Our findings indicated no compensatory rise in suicide rates by hanging or other methods from 1983 to 1993 in Taiwan, accompanying the marked decrease in solids/liquids poisoning suicide rates. The same pattern of change occurred in different sex and age groups.
Several previous studies illustrated that restricting access to lethal methods can reduce suicide rates [1–11]. The most often cited example was Kreitman's study , which demonstrated a reduction not only in gas-specific suicide rates, but also in the overall suicide rates because of the lack of a compensatory rise in the use of other methods. However, Gunnell et al., who offered the criticism that those analyses have either failed to examine trends in method-specific suicide rates or have not assessed age- and gender-specific effects, reanalyzed the suicide trends in England and Wales in 1950-75, and found that the effects of these reductions on overall suicide rates due to coal gas detoxification were partially offset by rises in drug overdose deaths in women and younger men .
In fact, some other studies showed that method substitution did occur in some specific subgroups and with certain methods, as summarized in Table 1[3, 8, 12–15]. Among those studies, method substitution seemed to be most likely found in young people [14, 15] or in males [3, 8, 12]. In addition, as seen in Table 1, we also found that non-violent methods (e.g., domestic gas) were often replaced by non-violent methods (e.g., car exhaust or drug overdose) [3, 12, 13] and that violent methods (e.g., firearms) were often replaced by violent methods (e.g., jumping or hanging) [8, 14].
If suicide rates in Taiwan followed the substitution hypothesis, we would expect a compensatory increase in suicide rates by overdose, domestic gas and other gas in specific demographic subgroups. Our analyses, however, did not find such results. Why was there no method substitution in Taiwan? And what are the factors affecting the occurrence of substitution?
Three possible factors proposed by Gunnell et al. , i.e., the accessibility to, and the acceptability and popularity of the lethal methods were found to have profound effects on method substitution.
As to accessibility, there were nearly no limitations on the availability of medications, since people in Taiwan were free to visit physicians and to buy medications in the drugstore. Similarly, domestic gas was also available to nearly all people, because it was the main heating source for cooking and bathing in every household. Also, car ownership had increased annually due to the 1983-1993 economic development in Taiwan.
In terms of acceptability, there was no stigma to using poisoning with medications, poisons and gas as suicide methods in Taiwan society. However, there was still no method substitution in Taiwan. This may be due to the lack of popularity of other suicide methods during the period 1983-1993 in Taiwan.
The most popular suicide methods in Taiwan were poisoning with solids/liquids (mainly pesticides) and hanging, which constituted 90% of all suicide deaths in the period 1983-1993 . The other suicide methods popular in Western countries, such as car exhaust, medicine overdose, and gassing, contributed little to suicide deaths in Taiwan. Therefore, the lack of popularity of other violent suicide methods similar to pesticide overdosing before 1993 may be one of the possible explanations for the phenomenon of no method substitution in Taiwan.
There were some limitations to our study. First, the E-codes of the ICD-8 and ICD-9, used in Taiwan during the study periods, only included three-digits. The code E950 (and its analogous counterpart E980) is a broad category that includes suicide from all types of solid and liquid substances, such as pesticides, prescriptions and other poisons. However, the data from the years 1987 and 1992 show that pesticides were the major ingested solid/liquid substances causing suicide death in Taiwan. In addition, no compensatory rise in suicide rates by medications and other solid/liquid substances was found, as the pesticide suicide rates decreased markedly from 1987 to 1992. In fact, the sex-specific suicide rates by poisoning with pesticides, medications and other substances all decreased from 1987 to 1992.
Second, suicide death is thought to be a complex phenomenon involving multiple factors. The influence of many other factors, such as mental health and the socio-economic environment on sex-, age- and method-specific suicide rates was not considered in this study. Further study is needed to test whether changes in psycho-socio-economic conditions would have differential effects on the method-specific suicide rates among particular subpopulations.
Thirdly, one may wonder why did authors not include more recent years for analysis? In fact, the suicide methods changed much in the recent decade in Taiwan. In a previous study, Lin and Lu reported , during the 1970's and 1990's, the two leading suicide methods in Taiwan were poisoning by solids/liquids and hanging. These two methods were responsible for about 90% of all suicide deaths before 1990. In the contemporary years of 1991-2005, nearly all method-specific suicide rates seemed to increase in both sexes. However, the distribution of suicide methods changed much. The contribution of the traditional suicide methods in Taiwan (i.e., hanging and poisoning by solids/liquids) decreased, while that of the new methods (i.e., jumping from heights and poisoning with other gases) markedly increased. In this study, we focused on the testing of substitution hypothesis. So, we chose the period 1983-1993, when the suicide methods were relatively simple, to test the substitution hypothesis.
Implications for suicide prevention
As Gunnell et al estimated that pesticide self-poisoning accounted for about one-third of the world's suicides and suggested that we might prevent many of these deaths by restricting the access to pesticides . Our results add to the evidence that restricting access to lethal methods is a promising strategy in suicide prevention, particularly in those countries where the "target method" contributed greatly to the suicide rate. Moreover, when we plan to restrict one lethal suicide method, we should also be aware of the popularity of other possible substitute methods with equal access and similar acceptability.