Methods of suicide: international suicide patterns derived from the WHO mortality database
Vladeta Ajdacic-Gross a, Mitchell G Weiss b, Mariann Ring a, Urs Hepp c, Matthias Bopp d, Felix Gutzwiller d, Wulf Rössler a
Restricting access to the means of suicide is an important component of comprehensive strategies for suicide prevention. To improve prevention efforts, better knowledge of national, regional and local suicide patterns is vital, and better understanding of underlying mechanisms is absolutely crucial.
National studies on suicide indicate that suicidal behaviour and, in particular, the preferred suicide method, varies between countries. Some patterns are well known, such as the high percentage of firearm suicides in the United States of America.1 In addition, the role of pesticide suicide in Asian countries became apparent in the 1990s.2,3 The emergence of a new method, charcoal-burning suicide, in Hong Kong Special Administrative Region (SAR), China and urban Taiwan, China,4,5 has been a surprise, but serves as a warning that research and prevention efforts should be reinforced.
While numerous factors contribute to the choice of a suicide method, societal patterns of suicide can be understood from basic concepts such as the social acceptability of the method (i.e. culture and tradition) and its availability (i.e. opportunity).6,7 International or intercultural comparisons of suicide methods help increase understanding of the interplay between these two factors and provide a basis for preventive strategies.8–10
The aims of this study were:
- to provide, for the first time, a comprehensive international comparison of suicide methods using WHO mortality data (i.e. data collected in a standard way);
- to use correspondence analysis (a data reduction technique) to identify underlying patterns in the data;
- to contribute to discussions on the impact of the availability and acceptability of suicide methods on suicide behaviour;
- to draw conclusions about suicide prevention.
The data presented below were derived from the WHO mortality database (http://www.who.int/whosis/mort/en/index.html) on 17 November 2006. The database includes data gathered since 1950 and is dedicated to collecting and making available data on the underlying cause of death in all Member States using a standard format. Several countries have not reported mortality data to the WHO, or have reported data covering only a few years.11 Others have reported only partial data, for example, on subpopulations or regions. However, the database also provides estimates of the completeness and coverage of mortality data for each reporting country. Data for industrialized countries are most complete, whereas those for developing countries and conflict regions are often incomplete or missing altogether.
For this study, detailed information about suicide methods was extracted from WHO mortality data only if it was encoded according to the International Classification of Diseases, 10th revision (ICD-10). Since not all countries report mortality data to this standard, the study relied on only a selection of countries. In particular, countries from Africa were absent, except South Africa. In addition, countries with small populations, and thus few suicides, were excluded. In total, 56 countries and all the years covered by ICD-10 data, 1994–2005 at most, were included in the analysis, though the time span varied slightly from country to country (Table 1, available at: http://www.who.int/bulletin/volumes/86/9/07-043489/en/index.html).
The ICD-10 codes for suicide are in the range X60–X849. We differentiated suicides due to a pesticide or an unspecified poison (X68–X699), other poisons (X60–X679), hanging (X70–X709), drowning (X71–X719), firearms and explosives (X72–X759), and jumping from a height (X80-X809), and other suicide methods. Some methods, such as hanging or firearm suicide, are reported more accurately than others, such as poisoning or drowning.12 This should be kept in mind when interpreting frequency distributions, particularly in countries with incomplete data.
To provide an overview of suicide methods by country, we calculated the number of suicides with each method as a proportion of the total number of suicides for each sex and country. In addition, we used correspondence analysis (CA)13,14 to summarize information from cross-tabulated data. In practice, CA converted data into a graphical form that grouped countries with similar suicide method patterns. Metaphorically, a new map of the world is emerging in which countries are grouped into quasi-continents on the basis of suicide methods (or groups of suicide methods). The analysis was repeated separately for European and non-European countries.
Correspondence analysis is an exploratory multivariate technique that is very similar to principal component analysis (PCA). In CA, the ‘extraction’ of dimensions is related to the |² distance, which is the measure of association (analogous to the correlation in PCA). A minimum number of dimensions is sought to account for the maximum amount of the inertia (analogous to the total variance in PCA). The inertia is a weighted measure of the total |² (that is, |²/N).
Correspondence analysis relies on standardized row or column profiles, or both. Because of standardization, the proportion of suicides by a particular method in each country contributes similarly to the CA results. Typically, the first or the first two dimensions extracted in CA are used to display the results in correspondence maps. Scores on these dimensions represent distances from the average row and column profile (i.e. the country and method profile, respectively). The distance between any two points is analogous to the weighted Euclidian distance.
