ORIGINAL PAPER

J. Arboleda-Flo rez á H. Holley á A. Crisanti

Understanding causal paths between mental illness and violence

Abstract The stigma associated with mental illness is amajor concern for patients, families, and providers ofhealth services. One reason for the stigmatization of thementally ill is the public perception that they are violentand dangerous. Although, traditionally, mental healthadvocates have argued against this public belief, a recentbody of research evidence suggests that patients whosu�er from serious mental conditions are more prone toviolent behaviour than persons who are not mentally ill.It is a point of contention, however, whether the rela-tionship between mental illness and violence is only oneof association, or one of causality; that mental illnesscauses violence. A proven causal association betweenmental illness and violence will have major consequencesfor the mentally ill and major implications for caregiv-ers, communities, and legislators. This paper outlines thekey methodological barriers precluding casual inferencesat this time. The authors suggest that a casual inferenceabout mental illness and violence may yet be hasty.Because a premature statement advocating a causal re-lationship between mental illness and violence couldincrease stigma and have devastating e�ects on thementally ill the authors urge researchers to consider thedamage that may be produced as a result of poorlysubstantiated causal inferences.

Introduction

Decreasing the stigma associated with mental conditionsis a major goal of many organizations that work forbetter health services, more research infrastructure, in-creased understanding and acceptance of patients intheir communities, and more respect for the rights of thementally ill [1±4]. These organizations have a tradition

of community activism aimed at reducing the stigmaassociated with mental conditions, demystifying mentalillness, and decreasing public prejudice. A stereotypedview of the mentally ill as dangerous, unpredictable, andviolent [5±7], however, seems to be supported by a recentbody of research that identi®es certain subgroups ofpersons with serious mental illness as prone to violentbehaviour.

The issue of whether violence is associated withmental illness is an important and complex problem thathas far-reaching consequences for patients and theircaregivers, for community support programs, and forlegislators. However, the literature on the nature of theassociation is contradictory. It ranges from statementsabout the burnout of persons with antisocial personalitydisorders [8], studies on the rate of arrest of mentalpatients [9], studies of mental illness in incarceratedpopulations [10], population-based and communitystudies [11, 12], risk assessment [13], biological studies[14], and review or position papers discussing the pur-ported association [15±17].

The growing complexity of the literature and recentuncertainty concerning the association of mental illnessand violence prompted the Mental Health Division ofHealth Canada to commission a critical appraisal of theliterature. The aim was to assess whether su�cient evi-dence existed to conclude that mental illness causes vi-olence. In their work the investigators received adviceand guidance from a national Steering Committeecomposed of representatives and stakeholders fromdi�erent organizations and agencies that had manifestedan interest in the subject matter of the review. Byagreement, the Steering Committee did not in¯uence oralter the authors' ®nal conclusions. The agencies thatparticipated included the Mental Health Division ofHealth Canada, the Canadian Mental Health Associa-tion, Canadian Psychiatric Association, John HowardSociety of Canada, National Network for MentalHealth, and the Schizophrenia Society of Canada.

This paper will not repeat the critical summary ofthe literature contained in the original report [18].

Soc Psychiatry Psychiatr Epidemiol (1998) 33: S38±S46 Ó Springer-Verlag 1998

J. Arboleda-Flo rez (&) á H. Holley á A. CrisantiDepartment of Psychiatry, Queen's University,Kingston, Ontario, Canada, K7L 3N6

Rather the goal is to discuss key conceptual andmethodological issues identi®ed as a result of the re-view and, where appropriate, illustrate these pointsusing selected examples from the literature. Based onthis discussion we will argue that conclusions about acausal relationship between mental illness and violencemay well be hasty and that the empirical evidenceconcerning a statistical association between mentalillness and violence remains largely equivocal. Apremature statement supporting a causal relationshipbetween mental illness and violence could increase thestigma and have devastating e�ects on the mentally ill.Therefore, we ask researchers to consider the damagethat could result from a general statement of causal-ity, and ask that they adopt a stricter standard ofproof in this ®eld of research than has recently beenthe case.

Overview of the ®eld

The reader is referred to the original report for thefull literature review on which our conclusions arebased and the details of the search strategy used toidentify relevant literature [18]. By way of an over-view, the relationship of mental illness to violence hasbeen studied from a variety of perspectives and thesehave o�ered a number of vantage points from whichto view the subject. Studies have included samples ofthe general population [19±23], birth cohorts identi®edand tracked using record linkage technologies [12, 24,25], psychiatric patients [26±37], incarcerated o�enders[38±48], as well as other groups [49±52]. In addition,the relationship between mental illness and violencehas been widely discussed in a number of key positionpapers [53±66].

