Section of Forensic Mental Health, Institute of Psychiatry, London
Department of Forensic Psychiatry, Institute of Psychiatry, London
Section of Forensic Mental Health, Institute of Psychiatry, London
Correspondence: Dr Elizabeth Walsh, Section of Forensic Mental Health, Institute of Psychiatry, De Crespigny Park, Denmark Hill, London SE5 8AF. E-mail: sppmemw{at}iop.kcl.ac.uk
Declaration of interest E.W. was funded by a Wellcome Training Fellowship.
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ABSTRACT |
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Aims To critically examine the epidemiological evidence for the association between violence and schizophrenia and estimate the impact of this association on society.
Method A selective review of the key literature on the epidemiology of violence and schizophrenia. Population-attributable risks for violence in schizophrenia are calculated from population-based studies.
Results Most studies confirm the association between violence and schizophrenia. Recent good evidence supports a small but independent association. Comorbid substance abuse considerably increases this risk. The proportion of violent crime in society attributable to schizophrenia consistently falls below 10%.
Conclusions Less focus on the relative risk and more on the absolute risk of violence posed to society by people with schizophrenia would serve to reduce the associated stigma. Strategies aimed at reducing this small risk require further attention, in particular treatment for substance misuse.
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INTRODUCTION |
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METHOD |
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VIOLENCE STUDIES |
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Studies estimating the prevalence of violent acts among those with
schizophrenia
Two main designs have been used: cross-sectional studies and cohort studies
using case linkage technology.
Cross-sectional studies
With violence being a main selection criterion for admission, studies of
violence committed before and during hospitalisation are of limited usefulness
because they will overestimate any association. Discharged patients are a
selected group because they are generally judged not to pose a threat or to
pose less threat than those retained in hospital. As such, one would expect
lower rates of violence to be recorded at this time than prior to
admission.
Before hospitalisation. Humphreys et al (1992) estimated that 20% of first-admission patients with schizophrenia had behaved in a life-threatening manner prior to admission. Volavka et al (1997) estimated that 20% of first-contact patients with schizophrenia had assaulted another person at some time in the past.
During hospitalisation. These studies have suggested relatively high rates of assaultativeness (Karson & Bigelow, 1987; Walker & Seifert, 1994). Results must be viewed with particular caution, however, because violence may be more of a response to the contextual setting of a confined ward than to an individual's mental state.
Following discharge. The two most comprehensive studies published to date on violence risk after discharge fail to provide separate data for schizophrenia (Steadman et al, 1998; Link et al, 1992, see below). Monahan & Applebaum (2000), as part of the MacArthur Risk Assessment Study, estimated the prevalence of community violence in discharged patients by diagnosis. Violence was measured from multiple sources every 10 weeks for a year. Of the 17% of patients with a diagnosis of schizophrenia, 9% were violent in the first 20 weeks after discharge. This compares with a violence prevalence of 19% for depression, 15% for bipolar disorder, 17.2% for other psychotic disorders, 29% for substance misuse disorders and 25% for personality disorder alone. The fact that this and other studies have found rates of violence to be lower in those with schizophrenia than in those with other diagnoses (Harris et al, 1993; Wallace et al, 1998) should not be misinterpreted to suggest that schizophrenia may be irrelevant or even a protective factor against violence. It is probably true that schizophrenia is less of a violence risk than substance misuse, personality disorder and possibly other mental disorders, but when compared with the general population, as this review amply demonstrates, the evidence is overwhelmingly in favour of an increased risk of violent behaviour.
Retrospective cohorts using case linkage
Three studies using slightly different methodologies have drawn similar
conclusions. In the first, 644 patients with schizophrenia followed for up to
15 years on a police register, were found to be four times more likely to have
committed a violent crime than the general population
(Lindqvist & Allebeck,
1990).
The second study compared the rate of criminal convictions among 538 incident cases of schizophrenia with that of non-psychotic psychiatric controls matched for age and gender (Wessely et al, 1994). Male patients with schizophrenia were twice as likely as men with other mental disorders to have a violent conviction. This was despite the control group containing a substantial minority of individuals with psychiatric disorders with an established association with crime. Women with schizophrenia were also significantly more likely to be convicted of violent crime than controls.
In the third study, Mullen et al (2000), in Australia, studied two groups of patients with schizophrenia first admitted in either 1975 (before major deinstitutionalisation) or 1985 (when community care was becoming the norm). Compared with general population controls, both groups were significantly more likely to be convicted for all categories of criminal offending, except sexual offences. Those with comorbid substance abuse accounted for a disproportionate level of offending. The increased number of convictions in those with schizophrenia in the 1985 group compared with the 1975 group seemed to reflect a general increase in offending in those of a similar age, gender and place of residence. As such, the shift to community care was not marked by any significant change in relative rates of conviction in schizophrenia. The effect of community care on risk of violence in schizophrenia requires further study. One study examining homicide statistics in the UK has reported little fluctuation in the numbers of people with mental illness committing homicide between 1957 and 1995 and a 3% annual decline in their contribution to the official statistics (Taylor & Gunn, 1999).
