Before discussing findings of this study, it is important to consider reliability of the data and other potential limitations. According to the Lithuanian regulations, registration of all cases of unnatural deaths must be based on medico-legal investigation, and the autopsy rate covers nearly all such cases. However, despite the high degree of formalization in assessment of unnatural deaths, suicide numbers derived from mortality statistics might generally underestimate the real numbers to a certain extent. Suicide is very specific cause of death, which may be masked by other causes, such as accidents or undetermined causes of death, due to the sensitivity of society to such a diagnosis. Nevertheless, quantitative and qualitative analysis of the reliability of the statistics of suicide in the republics of the former Soviet Union during the period of 1970–1990 demonstrated that suicide mortality statistics were reliable in the Baltic republics [
17]. The limitation of this study regards the possibility of incomplete counts, since a considerable number of suicide episodes does not result in death and were not included in the study. Although, at an aggregate level, it is impossible to assess the impact of particular factors on suicide, this type of surveys overcome sampling problems by using population-based data, over a long period of time, which is important in applying statistical analysis for monthly data separately for different suicide methods in both genders and drawing statistically significant results. Hence, the findings of this study may be helpful in developing in-depth research surveys and suicide prevention programs.
The pattern of seasonal variation in suicides in Lithuania displays similarities with that in other countries. Suicide seasonality differs greatly by suicide method. It is well known that violent suicides are more impulsive rather than premeditated, and they usually show considerable seasonality when compared to non-violent methods. This appears to be particularly true in the countries with a higher percentage of suicide by hanging, and strangulation [
18]. Several studies have shown that violent suicides depict a seasonality with peaks in spring and summer [
18,
19]. Nevertheless, some surveys report that suicide seasonality is absent or weak in some violent methods [
8]. Hanging, as the method of suicide has been historically popular in Lithuania. Already before 1940's hanging was the most common suicide method, though accounted for less than one third of all suicides [
20]. This highly lethal method was gradually increasing in Lithuania during the last decades, reaching around 90% among all suicides in 1998–2002. Prevalence of particular methods of suicide differs by the country. Hanging is also the most frequent type of suicide in countries such as Germany, Israel, Slovenia, Hungary and Estonia [
21-
23], however the proportions of the reported hanging are considerably lower than in Lithuania.
The present study suggests the obvious seasonality for hanging among men with peaks in June of over 24%, and more than 24% lower December frequencies, while for women those fluctuations were even greater. Direct international comparisons of variation amplitudes are rather complicated due to different methodologies applied. Nevertheless, the seasonal variation in hanging in Lithuania is of bigger amplitude compare to the recent Finnish data [
10]. Majority of studies report considerably greater suicide seasonality among men, explaining this fact by more frequent use of more violent methods among them [
10,
19,
24]. Based on spectral analysis, the Lithuanian dataset supports this observation, suggesting greater hanging seasonality among men during two periods under investigation. Nevertheless, large seasonality, though declining over the decade, but still remaining strong was reported in women, with 32.1% explained variance in 1998–2002. This might be related to extraordinarily high proportion of hanging among Lithuanian women. However, the reasons of choosing this highly lethal method of suicide among Lithuanian women have never been investigated.
The findings from different countries report controversial changes in seasonality of suicides over time, generally suggesting the decrease in the amplitudes in the recent decades [
14]. Lithuania also seems to belong to the group of countries with declining monthly variation of suicides throughout the last decade. Some authors conclude that stable or increasing suicide seasonality is characteristic to societies in transition [
15]; nevertheless, contradictions do exist. Seasonality of suicide in Hungary decreased over the last decades, while some highly stable societies, like Australia report even increasing suicide seasonality [
11,
25]. The positive association of seasonality and the incidence of suicide are well known [
25], therefore pronounced seasonality of suicides in a country with extremely high violent suicide rates should not be surprising.
