Cause-specific mortality differences across socioeconomic position of municipalities in Japan, 1973-1977 and 1993-1998: Increased importance of injury and suicide in inequality for ages under 75

Background: Socioeconomic inequality in health has been a major concern in public health. This study examined socioeconomic inequality in regional mortality and the impact on inequality by cause of deaths in 1973-1977 and 1993-1998 using municipal statistics in Japan. Methods: The municipalities across the country (N = 3244 in 1973-1977 and 3334 in 1993-1998) were classified into quintiles according to the index of socioeconomic position (SEP) obtained by principal component analysis of municipal indicators related to income and education. Mortality gradient by SEP for selected major causes of death in the population aged 0-74 years was examined using standardized mortality ratio by quintile and rate ratio of mortality across quintiles. As a measure of cause-specific impact on inequality, the number of excess deaths from each cause in the lower four SEP quintiles compared with the highest quintile was calculated. Results: Mortality gradient by SEP and excess deaths in the lower SEP quintiles due to injury and suicide markedly increased from 1973-1977 to 1993-1998 for both males and females. In contrast, stroke, especially cerebral haemorrhage, showed a decrease in mortality gradient and excess deaths. For females in 1993-1998, a negative gradient of mortality by SEP was found, and cancer contributed the higher all-cause mortality in the higher SEP quintile. Conclusions: The relative importance of socioeconomic inequality in regional mortality of stroke decreased, while that of injury and suicide increased. The prevention of injury and suicide, in addition to stroke, in socioeconomically disadvantaged regions, and cancer in urban areas with higher SEP should be given priority.