Despite symmetrical standardization along both the row and column profiles used in this study, interpretations based on the cross-comparison of distances between countries and suicide methods should be avoided. It is advisable to focus on similar directions from the zero point, however, and to consider the distance from the zero point separately for each variable.
Among the problems which may arise in applying CA is the possibility that a straight sequence of objects or data points may be displayed in a curvilinear or pyramidal way. This phenomenon is called the ‘arch effect’, which derives from unimodal response curves or from the compression of distances at the ends of an axis, generally the first axis. In such cases, only the first axis, rather than two axes, is included in data interpretation. Alternatively, another CA technique (e.g. detrended CA) can be used.
Table 1. Suicide by method (% out of all suicides by country) according to the WHO mortality database (as at November 2006); countries reporting ICD-10 data
Hanging was the predominant method of suicide in most countries included in the analysis (Table 1). The highest proportions were around 90% in men and 80% in women, as observed in eastern Europe (i.e. Estonia, Latvia, Lithuania, Poland and Romania). There were a number of deviations from the predominant pattern. As might be expected, firearm suicide was the most common method in the United States, but was also prevalent in Argentina, Switzerland and Uruguay, although only men used this method in Switzerland. Jumping from a height (designated as falls in the figures) plays an important role in small, predominantly urban societies such as Hong Kong SAR, Luxembourg and Malta. In contrast, in rural Latin American countries (e.g. El Salvador, Nicaragua and Peru), Asian countries (e.g. the Republic of Korea and Thailand) and also in Portugal, poisoning with pesticides was a major problem, notably among women. Poisoning with drugs was common in women from Canada, the Nordic countries and the United Kingdom. It also played an important role in male suicide in these countries.
The well-known differences between men’s and women’s preferred suicide methods can also be seen in Table 1. Violent and highly lethal methods such as firearm suicide and hanging are more frequent among men, whereas women often choose poisoning or drowning, which are less violent and less lethal.
Correspondence analysis for all countries yielded the correspondence maps shown in Fig. 1 for men, and Fig. 2 for women. In the men’s data, the first two dimensions accounted for 84.5% of the inertia; in the women’s data, it accounted for 77.1%. In the figures, pesticides, hanging (and other methods) and firearms form a triangle. In addition, the countries with the highest proportions of pesticide poisoning and firearm suicide listed above also define the triangle apices. Hanging is in an intermediate position since it is more frequent than firearm suicide in almost all countries with a high proportion of pesticide suicide. Conversely, hanging is more frequent than pesticide suicide in almost all countries with a high proportion of firearm suicide. Thus, there are two slopes: one from hanging to pesticide suicide and the other from hanging to firearm suicide. This is the typical arch effect observed in CA. To be safe, we should not interpret these CAs using the second axis. The first axis should be kept in mind, on which the poles are pesticide suicide and firearm suicide (and, to a lesser extent, other poisoning), with hanging and other methods lying in between.
Fig. 1. Correspondence map based on suicide frequencies, in men in the years close to 2000, for different suicide methods and countriesa
Fig. 2. Correspondence map based on suicide frequencies, in women in the years close to 2000, for different suicide methods and countriesa
The correspondence maps for non-European countries (Fig. 3 and Fig. 4, available at: http://www.who.int/bulletin/volumes/86/9/07-043489/en/index.html) show similar results. The amount of the inertia accounted for by the first two axes in the CA was 90.7% for suicide in men and 87.5% for suicide in women. Clearly, there is also an arch effect here. Again, the countries mentioned above defined the slopes between the middle and the poles.
Fig. 3. Correspondence map based on suicide frequencies, in men in the years close to 2000, for different suicide methods and non-European countriesa
Fig. 4. Correspondence map based on suicide frequencies, in women in the years close to 2000, for different suicide methods and non-European countriesa
The correspondence maps for European data (Fig. 5 and Fig. 6, available at: http://www.who.int/bulletin/volumes/86/9/07-043489/en/index.html) show a different pattern. Here, the amount of the inertia accounted for by the first two axes was 77.3% for suicide in men and 74.7% for suicide in women. Again, hanging occupied a central position in both men’s and women’s plots. The first axis divided countries in central and western Europe, where there was a more mixed pattern, from countries in eastern and southern Europe, where hanging distinctly predominated but was partly supplemented by a moderate proportion of pesticide suicide. On the second axis, the poles are represented by firearm suicide in men and other poisoning in women, and by jumping from a height (i.e. falls) and other methods. For men, the firearm suicide quadrant includes Croatia, Serbia and Switzerland. Firearms were probably widely available in Croatia and Serbia following the civil war in the 1990s. In addition, Finland, France and Norway also appear in this quadrant. For women, the analogous quadrant relates to drug suicide (i.e. other poisoning) and includes the United Kingdom and Nordic countries. The quadrant of the opposite pole of jumping from a height includes Luxembourg, Malta and Spain.