Unselected birth cohort studies
Hodgins (1992), in a
30-year follow-up of an unselected Swedish birth cohort, found that compared
with those with no mental disorder, males with major mental disorder had a
4-fold and women a 27.5-fold increased risk of violent offences. No separate
data were provided for schizophrenia. A later study using the same methodology
revealed similar findings (Hodgins et
al, 1996).
The first cohort study to demonstrate the quantitative risk of violent behaviour for specific psychotic categories followed an unselected birth cohort of 12 058 individuals prospectively for 26 years (Tiihonen et al, 1997). The risk of violent offences among males with schizophrenia was 7-fold higher than controls without mental disorder.
Brennan et al (2000) traced all arrests for violence and hospitalisations for mental illness in a birth cohort followed to age 44 years. Schizophrenia was the only major mental disorder associated with increased risk of violent crime in both males and females, adjusting for socio-economic status, marital status and substance abuse.
Arseneault et al (2000) studied the past-year prevalence of violence in 961 young adults who constituted 94% of a total city birth cohort. Three Axis I disorders were uniquely associated with violence after controlling for demographic risk factors and all other comorbid disorders: alcohol dependence, marijuana dependence and schizophrenic spectrum disorder.
Studies estimating the prevalence of schizophrenia in individuals who
have committed violent acts
Numerous studies have estimated the prevalence of schizophrenia among
prison inmates. Despite problems of unstandardised diagnoses and the frequent
absence of comparison data among the general population, the evidence suggests
an over-representation of those with schizophrenia among offender
populations.
Taylor & Gunn (1984), using validated diagnoses, studied the psychiatric status of male prisoners remanded to a prison in south London. Nine per cent of those subsequently convicted of non-fatal violence and 11% convicted of fatal violence had schizophrenia, which are substantially higher prevalences than would have been expected in the general population for the same area (0.1-0.4%).
Teplin (1990) compared the prevalence of schizophrenia among 728 male prisoners with that of the general population. The prevalence in the jail population (2.7%) was found to be three times higher than that of the general population (0.91%) after controlling for socio-demographic factors.
Eronen et al (1996), in a study of 693 people convicted of homicide in Finland, found schizophrenia to be associated with an 8-fold increase in homicide by men and a 6.5-fold increase by women.
Wallace et al (1998), in a study of individuals convicted of serious offences in Victoria County, Australia, searched for evidence of a psychiatric contact on the county psychiatric register. Those with schizophrenia were found to be over four times more likely to be convicted of interpersonal violence and ten times more likely to be convicted of homicide than the general population.
Community prevalence studies
The above studies, although valuable in making inferences about the
relationship between violence and schizophrenia, are subject to biases that
will be discussed below. Data on unselected samples of people from the open
community are needed to augment the findings. Probably the most important
study in the violence literature to date is that of Swanson et al
(1990). Using a sample of 10
059 adult residents from Epidemiologic Catchment Area (ECA) study sites
(Eaton & Kessler, 1985),
the authors examined the relationship between violence and psychiatric
disorder. Eight per cent of those with schizophrenia alone were violent,
compared with 2% of those without mental illness. Comorbidity with substance
abuse increased this percentage to 30%.
Two other community epidemiological studies, both finding increased risk of violence among psychiatric patients (Link et al, 1992) and those with major mental disorder (Stueve & Link, 1997), respectively, failed to provide data on schizophrenia as a separate diagnostic entity.
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METHODOLOGICAL LIMITATIONS OF VIOLENCE STUDIES |
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A brief overview of each of these limitations from an epidemiological viewpoint is outlined below.
Definition and measurement of exposure
Some studies include schizophrenia as part of a heterogeneous group of
psychotic disorders (Hodgins,
1992; Hodgins et al,
1996; Steadman et al,
1998) or do not give the diagnostic breakdown of subjects at all
(Link et al, 1992).
Fewer examine schizophrenia alone
(Lindqvist & Allebeck,
1990; Wessely et al,
1994) and those that do use varying diagnostic techniques.
Diagnoses are variously derived from case notes, psychiatric registers,
clinical interviews or research interviews. Case-note diagnoses are dependent
on individual clinical judgements. Those extracted from case registers are
usually those made at discharge and are subject to the same limitations. These
diagnoses may be more reliable, however, than those made at a single clinical
interview because they are usually based on a period of observation in
hospital, collateral information and previous history, which are likely to
increase the validity of diagnoses. The use of one agreed diagnostic procedure
in studies would allow comparisons of like with like.
