Table 2. Main limitations of studies and their associations between method of homicide and type of diagnosis.

Study	Key findings relevant to hypothesis	Main limitations of study
Hughes, Macaulay and Crichton (2012)	Evidence of association between unplanned homicide and perpetrators who have had contact with mental health services in England. Schizophrenia: Evidence of association between unplanned homicide and kitchen knife use in offenders with schizophrenic illness	Psychiatric reports available only in 96% of cases. Sharp weapons are will also be more used because of their immediate availability. Small sample and selection of cases may have focused on domestic unplanned homicide only. Kitchen knife use may be a broader problem within English society (not relating to a specific type of homicide) – as gun control is in America. Study focused on sharp instrument method only.
Rodway, Flynn, Swinson, Roscoe, Hunt, Windfuhr, Kapur, Appleby and Shaw (2009)	Perpetrators with a lifetime history of mental illness were more likely to use a sharp instrument. Schizophrenia: 96% used a violent method 55% used sharp instruments as a method of homicide as opposed to 40% when compared to other mental illnesses Mood disorders: 88% used a violent method 22% used strangulation/suffocation/drowning as method of homicide as opposed to 13% when compared to other mental illnesses 31% were mentally ill at the time of the offense	Psychiatric reports available only in 57% of cases. Of the 433 perpetrators with severe mental illness (schizophrenia/other delusional disorders/mood disorders) over half had co-morbid alcohol/drug dependence/misuse. Violent methods were more likely used by dual diagnosis perpetrators than by perpetrators with a severe mental illness, and no history of alcohol/drug misuse/dependence. Bipolar disorder and depression were joined due to the small number of offenders with a diagnosis of bipolar disorder.
Catanesi, Carabellese, Troccoli, Candello, Grattagliano, Solarino and Fortunato (2011)	100% of psychiatric reports obtained Schizophrenia: Results show significant correlation between delusional disorders and the use of sharp weapons (O.R. = 4.7; X2 7.23; p≤0.05) Mood disorders: Depressive disorders were found more associated with asphyxia (O.R. = 15.9; X2 = 13.19; p≤0.05) The use of sharp weapons was more frequent in cases where the homicide was the result of an impulsive reaction 36% were mentally ill at the time of the offense	Highly occurrence of disorders like schizophrenia and other delusional disorders were highly represented. Due to Italian law, availability of firearms is very low and this may affect the results in which this method was used by the offender.
Häkkänen and Laajasalo (2006)	100% of psychiatric reports obtained Use of sharp weapon was most frequent in schizophrenic group Schizophrenia: 58% used sharp instrument as opposed to 48% when compared to other mental illnesses	Some information was absent and coded as missing. Sample size is moderate and study should be replicated with a larger sample that can lead to more significant results. Dividing offenders between mentally ill and non-mentally ill is not sufficient; different diagnoses will differ in crime scene behaviors. More than one method of homicide per offender is possible and not specified in study data.
Matejkowski, Solomon and Cullen (2008)	100% of psychiatric reports obtained Schizophrenia: 30% used a sharp instrument as opposed to 27% when compared to other mental illnesses Mood disorders: 15% used strangulation/suffocation/drowning as opposed to 12% when compared to other mental illnesses	Small sample Firearms most common homicide method reported across all diagnoses for their easy availability More than one diagnosis per offender possible. More than one method of homicide per offender is possible.
Shaw, Amons, Hunt, Flynn, Turnbull, Kapur and Appleby (2004)	Perpetrators were found more likely to have a history of drug misuse than a history of contact with mental health services 32% were mentally ill at the time of the offense	Psychiatric reports available only in 73% of cases. Studies included over-represent homicide by strangers. The loss of the psychiatric reports that were not available may have inflated proportion of perpetrators with mental illness since reports with diagnosis of mental illness are more likely to be used in court.
Häkkänen-Nyholm, Repo-Tiihonen, Lindberg, Salenius and Weizmann-Henelius (2009)	100% of forensic examination reports obtained	Study mainly focused on reporting cases of sexual homicide, although full sample involves all types of homicide offenders, reported data concerns 18 cases of sexual homicide.
Putknonen, Collander, Marja-Liisa, Honkasalo and Lönnqvist (2001)	100% of psychiatric reports obtained Results show that women most often kill men by stabbing in a quarrel	The diagnostic reliability of personality disorders in general has been suggested to be somewhat poor. Figures for neuroses and alcoholism may also be underestimates.
Yoon, Kim, Choi, Lyu, Kwon, Jang and Park (2011)	100% of psychiatric reports obtained Suffocation was the most common method of homicide in the depressive phase group of Bipolar I disorder Stabbing was most common method of homicide in the mania group Mood disorders: 35% used strangulation/suffocation/drowning	Study only looks at offenders with bipolar I disorder. Only offenders with serious psychopathology were enrolled, hence it presents limitations when generalizing findings to all offenders with bipolar I disorder. Mental status examination in Korea is not a routine and is therefore requested only when previous psychiatric history was found; because of this, patients in manic phase can be over-represented since patients in depressive phase may be considered as normal at that moment. Data was collected though a retrospective chart review, instead of prospective which would result in more precise data.