Schipkowensky, ^{Reference Schipkowensky and Arieta1} Gudjonsson & Petursson, ^{Reference Gudjonsson and Petursson2} and Coid ^{Reference Coid3} suggested that the rate of homicide by the mentally ill is associated with the prevalence of mental illness and is thus unrelated to the rate of other homicides. Coid went so far as to propose two ‘epidemiological laws’ regarding the incidence of homicide due to mental disorder and suggested that there is a fixed rate of about 0.13 per 100 000 population per year in all countries, regardless of the overall rate of homicide. ^{Reference Coid3} Coid's epidemiological laws are still cited ^{Reference Taylor and Gunn4–Reference Coid, Yang, Roberts, Ullrich, Moran, Bebbington, Brugha, Jenkins, Farrell, Lewis and Singleton6} despite studies from Sweden, ^{Reference Fazel and Grann7,Reference Lindqvist8} the USA, ^{Reference Boudouris9,Reference Wilcox10} Barbados ^{Reference Evans and Malesu11} and some Japanese prefectures ^{Reference Hata, Kominato, Shimada, Takizawa, Fujikura, Morita, Funayama, Yoshioka, Touda, Gonmori, Misawa, Sakairi, Sakamoto, Tanno, Thaik-Oo, Kiuchi, Fukumoto and Sato12} that have shown annual rates between two to four times higher than 0.13 per 100 000. Furthermore, most studies that report a high rate of homicide due to mental disorder were conducted in countries with a high total homicide rate ^{Reference Boudouris9–Reference Hata, Kominato, Shimada, Takizawa, Fujikura, Morita, Funayama, Yoshioka, Touda, Gonmori, Misawa, Sakairi, Sakamoto, Tanno, Thaik-Oo, Kiuchi, Fukumoto and Sato12} and in the USA, with the highest rate of homicide among higher-income countries, a large proportion of incarcerated homicide offenders are thought to be mentally ill. ^{Reference James and Glaze13} The findings of these further studies suggest that sociological and legal factors affect both the total rate of homicides and the rate of homicides due to mental disorder.

The first of Coid's epidemiological laws holds that the higher the rate of homicide in a population, the lower the proportion of homicide offenders that are mentally disordered. ^{Reference Coid3} The existence of this law was supported by an interpretation of Home Office statistics from England and Wales that showed a decline in the proportion of homicides due to mental disorder, and a rise in the total number of homicides in England and Wales between 1957 and 1995. ^{Reference Taylor and Gunn4} However, if homicide due to mental disorder had been constant, the decline in the proportion of such homicides from 50% to 10% of all homicides could only have been explained by a 10-fold increase in total homicides. In fact, much of the decline in the proportion of homicides due to mental disorder appears to be due to a fall in the rate of such homicides since the 1970s.

In this paper, we re-examine the official homicide statistics from England and Wales and the proposition that statistics for homicide due to mental disorder have been stable over time.

Method

We located four sets of homicide statistics for England and Wales; Taylor & Gunn, ^{Reference Taylor and Gunn4} Gibson & Klein, ^{Reference Gibson and Klein14,Reference Gibson and Klein15} Richards ^{Reference Richards16} and Coleman et al. ^{Reference Coleman, Hird and Povey17}

Statistics for homicides due to mental disorder are more complete after 1957, when the defence of diminished responsibility was introduced and the mandatory death penalty was abolished.

For the purpose of consistency we used the same definition of mental disorder as Taylor & Gunn, ^{Reference Taylor and Gunn4} which is the total of the findings of Section 2 under the Homicide Act 1957 (diminished responsibility), infanticide, not guilty by reason of insanity and permanent unfitness to stand trial (see Appendix). We also chart the trends in the components of homicide due to mental disorder (Figs. 1 and 2) and homicide–suicides (Fig. 3). Taylor & Gunn did not include homicide–suicides in the mental disorder group, as it is not known what proportion of homicide–suicide offenders are also ‘mentally abnormal’ (e.g. those unfit to stand trial, not guilty by reason of insanity, with diminished responsibility or infanticide). ^{Reference Taylor and Gunn4} Homicide–suicide statistics are examined separately in this paper. Non-age-adjusted homicide rates per 100 000 were calculated using population figures from census data, ^{Reference Hicks and Allen18} using an assumption of linear population growth between each census.

Independent data extraction by M.L. and G.S. did not result in any discrepancies. The annual number of homicides attributed to mental disorder was taken from Taylor & Gunn (1957–1992) ^{Reference Taylor and Gunn4} and Coleman et al (1993–2004). ^{Reference Coleman, Hird and Povey17} The annual numbers of offences initially recorded as homicide were taken from Richards ^{Reference Richards16} (1946–1992) and Coleman et al ^{Reference Coleman, Hird and Povey17} (1992–2004), and total murder convictions were taken from Gibson & Klein ^{Reference Gibson and Klein14,Reference Gibson and Klein15} (1946–1956), Taylor & Gunn ^{Reference Taylor and Gunn4} (1957–1992) and Coleman et al ^{Reference Coleman, Hird and Povey17} (1993–2004). Figures for the finding of not guilty by reason of insanity, unfit to stand trial, infanticide and homicide–suicide were taken from Gibson & Klein ^{Reference Gibson and Klein14,Reference Gibson and Klein15} (1952–1956), Taylor & Gunn ^{Reference Taylor and Gunn4} (1957–1992) and Coleman et al ^{Reference Coleman, Hird and Povey17} (1993–2004). The 172 victims of Dr Harold Shipman, most of whom died in preceding years, were included in official statistics in 2003. ^{Reference Coleman, Hird and Povey17}

