The voluminous literature generated by Ronald Inglehart's thesis that a shift in values from materialist to post-materialist occurs among citizens as societies modernize indicates the seminal nature of the theory. The predictive power of the theory has become more reliable with the continuation of surveys over the decades since the theory's inception. Nevertheless, one problem remains unsolved using conventional methodology: how to determine the extent to which the proportion of post-materialists was already increasing during the years preceding the first survey in 1970. This article contributes to the solution of that problem by analyzing the values contained in 33,479 Japanese newspaper editorials from 1945 to 2000. By reflecting the public's values, Japanese newspapers are able to maintain the largest circulations in the world. Moreover, the lifetime employment system in Japan enables us to "control for" life-cycle effects on value change, because the age of editorial board members has been constant during the postwar period. Using qualitative and quantitative techniques, I confirm the thesis that value change occurs during the process of economic development. My findings support the socialization hypothesis of the original theory, that is, that the basic values of a person reflect the socioeconomic conditions that prevailed during his or her pre-adult years.