Age, period and cohort analysis of trends in mortality from major diseases in Japan, 1955 to 1979: peculiarity of the cohort born in the early Showa Era.

Japanese researchers have reported that recent mortality rates from diabetes mellitus, ischemic heart disease, peptic ulcer, cirrhosis of the liver and suicide for middle-aged Japanese men have increased by comparison with those for other age groups. There has been some controversy over the etiology of this unusual trend, and in particular whether it is due primarily to recent undesirable socio-economic factors (period effects) or to factors specific to these cohorts born in the early Showa Era, around 1925 to 1940 (cohort effects). A possible source of this controversy lies in the methods which have been used to describe the trends; these are mostly descriptive and graphical. To elucidate which factors are responsible for these trends, we analysed the mortality data quantitatively applying an age-period-cohort model modified so that period effects remain constant within certain age groups but may vary from one age group to the next. Although the identifiability problem still occurs in the modified model, estimable curvature components of time effects may be used to examine these unusual trends. In fact, the peculiarity of the cohort born in the early Showa Era was clearly detected by the curvature components of cohort effects for these major diseases. These findings are consistent with the 'cohort hypothesis' for the recent peculiar trend in Japanese male mortality.