Later legal redetermination of cases may have caused some discrepancies in homicide numbers between the four sets of homicide statistics. The total number of annual homicides after 1967 taken from statistics held by the House of Commons library and reported by Richards ^{Reference Richards16} were 17% (s.d.=14) greater than the Home Office statistics reported in Taylor & Gunn. ^{Reference Taylor and Gunn4} However, an analysis of the trends and associations using total homicide figures from Richards ^{Reference Richards16} was very similar to that of Taylor & Gunn ^{Reference Taylor and Gunn4} and was hence not reported. There were only minor differences between the figures reported by Gibson & Klein, ^{Reference Gibson and Klein14,Reference Gibson and Klein15} Coleman et al ^{Reference Coleman, Hird and Povey17} and Taylor & Gunn ^{Reference Taylor and Gunn4} for the periods in which the statistics overlapped. Where there was a discrepancy, the most recent statistics were used.

We examined changes in mentally disordered homicide over time using Kendall's tau, and in relation to total homicide using linear regression. Statistics were performed using SPSS version 15.0 for Windows.

Results

Changes in homicide due to mental disorder over time

The annual number of homicides due to mental disorder rose from under 50 in 1957 to well above 100 by the 1970s. The highest annual rate of 0.245 per 100 000 population was in 1973 and the absolute number peaked in 1979. During this period (1957–1980), homicides due to mental disorder and total homicides were strongly correlated (Kendall's tau=0.747, two-tailed significance P<0.0001).

In the subsequent 24 years (1981–2004) homicides due to mental disorder declined and were negatively correlated with the rate of homicide by people without mental disorder (Kendall's tau=–0.829, two-tailed significance P<0.0001). The absolute number of homicides due to mental disorder fell to levels not seen since the early 1950s and the rate has been at historic lows of 0.07 per 100 000 or lower since 2000 (Fig. 1).

Fig. 1

Homicides due to mental disorder in England and Wales, 1952–2004.

The true figures from 2000 onwards may eventually be higher as about 10% of legal proceedings from 2000 and 20% from 2003 were not concluded. ^{Reference Coleman, Hird and Povey17}

Most of the increase in homicides that were recorded as being due to mental disorder was a result of an increase in verdicts of diminished responsibility. However, there were also peaks in infanticide, homicide offenders who were found not guilty by reason of insanity and among those found unfit for trial in the late 1960s and early 1970s (Fig. 1).

Homicide followed by suicide

Homicide followed by suicide was considered separately and did not follow the pattern of homicide due to mental disorder. There was a small but significant fall in murder–suicides in the period after the abolition of the death penalty in November 1965. Between 1952 and 1965 there were 1928 homicides of which 521 (27%) were homicide–suicides, whereas between 1966 and 2004 there were 19 236 homicides of which 1357 were homicide–suicides (7.1%) (χ²=865, d.f.=1, P<0.001). The fall in the proportion of homicide–suicides is mainly explained by the rise in total homicides, but the total number of homicide–suicides also fell from 37.2 per year (s.d.=3.76) in the 14 years before the abolition of the death penalty to 30.9 per year (s.d.=8.32) during the following 14 years (unpaired two-tailed t-test, t=2.57, d.f.=26, P=0.016) (Fig. 3).

Fig. 2

Homicide rates in England and Wales, 1946–2004.

Fig. 3

Homicide–suicides in England and Wales, 1952–2004.

Discussion

Gibson & Klein ^{Reference Gibson and Klein14} and Morris & Blom-Cooper ^{Reference Morris, Blom-Cooper and Wolfgang19} reported that the homicide rate was fairly stable between 1900 and 1959 at about 0.3 per 100 000 per annum. Total homicides rose steadily in the next 45 years and by the year 2000 there were more than 1.5 homicide convictions per 100 000 population in England and Wales. Our analysis of official figures shows that the rate of homicide due to mental disorder also rose from the mid-1950s, but reached a peak in 1973 and then steadily declined to reach historically low levels. However, the reasons for the rise and fall in the rate of homicide by people who were legally classified as being mentally disordered after committing a homicide cannot be determined from these data.

Appendix

Types of homicide due to mental disorder ^{Reference Taylor and Gunn4}

Legal outcome	Approximate definition
Not guilty by reason of insanity	At the time of the commission of the acts constituting the offence, the defendant, as a result of a severe mental disease or defect, was unable to appreciate the nature and quality or the wrongfulness of his acts.
Diminished responsibility	Where a person kills or is a party to the killing of another, he shall not be convicted of murder if he was suffering from such an abnormality of mind (whether arising from a condition of arrested or retarded development of mind or any inherent causes or induced by disease or injury) as substantially impairing his mental responsibility for his acts and omission in doing or being party to the killing.
Unfit to plead	Where the accused is unable to understand the charge and possible penalties, or unable to understand court proceedings, or unable to give instructions to a defence lawyer.
Infanticide	When a woman or girl who unlawfully kills her child (under 12 months) under circumstances which, but for this section, would constitute wilful murder or murder, does the act which causes death when the balance of her mind is disturbed because she is not fully recovered from the effect of giving birth to the child or because of the effect of lactation consequent upon the birth of the child, she is guilty of infanticide only.
Homicide—suicide	When the homicide offender completes a suicide at the same time or after the homicide. Most suicides after homicide occur within a week of the homicide and suicides that occur after a conviction are not included in official statistics. It is not known what proportion of homicide—suicide offenders might have had a mental illness defence available to them had they survived.