The availability of information on all individuals living in Switzerland at the time of the census as well as on the composition of their households, combined with the linked mortality data, made it possible to identify homicide-suicide events that took place in households of two adults with or without children. We identified 73 such events, with a total of 158 deaths, during the period 1991–2008. The study showed that the perpetrators were predominantly men while the victims were women, confirming data from numerous case series 
. The age distributions were multimodal with peaks in middle and older ages, possibly reflecting events related to the dissolution of relationships (so-called ‘amorous jealousy’ 
) and the burdens of caretaking (‘mercy killing suicide’ 
), respectively. The incidence of homicide-suicides within households was similar in 1991 and 2001: around 3 events per million households and year.
The Swiss homicide rate of 0.9 per 100,000 is one of the lowest among European countries and has remained relatively stable for the last 20 years 
. However, the percentage of homicides occurring within family settings, as well as the percentage of homicides followed by suicide, is higher in Switzerland than in other western nations 
. About 60% of all homicides between 1980 and 2004 took place in private homes and 13% ended with the completed suicide of the perpetrator. This situation has been attributed to one of the highest percentages among European countries of households owning at least one firearm, i.e., approximately 28% 
. A large proportion of these firearms are of military origin – the Swiss compulsory militia system distributes army weapons among soldiers. These weapons remain in private homes for as long as citizens serve in the army, and often beyond that time 
. The prominent role of firearms and the high prevalence of gun ownership have also been observed in suicide deaths, especially among young males 
. Our study thus confirmed the leading role of firearms in the deaths of both victims and perpetrators. However, the role of military weapons could not be investigated because this information is not recorded in mortality statistics. Efforts are now underway to link criminological databases to the cohort data.
This is the first nation-wide cohort study of homicide-suicide. It allowed possible predictors of homicide-suicide events to be examined at the level of the individual, household and area: the Swiss censuses 1990 and 2000 included comprehensive data on demographic and socio-economic characteristic of residents, the composition of households and the geographical coordinates of buildings. Some of these variables were associated with the risk of committing a homicide-suicide act among men, or the risk of being killed in such an event in women. Among men, the risk was increased in divorcees and in foreigners without permanent residency in Switzerland. The risk was also increased in overcrowded households with two or more persons per room. Other studies have found that overcrowding increases the risk of violence, generally, and the risk of violence against women, in particular 
These associations reflect situations and living conditions that are often associated with general psychological stress and intimate partner strain. Sociological and criminological theories have repeatedly linked psychological stress and life pressures with violence. In 1938 Merton put forward what has become known as ‘strain theory’: by putting strain on some members of society, social structures will make it more likely for some to commit crimes than for others 
. More recently, Levin and Madfis 
proposed a sequential model of cumulative strain to explain the origin of mass murder committed by students at their schools. The stages of the model, including chronic strain (for example due to long-standing family conflicts, jealousy or financial difficulties) and acute strain (for example following an argument, a break-up, a new partner) that initiates the planning of the violent act, may also be pertinent to homicide-suicide within families. Feminist theories address the fact that men predominate as perpetrators in homicide-suicide, an aspect neglected in many conceptual models of interpersonal violence 
. They interpret homicide-suicide as a form of hegemonic masculinity, where the violent act is an extreme way of controlling female sexual partners and descendants and perceived as the only remaining option when relationships break down 
Interestingly, both in male perpetrators and female victims the risk was higher in individuals without religious affiliation, compared to Catholics. Women with no religious affiliation may on average be less amenable to authority structures (including patriarchy) and less bound by traditional relationship roles, than Catholic and Protestant women: the woman leaving the relationship is often the trigger for the escalation of violence 
. Normative integration, where individuals accept the social norms and dogmas of a faith, may also play a role. The Catholic faith condemns murder, and condemns suicide as self-murder. According to the Catechism, “infanticide, fratricide, parricide, and the murder of a spouse are especially grave crimes by reason of the natural bonds which they break” 
. Of note, in a recent analysis of the same cohort, we found that rates of suicide and assisted suicides were also substantially higher in those with no religious affiliation 
Factors unrelated to risk are also worth noting. In particular, current study found no evidence that homicide-suicide was associated with education or occupation, two variables that measure important components of socio-economic position (SEP) 
. An article on episodes of homicide-suicide in Yorkshire and the Humber described the typical perpetrator as a “white male from the lower middle to working class” 
. This may be a general misconception due to the shortcomings of much of the previous research in this area, which was generally based on case-series, without information on the population at risk. Alternatively, the epidemiology of homicide-suicide may differ in Switzerland. There was also no evidence for an association with nationality, the presence of children in the household, the language region and the degree of urbanization.
Homicide-suicides can be seen as essentially homicidal, with the killer subsequently dying by suicide, perhaps out of remorse. Alternatively, it can be seen as a suicide extended to intimate relations 
. Studies comparing perpetrators of homicide-suicides with individuals committing murders or people dying by suicide showed that in comparison with simple murderers those committing homicides and then suicide were older 
, more likely to be men 
, more likely to be married 
and less likely to be unemployed 
. A history of domestic violence 
, and acting under the influence of alcohol 
, was common in perpetrators of homicide-suicides, but less common than in perpetrators of homicides. Finally, firearms were more likely to be used in homicide-suicide cases than in simple homicides 
. In Europe, among homicide-suicide events with multiple victims, 80% of victims were shot, compared to 38% in single homicides 
. A history of attempted suicide was more common in individuals dying by simple suicide than in perpetrators of homicide-suicides 
. Homicide-suicide events thus appear to represent a distinct entity, with characteristics distinguishing them both from homicides and suicides.
Our study has several limitations. Firstly, it relied on decennial census information for information about eligible households and their composition. The analysis thus excluded events that involved individuals not sharing the same household at the time of one of the two censuses, or events concerning individuals never registered as living together. Interestingly, the homicide-suicide offending rates observed in current study (0.09 and 0.10 per 100,000 in 1991 and 2001, respectively) are closely similar to the rate from a study of cases where an autopsy had been performed (0.09 per 100,000 for the period 1992–2004) 
. The latter rate will, however, also be too low because an autopsy is not always ordered. It is therefore likely that the true incidence of homicide-suicide events in Swiss households is slightly higher. Secondly, the study relied on data from death certificates and we therefore will have missed all cases of homicide followed by attempted suicide. Finally, it cannot be excluded that the analysis may have matched homicide and suicide events that were in fact unrelated. In other words, the person committing suicide may not have killed the members of his household. The very short time span between violent deaths, most of them by firearms and within the same household, makes this possibility highly unlikely. Finally, we did not investigate the role of household income or the role of the SEP of neighbourhoods. Data on household income are not collected in the census and the index of the SEP of neighbourhoods recently developed for Switzerland 
was based on census 2000 data, whereas the present analysis is based on the 1990 census. We included education and occupation, which are important indicators of individual SEP, as well as crowding of households, which to some extent reflects the SEP of households.
In conclusion, this national longitudinal study in a country with wide-spread access to firearms shows that living conditions associated with psychological stress and lower levels of social support are associated with homicide-suicide events. Conversely, the study found no evidence that homicide-suicide was associated with education or occupation, nationality or the presence of children in the household. The linkage of criminological, forensic and psychological data with the national cohort study will overcome some of the limitations of the present study and allow more detailed characterisation of events, including the role of military and other firearms. Almost all incidents involved firearms and results thus add to a growing body of evidence that in Switzerland and elsewhere restricting access to firearms might prevent at least some homicide-suicide tragedies.