Studies conducted during the past 10±15 years havereported a statistical association between mental illnessand violence. Since they have adopted di�erent meth-odological approaches, but have come to similar con-clusions, it has been argued that the accumulatedevidence supports a causal relationship between mentalillness and violence; that the consistency of ®ndingsacross studies overshadows the methodological weak-nesses of any one [67].

We would argue that the methodological weaknessesinherent in the various approaches to study the issuedo not cancel each other out. Instead we would askreaders to consider the possibility that they build uponeach other to yield an inaccurate and distorted picture.Cohort studies of at-risk individuals (individuals whohave not already been violent) conducted in popula-tions representative of all mentally ill (combiningtreated and untreated) are needed before general causalstatements could be made with any con®dence. To dateno such studies have been conducted. In our review,empirical evidence could not be found to support thetwo key hypotheses on which a causal statement wouldhinge:

1. That representative samples of at-risk persons withmental illness were more likely to commit violencethan comparable controls

2. That people with mental illness accounted for a largeproportion of the violence experienced by the generalpublic

We consider that both hypotheses would have to be wellestablished in order to warrant the general conclusionthat mental illness per se causes violence. A number ofkey methodological ¯aws were encountered throughoutthe recent literature. These included a lack of adjustmentfor comparisons and resulting confounding, poor use ofstandardized psychiatric diagnostic conventions, lack ofcontrol for e�ects of psychiatric or other medications intreated samples, selection bias, information bias, mis-classi®cation of mental illness or violence, and de®cien-cies in the temporal ordering of purported casualfactors.

Considering the uncertainty resulting from these dif-®culties, only limited conclusions could be supportedfrom the available literature, such as:

1. The prevalence of mental illness (particularly sub-stance abuse disorders) among incarcerated popula-tions is high

2. When released into the community, mental patientsare at high risk of subsequent arrest and violence,especially if they have a history of prior arrests andviolence, or if they experience psychotic symptoms

3. Hospitalized mental patients are at high risk ofcommitting violence while in hospital

4. Family members, not the general public, are the mostlikely targets of violence from untreated mentally ill

Key conceptual and methodological issues raised duringour literature review are discussed in more detail below,followed by selected examples from the literature con-sidered to be broadly illustrative of the methodologicalpoints raised.

Conceptual and methodological issues

The nature of causality

Despite Lord Russell's suggestion that the concept ofcause does not play a useful role in science [68], andCook and Campbell's [69] observation that ``the episte-mology of causation is at present in a state of nearchaos,'' productive dialogue concerning whether mentalillness causes violence must give at least ¯eeting con-sideration to the nature of causality and the philosoph-ical issues surrounding causal determinations. However,any attempt to outline speci®c causal criteria must betempered by the knowledge that philosophers have beenabsorbed by this question for centuries. For example,David Hume [70] in his famous positivist analysis ofcause, concluded that cause cannot be directly demon-strated, but can be invoked when high correlations are

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involved, and inferred if three conditions are present:contiguity between the presumed cause and e�ect; tem-poral precedence; and constant conjunction, meaningthat the cause is always present whenever the e�ect isobtained. Hume's analysis has led to the equating ofcausation with high correlations and powerful predic-tion. However, he and other positivists have been criti-cized for their inability to consider uncertainties aboutcausal connections.

Judgement under uncertainty is in the nature of cause[71] because a cause is seldom both su�cient and nec-essary to produce an event [72]. This is apparent inMill's [73] three methods to determine causal inferences:the method of agreement, that an e�ect will be presentwhen the cause is present; the method of di�erence, thatthe e�ect will be absent when the cause is absent; and hismost important contribution to the issue, the method ofconcomitant variation, that when the ®rst two condi-tions are present, causal inference will be the strongersince other interpretations of the covariation can beruled out.

