These results demonstrate that low 5-HT transporter density and high ambient temperatures are associated with higher rates of violent crimes in Finland. Over a 16-year period, increases in ambient temperature accounted for 24% of the seasonal variation in violent crime without monthly stratification, and 10% when stratified by month. An additional analysis during a 12-month period showed that ambient temperature correlated with a one month delay in the seasonal changes in peripheral SERT density (r = −0.64 among healthy subjects and −0.68 among violent offenders), which itself correlated strongly with monthly violent crime rate (r from −0.56 to −0.63, respectively). This implies that the seasonal variation in violent offending is influenced by natural fluctuations in the serotonergic system, and that a 2 °C increase in ambient temperature could increase the violent crime rate by more than 3% if other contributing factors remained constant. This estimate is close to the results published by Ranson10, which suggest that climate change will increase the incidence of murder in the United States by 2.2%, and the incidence of aggravated assaults by 2.3%, by end of the century. Concerning absolute numbers of violent crime, Anderson et al. have estimated that 8 °F (corresponding to about 4 °C) warming will result to 100,000 additional murders/assults in US26. Two previous studies on this topic have suggested that the positive correlation between ambient temperature is non-linear, and that incidence of violent crime does not increase further when the ambient temperature is higher than 27–32 °C8, 27. However, these findinds may be explained by shortcomings in analytical methods, and the correlation is more linear when time of the day is taken into account in the statistical analysis28. Although it seems plausible that the correlation between ambient temperature and aggressive behaviour exists also beyond temperatures higher than 27 °C, our results can be generalized with reasonable quantitative certainty only to non-tropical and non-subtropical areas, where mean temperatures for all months are below 25 °C, with substantial seasonal variations in temperature.

Global warming may increase violent behavior by worsening of developmental environment of children, by causing war and civil unrest, and by direct effect on the human physiology29. Our finding of a substantial inverse correlation between monthly mean B max and monthly assault occurrence rates is in line with reports of reduced platelet imipramine B max among adolescents with conduct disorder22 and impulsive-aggressive children21, as well with decreased cerebral SERT densities among suicidal individuals30, 31. On the other hand, our violent offenders had higher B max values than healthy individuals. A similar finding has been reported by Coccaro et al.20 who obtained higher SERT densities from patients with personality disorders and aggressive behaviour, when compared to healthy individuals. However, within the aggressive patient group the B max values had an inverse correlation with aggressivity20. Because most all of our patients were habitual violent offenders, it was not possible to use standard aggression scales to analyze the correlation between B max and aggression, as the majority of them would have had maximum scores. In conclusion, alterations in SERT levels differ between depressed suicidal patients, impulsive children and adult antisocial individuals, thus reflecting heterogeneous dysfunctions of serotonergic system. Our results indicate that the serotonergic system responds to increasing ambient temperature with a one month delay. It is plausible to suggest that the gradient in the seasonal change of SERT levels is a major factor related to aggression, while the absolute amount of SERT activity (as B max ) does not play a major role. We presume that alterations in platelet SERT density reflect analogous alterations in the brain, since previous studies have shown that SERT densities in the brain are higher during the autumn and winter when compared to spring and summer, and that there is a negative correlation between these densities and the duration of daily sunshine32. Also, light therapy is known to decrease SERT density in the brain33. Although hours of sunshine had a somewhat similar correlation with peripheral SERT densities in our study, ambient temperature had substantially stronger correlations with both SERT densities and the occurrence of violent crime. Moreover, when the months were stratified, the correlation became weakly negative between sunshine and the violent crime rate. This suggests that ambient temperature may be the most important environmental factor that modifies aggressive behavior in humans. However, it must be acknowledged that we had data on SERT only for one year period, and it was not possible to stratify month in the one-year analysis to control the effect of other seasonal variables than temperature such as summer vacation. In the context of criminality, ambient temperature does not provide only the motive to commit a crime (increased impulsivity and irritability influenced by serotonergic system), but it does influence also opportunity. Put simply, violent crimes tend to be committed during nice weather (i.e., summer vs. winter).

To our knowledge, this is the first report to demonstrate that a seasonal variation in violent crime is associated with weather conditions and 5-HT function. However, this association does not necessarily indicate causation. In order to address the question of causation, we must consider the relationships that concern the associations between ambient temperature, serotonin transporter density and the occurrence of violent crime. To this end, we stratified the months to eliminate the effect of factors other than weather, such as the holiday season (i.e., we compared the results for cold January vs. mild January, or hot July with chilly July, and so on). By logic, it is extremely unlikely that a) SERT density or violent crime rate could affect ambient temperature, b) violent crime rates could affect SERT densities, or c) there could be a common causal factor other than the seasonal variation of solar radiation that could influence ambient temperature, SERT density, and violent crime. Therefore, it seems to be obvious that ambient temperature drives variations in SERT levels and the occurrence of violent crime. Either high ambient temperature and/or related sunshine has a solely direct effect on brain serotonergic activity, or the effect is mediated partly through intermediating factors. The analysis of 18-year period showed that although there was a substantial correlation between the mean ambient temperature of the preceding month versus the monthly violent crime rate (r = 0.40), agreeing with the assumption that ambient temperature modifies SERT-levels and violent behavior with one-month delay. However, the correlation was even stronger between mean ambient temperature of the current month versus the violent crime rate of the same month (r = 0.51). This indicates that the association between ambient temperature and violent behavior is multifactorial. Since a large body of literature implies that serotoninergic activity is strongly related to impulsive and aggressive behavior16, 17, 20, 34, it is reasonable to suggest that changes in serotonin transporter density modify the incidence of violent behavior, and that seasonal effects of solar radiation influence both. Recent studies indicate that, in animals, whole-body heating activates subdivisions of the dorsal raphe nucleus implicated in mood regulation, and whole-body warming has a substantial antidepressant effect in humans35. It is plausible that when temperature rises, people go out more and meet other people, which increases the rate of violent incidents. Therefore, the increase of violent crime rate is attributable also to serotonergic activation of the victims, and not only that of offenders. From an evolutionary point of view, ambient weather conditions have a direct impact on an eternal question for our species; should I stay or should I go? In particular, seasonal “bad” weather conditions (i.e., low ambient temperature, low amounts of sunshine) up-regulate 5-HT function, thereby decreasing impulsivity and risk-taking; e.g., influencing one to not wander too far from the homesite in bad weather. In contrast, “fine” weather (i.e., high ambient temperature and sunshine) may down-regulate 5-HT function and contribute to higher impulsivity and novelty seeking, such as hunting and other forms of useful aggression.