicide. We focused on violent convictions at ages 2126
years, because there were too few violent convictions in the year prior to the
interview, when the participants were aged 26 years, to analyse separately.
Forty-eight men and nine women (5.9% of the sample) were defined as violent
offenders according to official records for this 5-year period.
To ascertain whether violent behaviours were occurring during the past-year reporting period for schizophrenic symptoms, self-reports of violence committed during the past year were obtained using a private standardised interview developed for the National Youth Survey and National Institute of Justice multi-site surveys (Elliott & Huizinga, 1989). Violent offences included information about seven different types of violence: simple assault, aggravated assault, gang fighting, robbery, forced sex, domestic violence, and hitting or otherwise hurting a child out of anger (with follow-up questions ruling out situations of physical discipline). A single simple assault was quite common (13.5%), but thereafter the distribution of the violence measure was strongly skewed to more serious offending; therefore individuals who reported two or more different types of violent offences, 4.4% of the sample (34 men and 9 women), were defined as self-reported violent offenders.
Self-reported and court-recorded violence overlapped in the sample. The odds of conviction were 18 times greater for people self-reporting two offence types than for people who did not (95% CI 9.336.7). A combined violent group was constituted of 81 persons who either had self-reported committing at least two different types of violent offences in the past year or had been convicted of one (8.4% of the sample) in the past 5 years.
Childhood psychotic symptoms
At age 11 years, cohort members were administered the Diagnostic Interview
Schedule for Children (DISCC;
Costello et al, 1982)
for DSMIII criteria (American
Psychiatric Association, 1980) by a child psychiatrist
(Anderson et al,
1987). The schizophrenia section of the DISCC asked five
questions about possible psychotic symptoms. The items were scored by the
psychiatrist (0 no; 1 yes, likely; 2 yes, definitely). Participants were
divided according to the strength of the symptoms: no symptoms
included children who did not report any symptoms (n=653, 86.0% of
the 759 participants with complete childhood data); weak
symptoms included children who answered yes, likely to
one symptom (n=94, 12.4%); and 'strong symptoms' included children
who answered yes, likely to two symptoms or yes,
definitely to one symptom (n=12, 1.6%). We have previously
shown that these self-reported psychotic symptoms at age 11 years predicted a
schizophreniform diagnosis at age 26 years
(Poulton et al, 2000;
Cannon et al,
2002a).
Childhood physical aggression
Childhood fighting was assessed by the children's teachers at cohort ages 7
years, 9 years and 11 years, using the Rutter Child Scales
(McGee et al, 1985).
At each assessment age, teachers reported whether the child had fought with
other children in the past year (0 no, doesn't apply; 1 yes, applies somewhat;
2 yes, certainly applies). We divided the sample into three discrete groups
based on the levels of physical aggression between ages 7 years and 11
years:
Parental socio-economic status
The socio-economic status of participants' families was coded into one of
six categories based upon the educational level and income associated with
occupations in data from the New Zealand census
(Elley & Irving, 1976).
The scale ranged from 1 (unskilled labourer) to 6 (professional). The variable
used in our analyses, parental socio-economic status, is the average of the
highest socio-economic status level of either parent across the seven
assessments of the Dunedin study from birth to age 15 years; thus, it reflects
the socio-economic conditions experienced by the children while they grew
up.
Statistical analyses
Logistic regression techniques were used to investigate the association
between schizophreniform disorder at age 26 years and different measures of
violence (self-reported, violent convictions, and the combined measure). In
this analysis, we included all participants with complete data in adulthood
(n=970). We report the risk, estimated by odds ratios with 95%
confidence intervals, for people with schizophreniform disorder to be violent
according to each of the three measures of violence. We report unadjusted odds
ratios, odds ratios adjusted for gender and parental socio-economic status,
and finally odds ratios adjusted for gender, parental socioeconomic status and
substance dependence disorders at age 26 years, simultaneously. The addition
of gender interaction terms did not yield significant improvements in the fit
of models predicting violence above and beyond models with main effects only.
Consequently, analyses were performed on the whole sample.
We then investigated whether the association between schizophreniform disorder and violent behaviour (using the combined measure) could be accounted for by childhood psychotic symptoms or childhood physical aggression. First, we ascertained the association between schizophreniform disorder and violence within the subsample of individuals with complete data at age 11 years (n=759) using logistic regression analyses. Second, we adjusted the model for self-reported psychotic symptoms at age 11 years (two groups: weak and strong symptoms). The hypothesis was that if violence were significantly associated with schizophreniform disorder at age 26 years and this association were reduced in the second analysis, this change would indicate that the initial association was partly due to the presence of psychotic symptoms prior to diagnosable schizophreniform disorder. Third, we adjusted the model for childhood physical aggression (two groups: low and high aggression). Finally, we adjusted the model for both childhood psychotic symptoms and childhood physical aggression simultaneously.
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View this table: [in a new window] | Table 1 Proportions and risks of violent behaviour among participants with schizophreniform disorder at age 26 years |
Childhood risk factors for violence among adults with
schizophreniform disorder
Reduction in the sample size because of missing data at age 11 years did
not affect the association we observed between violence and schizophreniform
disorder within the complete sample: the risk of violent behaviour adjusted
for gender and socio-economic status in the full sample was 4.71
(Table 1), compared with a risk
of 4.07 in the reduced sample (Table
2, model 1). When we controlled for psychotic symptoms at age 11
years (Table 2, model 2), the
risk of violence in individuals with schizophreniform disorder at age 26 years
was reduced to 2.80, a reduction of 31.2%. This indicates that one-third of
the risk of adult violence associated with diagnosable schizophreniform
disorder at age 26 years was accounted for by psychotic symptoms in
childhood.