Covariations naturally introduce uncertainties, espe-cially since there is a di�erence between events andconditions. As Mackie [74] pointed out, events are morecausal than standing conditions; intrusive events aremore causal than events that occur generally; andsomething abnormal or wrong is more causal than whatis normal and right. His characterization is applicable tothe issue of mental illness and violence in that mentalillnesses are usually chronic conditions that involve fewimmediate and intrusive events of the kind that, inMackie's view, would facilitate causal interpretation.More recently, this theme has been picked up by Einhornand Hogarth [75]. These authors have singled out fourcomponents to judge probable cause: a causal ®eld orcontext; use of various probabilistic indicators of ``causalrelations''; judgmental strategies employed to combinethe causal ®eld with cues-to-causality; and discountingof causal strength by alternative explanations.

Thus, causal inferences are supportable from empir-ical evidence from well-designed and executed researchonly if no compelling discon®rmatory evidence can befound. Judgements of causality are just that: judgements.Thus, by de®nition, and by the very nature of the ob-servational data on which they rest, they must remainuncertain. Whether scientists accept a more lenientstandard of proof based on probable cause, or whetherthey will require overwhelming proof beyond a reason-able doubt, is a matter that varies by context. Becausethe standard is often implicit and varies from scientist toscientist, it should be clearly articulated and openly de-bated, particularly in areas where there is the potentialto cause great good or great harm. In other words,standards of proof should be accepted with full know-ledge of the human impact that a type I error will have,in this case on people with mental illnesses and theirfamilies.

Because of the stigma and harm that could resultfrom a hasty causal determination, our preference in

reviewing the literature was for the stricter standard. Ajudgement of whether the accumulating research sup-ported a causal determination was made on the qualityof the methodology (and therefore the evidence pro-vided), the absence of threats to validity, the possibleimpact of biases, and the respect for temporal orderingof factors. We also followed Popper's emphasis on theambiguity of information and the consequent need foreach researcher to explicitly rule out competing expla-nations [76]. We were also aware that internal validity inany one group of studies, such as those focussing on aselected population, was not su�cient to make a gen-eralized statement of cause applicable to all populations.Finally, consistent with an epidemiologic understandingof cause [77] that may have found its best expression inthe US Surgeon General's Report on Smoking andHealth [78], we recognized that statistical methods,however well executed or precise, can never establish acausal relationship. Rather, as previously discussed, thecasual signi®cance of an association is a matter ofjudgement that goes beyond any statement of statisticalprobability. Working from this philosophical andmethodological foundation, we identi®ed a number ofmethodological issues that precluded us from making acausal determination at this time.

Confounding

In 1983, Monahan and Steadman [16] conducted a lit-erature review now regarded as classic in this ®eld. Theirconclusion informed the discourse and served astouchstone for mental health providers and scientistsalike in their understanding of the relationship betweenmental illness and criminality:

While the unadjusted crime rate of the mentally ill is indeed higherthan that of the general population, and the unadjusted rate of mentaldisorder among criminals is indeed higher than among the generalpopulation, both relations tend to disappear when the appropriatestatistical adjustments are made for age, social class, and prior ex-posure to the mental health and criminal justice systems. (p. 181)

In 1993, Monahan [17] questioned this conclusion and,indeed, reversed it: ``I now believe that this conclusion isat least premature and may well be wrong'' (pp. 287±288). His reasoning highlighted the fact that social classand previous institutionalization are highly related tomental disorder; mental disorders cause people to de-cline in social class and also to become violent. In hisopinion, therefore, these two factors should not be sta-tistically controlled in the analysis. To control for lowsocial class, he reasoned, would attenuate the relation-ship between mental disorder and violence to some un-known degree. If, on the other hand, a mental disorderalso causes a person to become institutionalized, then, tocontrol for institutionalization would mask the rela-tionship between mental illness and violence to someunknown degree. This discussion reveals that the sta-tistical relationship observed between mental illness andviolence in any particular study will hinge on the in-

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vestigator's understanding of confounding factors andtheir appropriate statistical treatment; in other words,whether key variables are controlled out of the analysis,or whether they are considered to be part of the causalchain of events and left in.

