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The result was rapid growth in the Japanese population during the Yayoi period and subsequent Kofun period. Japanese people also began to use metal tools, arrowheads, new forms of pottery, moats, burial mounds, and styles of housing which were of peninsular origin. A significant cause of these dramatic changes in Japanese society was likely an influx of immigrants from southern Korea. Historian Hiroshi Tsude estimated that as many as 1.8 million Koreans immigrated to Japan during the Yayoi period. According to Satoru Nakazono, this period was "characterized by the systematic introduction of Korean peninsula culture".