The danger of physical harm faced by those with a low IQ is not restricted to the battlefield. A 2016 study by four psychologists using data from the Danish Conscription Database (containing 728,160 men) revealed low IQ to be a risk factor for almost all causes of death. A drop in IQ by a single standard deviation (roughly 15 points) was associated with a 28 percent increase in mortality risk. The association between low IQ and mortality was particularly great for homicide and respiratory disease (such as lung cancer). The high homicide rate could reflect a predisposition for those of low IQ to find themselves in dangerous situations, perhaps due to a lack of economic opportunity or an increased likelihood of being victimized by predatory individuals.