``Confounding'' is technically de®ned as confusingthe e�ects of an extraneous variable with the e�ects of astudy factor. To confound a relationship a factor mustbe causal for the outcome under study and correlatedwith the exposure, as would be the case when di�eren-tially distributed across study groups [79]. Monahan [17]postulates a mechanism whereby socioeconomic statusand institutionalization intervene in the causal chainbetween mental illness and violence. Any factor thatrepresents a plausible step in the causal chain of eventsunder study cannot be considered to be extraneous tothe analysis and therefore should not be statisticallycontrolled. If it is true that mental illness causes adownward social drift and that social causation ofmental illness is entirely implausible, then Monahan iscorrect; removing socioeconomic status through statis-tical analysis could seriously distort the results. Similarreasoning could be applied to institutionalization, butnot age, gender, and race, for it could not be argued thatthese lie on the causal pathway between mental illnessand violence, nor was this Monahan's intention. Yet, itshould also be recognised that mental illness cannot beregarded as a su�cient cause for institutionalization.Hospitalization is more a re¯ection of availability andaccess to beds and this varies by region and by country.Institutionalization will occur relatively infrequently inmental health systems that are oriented toward com-munity treatment. In these systems, violence may be themain predictor of institutionalization, particularly in-voluntary hospitalization.

When analyzing data on mental illness and violence,therefore, it would be appropriate to consider age,gender, and race as confounding factors that warrantstatistical control, and socioeconomic status and insti-tutionalization as potentially intervening factors that donot warrant control. In the presence of uncertaintyabout the causal mechanism, careful investigators mightassess the relationship between mental illness and vio-lence with and without statistical controls for socioeco-nomic status and institutionalization, in order tocompare di�erences in the results. Large discrepanciesbetween statistical models would preclude strong causalinferences until a more detailed etiologic theory, incor-porating notions of biological plausibility, could be re-®ned and tested. While a recent attempt to articulate theetiologic model underlying potential statistical associa-tions between mental illness and violence is presented byHiday [80], this area remains underdeveloped.

Confounding by de®nition

Confounding by de®nition, a form of confounding byindication [81], results from there being shared risk

factors between the two conditions to be compared. Inthe case of mental illness and violence, the likelihoodthat mental disorders will be diagnosed simply becauseof the presence of violent behaviour is an issue that hasnot been fully addressed or resolved. Yet it has majorimplications for the ®eld. A number of mental disordersincluding antisocial and borderline personality disor-ders, intermittent explosive disorder, ®resetting, or sex-ual sadism include violence as a key diagnostic feature.Other disorders such as schizophrenia, bipolar disorder,and substance abuse identify violent behaviour as anassociated feature. Mental disorder and violence may bestatistically related simply because of our overlappingde®nitions.

According to Harry [50], the Diagnostic and Statis-tical Manual (DSM) of the American Psychiatric As-sociation has insidiously enshrined violence as ade®ning characteristic of mental illness over its succes-sive editions and revisions. Harry content-analyzedDSM-I, DSM-II, and DSM-III to assess to what degreeour diagnostic concepts have changed vis-a-vis the re-lationship between mental illness and violence. He cat-alogued and counted words pertaining to violentbehavior in the descriptive paragraphs and diagnosticcriteria for each mental disorder. In DSM-I, 6 of the276 possible disorders were ``violent'' (2.17%). In DSM-II, 9 of the 337 possible disorders were ``violent''(2.67%). By DSM-III, however, the proportion of vio-lent disorders jumped to 162 out of 348, or 46.6%.Ninety-one of these (26.15%) included violent words aspart of the diagnostic criteria. While Harry did notdistinguish violence to self from violence to others, thepossibility that changing de®nitions of mental illnessexplain the statistical associations noted in the morerecent literature cannot be overlooked. This argumentbecomes more cogent when one considers that com-mitment criteria, once emphasizing the ``need fortreatment and care'' have moved toward explicit rec-ognition of ``dangerousness to self or others'' [82]. Thelegal criteria intended to safeguard the rights of thementally ill appear to have developed simultaneouslywith our reconceptualization of mental illness. Consid-ering that most pre-DSM-III studies did not ®nd anassociation between mental illness and violence [11], aprior scienti®c explanation for such a reconceptualiza-tion cannot be clearly articulated. Therefore, currentresearch pointing to an association between mental ill-ness and violence may be more revealing of social cir-cumstances than of a ``causal relationship'' between thetwo.

Confounding by de®nition seriously mars any causalinferences that could be made based on empirical evi-dence showing a statistical association between mentalillness and violence. By embedding etiologic inferencesin diagnostic formulations we seriously undermine ourability to conduct risk factor studies. Indeed, the desireto remove etiologic inferences from psychiatric diag-noses has been articulated as an important goal of DSMdevelopment [83].