There is an evidence about the meteorological, biological and physiological mechanisms underlying seasonality of suicides [
2,
26]. December decline and spring-summer increase in suicides is widely discussed in the literature [
8,
10,
18]. The latitude and climatic factors, such as day length, daily temperature, daylight, and humidity may influence mood [
27,
28] and impulsive behaviour [
29,
30]. The monthly averages of humidity, ambient temperature, duration and intensity of sunlight are positively correlated with the number of monthly suicide attempts, while cloudiness and atmospheric pressure are negatively correlated [
31]. Since the climatic conditions vary substantially between seasons in Lithuania, the effect of season on violent methods of suicides should be seen more clearly there than in countries nearer equator.
It has been suggested that seasonal vulnerability is biologically determined and associated with the circannual rhythms of central serotonin neurotransmission [
26]. A combination of hopelessness with aggressive and impulsive behaviour, anhedonia is related to low brain serotonin levels and is a predictor of suicide [
32]. In recent years, it has been demonstrated that serotonin production and serotonin-related medical and biological events (e.g., severity of migraine attacks, suicide rates, and blood coagulation) are related to environmental physical activity. Several studies have shown that suicide rates are inversely related to cosmophysical (i.e., solar and cosmic-ray) and geomagnetic activity [
5,
6]. Evidence have been provided that violent suicides and suicide attempts are higher among those with presence of depressive disorders [
33]; while depressed suicides are known to display seasonal variations with peaks occurring in spring and summer [
34]. Unfortunately, only limited data on the prevalence of depressions in Lithuania are available. The study on Lithuanian schoolchildren's self-reported explanations of suicidal ideation demonstrated that the most common reasons for suicidal inducements were depressing feelings and experiences [
35]. Surveys on influence of depression on suicides in Lithuania, however, have not been accomplished yet, and is a subject of further research.
It is difficult to distinguish biometeorological and endogenous effects from the social and cultural factors. Sociological arguments assuming that a higher level of social tensions and thus the seasonal peak in the second quarter of the year was developed by Durkheim [
36]. A more recent sociological approach to suicide seasonality labelled "broken promise theory" has been presented by Gabennesch [
37]. Vulnerable individuals may be influenced by approaching holidays, because it tends to promote hopes or expectations in people that they will feel better after holidays than they did before. When these expectations are not met, the resulting disappointment may trigger a suicidal reaction [
26]. However, the annual peak of suicides, reached before the end of usual summer vacations in Lithuania, points at other possible factors, occurring during summer months. Interestingly, the findings of this survey link to our previous report on daily variations of suicides in Lithuania, where Monday and after holidays peaks in suicides were presented. Although there were no individual data on alcohol consumption available for this survey, alcohol seemed to be strongly linked to weekly variations in mortality from suicides [
7]. The reports of other authors support our suggestions [
38]. Lithuania is a country that has been undergoing intensive social, political and economic transition over the last decade, which inevitably affected population of the country, requiring huge psychological adaptability efforts. The case for alcohol playing an important role in fluctuating mortality in Lithuania over the last decade of 20
th century has been widely discussed [
7,
39]. The binge drinking culture that exists in Lithuania and the growth of alcoholism is one of the most distressing social problems, directly related to the mortality from suicides. The recent Lithuanian forensic research data suggested that 53.3% of suicides were committed in the state of alcohol inebriation, with alcohol concentration exceeding 0.4‰. The proportion of suicides under alcohol inebriation reached the highest level in summer (60%) [
40]. In Moscow, a study of seasonal variation in mortality also pointed at harmful consequences of alcohol consumption to address the observed summer peak [
41]. Reliable data on seasonality in drinking pattern is not available for Lithuania, nevertheless, indirect observations suggest that alcohol is more commonly used during summer vacations, which might consequently contribute to a higher risk for destructive behaviours. However, the data available for this survey were not sufficient for implying alcohol as a possible causal factor for seasonality of suicides.
1 Skirmante Starkuviene,1 and Jadvyga Petrauskiene1
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