Fig. 5. Correspondence map based on suicide frequencies, in men in the years close to 2000, for different suicide methods and European countriesa
Fig. 6. Correspondence map based on suicide frequencies, in women in the years close to 2000, for suicide methods and European countriesa
The European data differ from those in the previous analyses. Instead of forming an arch-like pattern, suicide methods and countries are distributed more loosely over all four quadrants. The polarization between pesticide and firearm suicide has mostly vanished due to the modest proportion of pesticide suicide. In addition, distances along the figure axes are clearly smaller than in previous plots; that is, the country patterns are distinctly more similar. Moreover, it is noteworthy that the position occupied by firearm suicide in men is replaced by other poisoning (i.e. mainly drug suicide) in women. It seems that there is some functional equivalence between these suicide methods in Europe.
This paper reports the first comprehensive compilation of methods of suicide worldwide based on WHO mortality data, with the caveat that data for Asian and, above all, African countries are incomplete. Data were submitted to a correspondence analysis to identify typical configurations across countries.
International comparison of suicide methods
In sum, the differences in the suicide methods used in different countries are remarkable. Three methods – hanging, pesticide suicide and firearm suicide – dominate country-specific suicide patterns. Jumping from a height and other methods of poisoning (i.e. mainly poisoning by drugs) occasionally appear as important alternative methods.
The analysis indicates that hanging is the main suicide method when no other major method is available. The proportion of hangings typically decreases as either pesticide suicide or firearm suicide increases. Pesticide suicide has been recognized as a major public health problem in developing Asian countries.15–17 It has been known for some time that firearms affect the suicide frequency, and firearm suicide predominates in several countries in the Americas and also in some European countries. Firearm suicide is frequent in countries where firearms are common in private households.18,19
Underlying patterns identified by the correspondence analysis
Correspondence maps make large tables easier to interpret and are helpful in developing new hypotheses. Moreover, they can demonstrate valuable patterns despite data being noisy or incomplete. And, countries absent from the list can be added afterwards.
The first, and surprising, finding from the CA is that the correspondence maps for men and women are very similar. This is because there is a much larger difference in suicide methods between countries than between genders. Where a suicide method is particularly popular in men, it will often also be popular in women, and vice versa. In general, underlying suicide patterns tell us more about the availability and acceptability of suicide methods than about gender disparities.
The European data, however, provide a notable exception to the congruence between men and women. Here, in the women’’s correspondence map, suicide by other poisoning (mainly poisoning by drugs) has replaced firearm suicide in the men’s map. Assuming there is functional equivalence, we are drawn to the conclusion that unplanned or impulsive suicide in European women is mainly achieved by poisoning using drugs.
Correspondence analysis of all countries and of the subset of non-European countries provides evidence for a polarization between suicide methods. The poles are pesticide suicide and firearm suicide, while hanging is in an intermediate position. This polarization has several implications.
Firstly, at the poles, hanging and other more traditional suicide methods are displaced by suicide methods whose main characteristics are utilization of a technical means, ready availability, quick use and high lethality. For pesticide and firearm suicide, the impact of availability seems to be greater than for traditional suicide methods.
Secondly, the opposite slopes seen in the correspondence maps and the degree of displacement indicate that there is a substantial substitution effect: pesticide suicide and firearm suicide are not only associated with new suicide behaviour, but also tend to substitute hanging. The introduction of specific prevention programmes focusing on pesticides or firearms would be expected to reverse this substitution to some extent. In fact, this reversal was observed when firearm availability was restricted in Australia20 and Canada:21 at the same time, the proportion of suicides due to hanging increased.
Readily available poisons and firearms facilitate unplanned suicide acts,22,23 which are typical of impulsive suicide. Consequently, they increase the suicide frequency. It is noteworthy that the proportion of suicides in individuals with a background of severe mental illness is distinctly below average in firearm suicide.24,25 This observation holds for both Asian and European countries.