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Relative risk

Epidemiologic models that examine the link betweenexposures and outcomes in de®ned populations use as ameasure the relative risk, de®ned as the ratio of the in-cidence rate for persons exposed to a factor to the inci-dence rate of those not exposed to the factor [85]. Relativerisk must be calculated from prospective studies becauseretrospective studies lack the appropriate denominatorsdescribing the populations at risk. More speci®cally,these studies must compare a representative sample ofthose at risk of the outcome and exposed to the studyfactor of interest. In the present case the study groupwould be composed of a representative sample of bothtreated and untreated mentally ill individuals at risk ofcommitting violence. Technically de®ned, ``at risk'' refersto those who have not previously experienced the out-come of interest ± in this case, those who have no pre-vious history of violent behaviour. Comparison subjectswould be similarly representative of non-mentally ill atrisk subjects. An approximation of relative risk, the oddsratio, can be calculated from retrospective case-controlstudies, but only if (a) the controls are representative ofthe general population, (b) the cases are representative ofall cases, and (c) the frequency of the disease in thepopulation is small. Thus, the strongest epidemiologicdesigns are based on unselected samples, incorporate thepassage of time, include careful measurement and clas-si®cation of both the exposure and outcome under study,and make appropriate adjustments for confoundingfactors. Our literature review revealed that these meth-odological requirements have not yet been met in thestudy of mental illness and violence [18].

Selected examples from the literature

Three examples have been chosen to more concretelyillustrate the key methodological points being discussedin this paper.

The ®rst study, by Sosowsky [85, 86] was chosenbecause it represents a strong and often used study de-sign, and because it examines the issue from the vantagepoint of the psychiatric patient (the exposure). Sosowskycompared the arrest rates of former mental patients withthose of the general population using a historical cohortstudy design. Patients who were discharged from theNapa State Hospital between 1972 and 1975 were fol-lowed in local crime registries for up to 6.5 years fol-lowing discharge. Crime data for the surrounding SanMateo County comprised the standard for comparison.Findings were strati®ed according to the number of ar-rests each patient had prior to discharge. Compared tothe general population, ex-patients with no prior arrestswere 5.3 times more likely to be arrested for a violentcrime. Ex-patients with one prior arrest were 12.4 timesmore likely to be arrested for a violent crime, and thosewith two or more arrests were 14.1 times more likely to

be arrested for a violent crime. The author concludedthat mental status is causally related to arrest rates.

These ®ndings suggest that prior criminality, al-though a powerful predictor, does not entirely explainviolence among ex-mental patients. Also, this study isnoteworthy because it is one of the few demonstrating ahigher arrest rate among patients with no prior criminalhistory. This latter ®nding could support the purportedcausal interpretation if plausible competing explanationscould be ruled out.

Diamond [87] points out that the Napa State Hos-pital patients were a selected group known to be moreprone to violence. Patients with arrest histories or clin-ical indications of violent behavior tended to be sent tothis hospital. Indeed, this would be true of almost anypsychiatric hospital, especially in the post-deinstitu-tionalization era. Adams [88] argues that most patientsare sent to state hospitals based on psychiatric judge-ments that they will be dangerous. Thus, in general,hospitalized groups comprise a selected, more violentsubgroup of the mentally ill. On the basis of these se-lection factors alone one would expect to see higher ratesof violence or arrests among ex-patients compared to thegeneral population. In fact, the increased incidence ofarrests among these ex-patients could be viewed as anindicator of psychiatrists' ability to predict dangerous-ness, rather than any indication that mental illness was acause of violence.

To make a causal determination we would have to beequally convinced that the statistical association be-tween mental illness and violence was also apparentamong both nonhospitalized and untreated groups ofmentally ill (the bulk of the mentally ill) and that com-peting explanations had been ruled out.

The problems caused by selection bias are so perva-sive in this ®eld of inquiry that they remain importanteven in those rare instances when it has been possible toselect mentally ill from among large population-basedbirth cohorts. For example, Hodgins [89] identi®edstudy subjects from among 15 117 persons born inStockholm in 1953 and still residing there in 1963.Mentally ill were de®ned as those having had a previouspsychiatric admission and comparison subjects werethose who had never had a psychiatric admission, de-®ned on the basis of hospital admission records. Crim-inal justice data were used to de®ne violent crimes.While this study permits strong generalizations con-cerning the relationship between mental illness and vi-olence within the hospitalized mentally ill, selectionfactors preclude a more general casual statement be-cause we still cannot tell whether the relationship wouldhold in nonhospitalized and in untreated populations.

