It has been commonly claimed that prehistoric warfare in Japan began in the Yayoi period. Population increases due to the introduction of agriculture from the Korean Peninsula to Japan resulted in the lack of land for cultivation and resources for the population, eventually triggering competition over land. This hypothesis has been supported by the demographic data inferred from historical changes in Kamekan, a burial system used especially in the Kyushu area in the Yayoi period. The present study aims to examine the previous claim by using an expanded dataset of human skeletal remains and Kamekan. First, in order to quantify the intensity of warfare, we developed a database of injured individuals found in the middle phase of the Yayoi period in two large populations in the northern Kyushu area, the Fukuoka plain and the upperand middle-stream of the Homan River. Second, we collected Kamekan data from site reports published after 1990 and constructed a comprehensive database to infer the demography in these areas. Finally, we compared the frequency of injured individuals and the inferred demography. The results suggest that the frequency of injured individuals and the population increase tended to be higher at the upper- and middle-stream of the Homan River than on the Fukuoka plain.
Different assumptions of the lifetime of each type of Kamekan can produce mixed results on the relationship between demography and the frequency of injured individuals. They were positively correlated under the traditional assumption of constant time intervals, while there was no correlation using time intervals based on carbon dating by the National Museum of Japanese History. Thus, our results are partially consistent with the previous claim that the population increase and the lack of land and resources due to the introduction of agriculture were causes of warfare in the northern Kyushu
area in the middle phase of the Yayoi period.