Availability and acceptability of suicide methods
It is generally assumed that the use of hanging and other traditional suicide methods is largely governed by their acceptability and by sociocultural norms. While acceptability provides a general framework of beliefs about whether or not to commit suicide and which method to use, sociocultural norms provide a framework for how to proceed, for example, in how to deal with obstacles to suicide.
Hanging, for example, is a selective method because:it is violent; it needs some preparation; it needs some degree of courage and determination.26Each method has its own particular obstacles. Typically, the greater the obstacles, the lower the acceptability of the method and the greater the proportion of suicides associated with psychosis and other severe mental disorder.
This study indicates that the availability of technical means has a large influence on the acceptability of a specific method and, indirectly, of suicide in general. In historical terms, the introduction of pesticides in developing countries is too recent to have contributed to the emergence of traditional sociocultural norms. It has, though, lead to new norms and to new links with acceptability. A theoretical background to such processes is provided by the concept of an ‘opportunity structure’, as introduced by Cloward and Ohlin.27 The potential a new technical means has to win favour quickly among suicidal individuals is a cause of concern. Charcoal-burning suicide might not be the last such phenomenon.
Implications for suicide prevention
Given that the relationship between the availability of suicide methods and the level of suicide is principally mediated by firearm and pesticide suicide, it could be concluded that these two methods should be the main targets for prevention. In practice, many deaths due to pesticide poisoning and firearm suicide could easily be prevented if progress in public health were to outweigh the inertia of political and economic interests.28 However, the outcome is not certain. The above analysis indicates that we should differentiate between preventable suicides and suicides that will be carried out by hanging instead. With regard to poisoning and firearm suicide, preventive efforts are likely to have the greatest impact on the subgroup who carry out unplanned impulsive acts. Perhaps 20–30% of all suicides in industrialized countries belong to this subgroup and might be preventable, and a similar figure has been reported for China.23
In suicides that occur after a period of suffering, the relationship is more complex. As a first approximation, the balance between prevention and suicide method substitution depends, firstly, on the transition probability between suicide methods and, secondly, on the lethality of the particular method. The transition probability between two violent methods, such as firearm suicide and hanging, is presumably quite high and certainly much higher than between poisoning and a violent method.29 The lethality of firearm suicide and hanging are similar, with 80–90% of attempts proving fatal, while lethality is markedly lower for poisoning.8,30 Therefore, for suicide that occurs after a period of suffering, the prevention of firearm suicide would be expected to have little effect, whereas the prevention of poisoning may be well worth considering. Nevertheless, there remains the question of whether the low transition probability between poisoning and a violent method outweighs the higher lethality of the latter. Studies on the detoxification of household gas9,31 suggest that there is a substantial preventive effect in “closing such exits”.26 However, analysing longitudinal data on changes in suicide methods involves considerable methodological problems, particularly when sophisticated statistical techniques and long time series are not available.10,32,33
Study strengths and limitations
Although the WHO mortality database provides the most comprehensive source of mortality data collected in a standard way, data from many countries are incomplete. In particular, ICD-10 encoded data cover only certain parts of the world. It should be noted that ICD-9 encoded data in the WHO database do not distinguish between suicide methods.
In addition, there is a degree of underreporting of suicide. A useful indicator of underreporting is the number of violent deaths which cannot be classified as either intentional or unintentional. This number varies between countries.34 However, in the present analysis, the distribution of suicide methods is more likely to be influenced by differences in the way methods are reported. Specifically, violent methods, such as hanging, are reported more accurately than nonviolent methods, such as poisoning. Less violent suicide methods are more likely to be underreported. Pesticide poisoning may, therefore, be more important than the data indicate.
Another limitation stems from the different time periods covered by ICD-10 coded mortality data in different countries. However, as patterns of suicide methods typically change very slowly, this is a minor issue. Charcoal-burning suicide is an exception, which should be borne in mind.
There are substantial differences in the pattern of suicide methods internationally. These reflect the interplay of different determinants of suicidal behaviour,6,8 primarily the availability of suicidal means. The present findings indicate that restricting access to the means of suicide is more urgent and more technically feasible than ever. Restriction would help to prevent mainly unplanned impulsive suicide. ■
Competing interests: None declared.
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- Swiss Tropical Institute, Basel, Switzerland.
- Psychiatric Services of the Canton of Aargau, Baden, Switzerland.
- Institute of Social and Preventive Medicine, University of Zurich, Zurich, Switzerland.