Kofun volumes suggest resource concentration like modern firms

Kofun across Japan, including Gunma Prefecture’s Hodota Hachimanzuka Kofun, served as a yardstick for measuring “resource mobilization” in ancient society. Associate Professor Junshi Watanabe of Seijo University used the March 2025 edition of Nara Women’s University’s nationwide kofun database to estimate volume from the length and height of burial mounds. The dataset included 4,545 keyhole-shaped kofun, among other types, allowing researchers to compare mathematically the mobilization capacity of groups that built them from the mid-3rd to the 7th century.

The results showed a power law with a cumulative exponent of nearly 1: the number of larger keyhole-shaped kofun in the top tier declined rapidly. This resembles the distribution of modern companies, where only a small number have high sales and a large share is concentrated among the top firms. The middle tier was close to a log-normal distribution, which takes on a bell-curve shape when plotted logarithmically. A pattern previously known in societies with established written administration, monetary economies and tax systems thus also appeared before those institutions had taken hold.

The size of kofun itself varied by period and region, but after normalizing by the median to adjust for differences in scale, many distributions overlapped on a common curve. However, in Kinki, the political center, and parts of Sanyo, including the semi-central region of Kibi, the exponent fell below 1, indicating stronger resource concentration in a small number of giant kofun. A structure common across Japan and biases specific to central regions emerged at the same time.

So what changes? Using kofun volume as a clue makes it possible to compare disparities in mobilization capacity among groups on a nationwide scale, even in periods before written administrative records and a monetary economy became established. Overlaying the statistical perspective used for company distributions on the observation of individual archaeological sites opens a new way to examine the organization of ancient society.

However, the findings indicate only a possibility. Volume is a rough indicator of labor and resources, and the research has been reported in a preprint paper by Associate Professor Watanabe. This analysis alone does not establish the mechanisms or causal relationships underlying resource mobilization in ancient society.