Definition and measurement of outcome
How violence is defined varies greatly and reported rates differ, depending
on the levels of violence measured. Unsurprisingly, studies that include
threats as well as physical contact record higher rates than those that
include contact alone. It is virtually impossible to find violence defined in
the same way in any two studies by different researchers. This highlights the
need for the development of a standardised, validated, reliable and acceptable
rating instrument that could be adopted across studies.
Measurement of violence in studies has relied upon different single (self-report, informant, case notes, official records) or combined sources of information. All sources have inherent limitations. Self-report measures may underreport violence because of the desire for social acceptability or fear of adverse consequences of reporting. Additionally, retrospective designs produce problems with recall of sometimes distant events. Informants, who are often nominated by patients, may not be the most suitable people to provide information or be aware of incidents. Case notes are of limited usefulness because they are often incomplete. With regard to police contacts or arrest records, the proportion of violent acts that leads to arrest and prosecution varies as a function of the intensity and quality of policing, behaviour of the suspect, the availability of diversion to the mental health systems and the severity of the offence.
Records of criminal convictions are a widely used data source across studies. Most violent individuals are not convicted (Elliott et al, 1986). The mentally ill tend to be diverted to the mental health care system at various stages from apprehension to conviction. As such, it is likely that only the more serious crimes will lead to conviction. For this reason, the association between schizophrenia and more minor forms of violence is impossible to estimate from this source. For more serious offending such as homicide, individuals are more likely to be brought to trial and convicted, thus justifying the dependence on criminal registers. Unfortunately, as with all such registers, they are prone to data errors, are not inclusive of all convictions and often relate to one geographical area, taking no account of crimes committed outside that jurisdiction.
The more recent use of multiple combined measures for violence has highlighted the limitations of the majority of previous studies that relied on a single source. Steadman et al (1998) used agency records, self-report and collateral informants to collect information on violent acts. The one-year period prevalence for violence was 4.5% using agency records (arrest and rehospitalisation records) alone, 23.7% by adding patient self-reported acts that had not been in agency records and 27.5% by adding collateral informant-reported acts that had not been in either agency records or patient self-reports. Thus, the final prevalence was six times higher than it would have been if estimated from agency records alone. Mulvey et al (1994a) specifically set out to compare the yield of violence when different sources were used. A dramatically different picture emerged, depending on the source. These results support the previous observation that self-report methods consistently produce a higher frequency of violence than official records (Elliott et al, 1986). Thus, to provide accurate empirical data, it is crucial that it be based on self-report in conjunction with collateral informant and official records. One problem inherent to the use of multiple measures is that judgement must be made about what constitutes a single episode of violence and how the inconsistencies that may exist between reports should be handled.
Bias
Selection bias can occur whenever the identification of individual subjects
for inclusion into a study, on the basis of either exposure or outcome status,
depends in some way on the other axis of interest. This bias will result in an
observed relationship between exposure (schizophrenia) and outcome (violence)
that is different among those who are entered into the study than among those
who would have been eligible but did not participate. For example, a psychotic
individual's refusal to participate in a study or follow-up interviews might
be related to his or her propensity for violence. If so, the rates of violence
for those included in the samples may be lower than the true rates for
individuals with schizophrenia.
Location of recruitment is a crucial factor in interpreting any such association. Research on violence and mental illness is dominated by data on hospitalised/discharged patients, but most individuals with mental disorder are not hospitalised (Robins & Reiger, 1991). Cross-sectional prevalence studies in representative samples of community residents with both treated and untreated mental disorders largely overcome the problem of selection bias, although not completely. They frequently exclude those in jail (Steadman et al, 1998) and, as such, will underestimate any association.
It is not unusual to find high refusal and attrition rates in these studies, also leading to selection bias. In one study, only 50% of subjects completed all five follow-up interviews. These compliant subjects were found to be significantly less likely to have a history of previous violence a major predictor of future violence than those lost to follow-up (Steadman et al, 1998).
In analytical studies, the risk of violent offending in cases is expressed relative to the risk in controls. It is thus important that the results be interpreted with specific reference to the control group chosen. If, for example, risk of offending in schizophrenia is estimated relative to non-psychotic psychiatric controls (Wessely et al, 1994), the risk ratio will depend on whether or not that group contains an excess of patients with personality disorder and substance abuse disorders, both of which are linked to violent behaviour. If national or population-based figures are used for comparison, they may not take into account the confounding effect of social class on violence (Wallace et al, 1998). Alternatively, if neighbourhood controls are chosen, the estimated risk may not be generalisable to the population at large (Steadman et al, 1998).
Other possible biases include interviewer bias and recall bias. On reading most violence studies it is unclear whether interviewers were blind to subject status. If not, selective probing for symptoms of mental illness and/or violent episodes may result in interviewer bias.