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View this table: [in a new window] | Table 2 Association between schizophreniform disorder and violence at age 26 years: results of hierarchical logistic regression analyses controlling for childhood psychotic symptoms and childhood physical aggression (n=759) |
When childhood physical aggression was entered into the model (Table 2, model 3), the risk slightly decreased from 4.07 to 3.76, indicating that childhood physical aggression accounted for a small proportion of the association between adult schizophreniform disorder and violence. Nevertheless, in model 4, when both childhood psychotic symptoms and childhood physical aggression were entered simultaneously, the risk of violence in people with schizophreniform disorder decreased to 2.51, a reduction of 38.3% of the initial violence risk, and became nonsignificant. This suggests an additive effect of childhood psychotic symptoms and childhood physical aggression in accounting for the association between violence and schizophreniform disorder in adulthood.
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Stability of the risk of violence in adults with schizophreniform
disorder
The Dunedin study is the only existing longitudinal cohort study to report
on the violence risk in people with schizophreniform disorder at two different
assessment points. A comparison of this study with our previous report, when
the cohort members were aged 21 years
(Arseneault et al,
2000), reveals that the violence risk is relatively similar 5
years later (OR = 4.60 at age 21 years v. OR = 4.71 at age 26 years)
and the population attributable risk remains non-trivial (10% at age 21 years
v. 8% at age 26 years). Thus, the association between
schizophreniform disorder and violence appears to be strong and stable in
early adulthood.
Risk factors for violence among individuals with schizophreniform
disorder
Researchers are now starting to examine potential risk factors and causal
mechanisms for the association between schizophrenia and violence. Since the
sine qua non of a cause is that it precedes the outcome, such
research requires prospective longitudinal data. Cannon et al
(2002b) found that
childhood attentional impairment was a risk factor for later violent and
criminal behaviour in schizophrenia, and there was a marginally significant
increased risk of violence among males with schizophrenia who had a history of
labour or delivery complications. Hodgins et al
(2002) found that a history of
neonatal complications increased the risk of offending by men with major
mental disorders. In our previous study using the Dunedin cohort, we showed
that a juvenile history of conduct disorders and an adolescent personality
trait characterised by excessive perceptions of threat in the environment
partly accounted for violent behaviour in adults with schizophreniform
disorder (Arseneault et al,
2000). Supported by the study reported here, evidence is thus
accumulating to suggest that at least some individuals with schizophrenia may
be at risk of later violence for a variety of reasons (both neurodevelopmental
and behavioural) long before a diagnosis is made.
Childhood psychotic symptoms
What role do psychotic symptoms have in the aetiology of violent behaviour?
Although findings are not always replicated, cross-sectional studies show a
significant association between violent behaviour and certain psychotic
symptoms, particularly threat/control override symptoms
(Link et al, 1992;
Link & Stueve, 1995;
Appelbaum et al,
2000). These symptoms refer to a feeling of threats by others
(have you felt that there were people who wished to do you
harm?) and diminished self-control mechanisms (have you felt
your mind dominated by forces beyond your control?). Both the nature of
these symptoms and the specificity of the association with these symptoms only
were taken to suggest that psychotic disorder causes violence
(Link & Stueve, 1995), but
in fact the temporal relationship could not be assessed in previous
cross-sectional studies. To our knowledge, our study is the first to examine
the association between psychotic symptoms in childhood and later violent
behaviour. We found that a history of self-reported psychotic symptoms in
childhood accounts for a substantial proportion of the association between
violence and schizophreniform disorders at age 26 years.
Childhood physical aggression
Our findings suggest that individuals with psychotic disorders do not
manifest violent behaviour suddenly as their psychiatric disorder appears.
Rather, some such individuals are likely to have had past experiences of
fighting when they were children. It has been repeatedly demonstrated that a
history of violence in childhood is associated with violence in adulthood
(Farrington, 1994; Tremblay et al, 1994;
Nagin & Tremblay, 2001;
Stevenson & Goodman,
2001), and individuals with schizophrenia are unlikely to be an
exception. Further examination of our data indicated that 6.7% of participants
with a high level of physical aggression in childhood met diagnostic criteria
for schizophreniform disorder at age 26 years, compared with 3.7% of
participants with a low level of physical aggression in childhood and 3.1% of
participants with no history of childhood aggression. Indeed, we found that
violence among adults with schizophreniform disorder could be partly accounted
for by a history of early childhood physical aggression. However, a history of
self-reported psychotic symptoms at age 11 years appeared to have a stronger
effect.
Implications of the findings
This study provides support to previous findings from the Dunedin study in
showing that violence by adults with schizophreniform disorder is accounted
for by two different sets of early risk factors
(Arseneault et al,
2000): conduct disorders and psychotic symptoms. Findings from
this report go further in showing that these risk factors are present even in
childhood.
Early signs of psychotic symptoms and childhood physical aggression ought to be taken seriously by mental health practitioners, parents and teachers interacting with children. Children's bizarre thoughts that indicate a sense of lack of control over their mind and body should be the target of screening. These symptoms not only represent a risk factor for later schizophreniform disorder (Poulton et al, 2000), they also account for violent behaviour among these individuals. Early preventive strategies to reduce physical aggression among young children (Dodge, 2002) are likely to be more effective in decreasing violent behaviour among people with psychosis than is intensive case management in adulthood (Walsh et al, 2001).
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LIMITATIONS
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