In the interests of space we have highlighted selectionbias within the context of psychiatric patients. However,similar issues arise when study populations are drawnfrom criminal o�enders.

A second issue raised by Sosowsky's study [85, 86]relates to the lack of comparability of study and com-parison groups and lack of control for confounding. The

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Napa State Hospital is described as admitting patientsfrom a wide catchment area, yet the general populationcomparison data for this study were derived from aspeci®c county comprising only one part of the hospital'scatchment area. There could be signi®cant social, eco-nomic, and demographic di�erences between San MateoCounty and the larger catchment area from which thepatient group was drawn ± su�cient to confound thecomparison. If, for example, the patients represented alower socioeconomic group than San Mateo Countryresidents, then we would expect them to take on thehigher crime rate typical of their socioeconomic group.Socioeconomic di�erences were not controlled in theanalysis, so it is impossible to assess whether con-founding by socioeconomic status is another plausibleexplanation for the observed associations. It is not clearwhether the higher arrest rate in the patient group was aresult of their being a violence-prone group of mentallyill (selection bias), or a consequence of a greater con-centration of some social or economic factor associatedwith arrests in this group (confounding). The lack ofassessment for the e�ects of potential socioeconomicdi�erences argues against there being a valid statisticalassociation between mental illness and violence. In ad-dition, the potential selection bias precludes any gener-alization from the results noted in this sample, to thebroader etiological realm of mental illness and violence.

Community-based studies address the question ofwhether mental illness and violence are related in non-institutionalized (treated and untreated) mentally ill.While existing cross-sectional studies have overcome theimportant issue of selection bias, they have encountereddi�erent problems centering on their inability to tem-porally order exposure and outcome factors. For ex-ample, Swanson et al. [90] report the ®ndings from alarge representative community survey of 10 059 adultsstudied as part of the Epidemiologic Catchment AreaProgram in the United States. Psychiatric disorder wasmeasured using a structured diagnostic schedule in aface-to-face interview conducted by a lay interviewer.One-year period prevalence was used such that a personwas counted as a case if he or she met DSM-III criteriaduring the 12 months prior to the interview. Five itemsfrom the diagnostic interview schedule were also used tode®ne physical violence during the 12 months prior tothe interview. Findings reveal a strong statistical asso-ciation between mental illness and violence. For exam-ple, more than half of the individuals reporting violencein the previous year also met the criteria for a psychiatricdisorder in the previous year, compared to 19.6% ofthose who did not report violence. Conversely, amongthose with no diagnosis in the previous year, 2.1% re-ported a violent act in the previous year. This rose to6.8% among those with one diagnosis, 17.5% for twodiagnoses, and 22.4% for those with three or more di-agnoses. The authors conclude that individuals in thecommunity with psychiatric disorders are more likely toengage in assaultive behaviors compared to those with-out psychiatric disorders.

This study represents the state of the art in psychi-atric epidemiology. Yet, because both mental illness andviolence were measured cross-sectionally, and both re-¯ect the same 12-month period prior to the survey, it isnot clear whether the reported mental illnesses actuallypredated any expressions of violence, as would be re-quired for a causal relationship, or the reverse.

Secondly, as the authors point out, the measure ofviolence used was less than ideal because it re¯ectedinterview questions that were used, in part, to arrive at adiagnosis of mental disorder. Given the overlap in theconceptualization and measurement of mental illness,some level of statistical association would be expected byde®nition. As previously discussed, confounding byde®nition has become an increasing problem with theevolution of the DSM system. Finally, such studies forceus to assume that violence committed by a person whopresently has a mental illness, or has had a history ofmental illness, is always due to the mental condition.Many other reasons, some of them context-dependentand totally independent of the mental condition, mayhave led the person to violence [91]. The nature of thisstudy precludes our ruling out this important possibility.

Are we asking the right question?

When members of the general public consider thatmental illness causes violence, they believe they are atrisk of violence from the mentally ill. Although relativerisk is the measure of association most frequently usedto make statements of causality, from the point of viewof public health relative risk is not the best measure toquantify the risk to the population. Population-attrib-utable risk is de®ned as the proportion of all cases thatcan be ascribed to a factor [84]. It answers the importantquestions of how much of the risk in exposed individualscan be attributed to the exposure, and how much of therisk in exposed individuals can be eliminated if the ex-posure could be reduced or eliminated [92]. In otherwords, population-attributable risk measures ``the the-oretically achievable reduction in risk, if the risk factorwere entirely removed from a population'' [93].