Confounding
A confounder is a factor that is associated with the exposure
(schizophrenia) and, independent of this exposure, is a risk factor for
outcome (violence). Additionally, it should not be on the causal pathway
between exposure and outcome. Statistical relationships observed between
schizophrenia and violence in any particular study will hinge on the
investigator's understanding and statistical treatment of confounding factors
(Arboleda-Florez et al,
1998). Because of the uncertainty of the causal pathway between
schizophrenia and violence, it is unclear what variables should be considered
as confounders. The more robust studies do control for a range of possible
confounding factors, but these are by no means uniform. The relationship is
even more complex than this, however, with a wide range of personal and
situational factors that must be important in the mediation of violence being
impossible to measure.
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PREDICTORS OF VIOLENT BEHAVIOUR IN SCHIZOPHRENIA |
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Two factors appear to discriminate those with schizophrenia at increased risk of committing violent acts: comorbid substance abuse and acute psychotic symptoms.
It has been demonstrated repeatedly that schizophrenia with comorbid substance abuse increases the risk of violence considerably compared with schizophrenia without comorbidity (Swanson et al, 1990; Cuffel et al, 1994; Tiihonen et al, 1997; Wallace et al, 1998). It is important to note that because there is an increase in violence risk in those without comorbidity, substance abuse merely increases the level of risk rather than causing it (Arsenault et al, 2000; Brennan et al, 2000). Hence, the risk from substance abuse appears to be additive.
With regard to acute symptomatology, Taylor estimated that 46% of a sample of psychotic offenders were definitely or probably driven by delusions (Taylor, 1985). But delusions are an extremely common psychopathological phenomenon in psychosis and serious violence is not, so other factors must be operating (Taylor, 1998).
In a methodologically robust study, Link et al (1992) compared arrest rates and self-reported violence in a sample of community residents with no history of psychiatric contact with current and former patients with heterogeneous diagnoses from the same area. Former patients invariably were more violent than the never-treated community sample and almost all the difference between the groups could be accounted for by active symptoms. A further study revealed that specific threat/control override symptoms largely explained the relationship. These threat/control override symptoms represent experiences of patients feeling that people are trying to harm them and experiences of their minds being dominated by forces outside their control. These results have been replicated subsequently (Swanson et al, 1990, 1996, 1997; Link et al, 1998). The data in these studies, however, have been criticised for being retrospective, having been gathered for other purposes and having weak measures of delusions and violence. The MacArthur Violence Risk Assessment Study has largely overcome these methodological limitations and casts doubt on the importance of threat/control override delusions as mediators for violence (Appelbaum et al, 2000). Neither delusions in general nor threat/control override delusions in particular were found to be associated with an increased risk of violence in this study. The authors suggest that the reliance on self-report in previous studies may have resulted in the mislabelling of other phenomena that can contribute to violence as delusions.
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RISK OF VIOLENCE IN SOCIETY |
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Most of the above figures represent a fairly small percentage of the total violence in these populations. The problem is that the PAR% assumes that causality has been established. It thus fails to take into account other risk factors or confounding factors that may be operating in the association between a particular risk factor and disease. As we have seen previously, for example, comorbidity substantially increases the risk of violence in schizophrenia and it is thus possible that if substance abuse was to be eliminated from the population, the contribution to violence made by schizophrenia alone would be much less.
To prevent unnecessary stigmatization of the seriously mentally ill, it is the duty of researchers to present a balanced picture. By neglecting to report measures of both relative and absolute risk, a skewed picture may emerge. One example of a balanced report found that men with schizophrenia were up to five times more likely to be convicted of serious violence than the general population (Wallace et al, 1998). Results also presented indicated that 99.97% of those with schizophrenia would not be convicted of serious violence in a given year and that the probability that any given patient with schizophrenia will commit homicide is tiny (approximate annual risk is 1:3000 for men and 1:33 000 for women).
Risk is generally presented in terms of odds ratios, yet research has shown that people find it difficult to digest such measures. Better ways are required for presenting risk magnitudes in a digestible form, and a logarithmic scale provides the basis for a common language for describing risk (Calman & Royston, 1997). It has been suggested that community risk scales that describe the magnitude of risk in relation to an individual's community may be most useful. If communities are grouped into roughly logarithmic clusters (e.g. individual (1), family (10), village (1000), etc.), then such a classification allows individuals to think in terms of level of risk to themselves, their family, their town and so forth. This system also allows a consideration of how the risk of violence by people with schizophrenia compares with other risks, if also presented in the same way.
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CONCLUSION |
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Clinical Implications and Limitations |
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LIMITATIONS
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Received for publication March 8, 2001. Revision received August 14, 2001. Accepted for publication August 14, 2001.
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