Attributable risk is the measure of choice to assess thepotential bene®t of preventative e�orts and a more di-rect and sound measure to assess the risks incurred bythe population at large. In regard to mental illness andviolence, therefore, the most important question for thegeneral population and for health planners interested inreducing the hazard of the risk of violence to the pop-ulation may be, ``How much of the violence in thecommunity can be attributed to mental illness? not``Does mental illness cause violence?''

Although no paper in the literature on mental illnessand violence has addressed this sensitive epidemiologicaland public health issue, two authors have mentioned itperipherally. Monahan [94] suggests that ``mental healthstatus makes at best a trivial contribution to the overalllevel of violence in society.'' Similarly, Wessely et al. [37]

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have stated that ``viewed in terms of attributable risk,the contribution made by those with schizophrenia tothe level of recorded crime in the community is slender''.Perhaps we are collectively guilty of a type III error(obtaining an answer to the wrong question) when wefocus on risk factor analyses instead of population-attributable risk.

Conclusion

In light of the foregoing discussion, we argue that acausal interpretation of the currently reported statisticalassociations between mental illness and violence remainsuncertain. Directions for future research that couldprovide more de®nitive answers must begin with thedevelopment of independent measures of mental illnessand violence, so as to avoid confounding by de®nition.Study populations that are representative of all personswith mental disorders, not just those receiving treat-ment, must be viewed as a basic sampling requirement.To restrict the analysis to those ``at risk'' of violence,study populations could be further restricted to excludeindividuals with a prior history of violent behavior (or atleast this must be statistically controlled in the analysis).Finally, cohort designs capable of establishing temporalordering of factors and careful assessment and controlof potentially confounding factors are mandatory.

The notion that methodological weaknesses in onearea cancel out those in another should be re-evaluatedwith respect to the literature on mental disorder andviolence. This position overlooks the possibility thatconsistent design ¯aws are possible, and perhaps evenlikely, when attempting to address such a complex issue.We would argue that the combined e�ects of selectionbias, confounding by de®nition, and poorly controlledcomparisons potentially o�er rival explanations forcurrent statistical associations which cannot be so easilyexplained away by a law of averages. Greater attentionto identifying alternate explanations for ®ndings wouldhelp to elucidate the role of these and other factors andadvance the ®eld of inquiry. Finally, we would arguethat the more important question may be to determinethe population-attributable risk of violence.

Coda

What would be the damage caused to the mentally ill ifscientists conclude that mental illness causes violence?This important social question must be answered beforeany conclusion is reached, especially when the evidencemay be ¯awed.

In order to answer this question, readers, researchers,legislators, planners, and the public should be invited toenter into a gedanken exercise by constructing a hypo-thetical risk control ratio (RCR). Such could be ac-complished by quantifying the potential damage thatcould be caused to those with the disease in the nu-

merator, and the attributable risk in the denominator.Low values of RCR could then be used to indicate mi-nor damage whereas high values could be used to indi-cate unacceptable damage to the population a�ected bythe disease. In the case of the mentally ill, this hypo-thetical RCR could have in the numerator factors suchas increased stigmatization and discrimination, restric-tive legislation, and calls for more institutionalization.These potentially high damages to the interests of thementally ill will be counterbalanced by placing inthe denominator the attributable risk for violence in thecommunity that they could present, and which is alreadyknown to be low. If this RCR could be quanti®ed, itwould likely have high values indicative of unacceptabledamages to the population a�ected by mental illness.

Weed [95] reminds us that how knowledge is acquiredand how moral judgements are made regarding the ac-quisition and use of knowledge are issues that residewithin the domain of epistemology and ethics, and arecentral to the epidemiological discourse. Levine places``considerations of justice'' as one of the legs of animaginary ethical tripod of research on human subjects,with the other two being respect for persons, and be-ne®cence. Levine intimates that the common expression``that's not fair'' captures the feeling of concern when aninnate perception of justice has been trespassed [96].Given all the scienti®c ¯aws in the reported studies andthe dire consequences to mental patients, ``is it fair'' toconclude that mental illness causes violence?

Acknowledgements The authors wish to express their appreciationto Health Canada, Division of Mental Health, for their support incommissioning the literature review.

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