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群馬県の保渡田八幡塚古墳を含む全国の古墳が、古代社会の「資源動員」を測る物差しになった。成城大学の渡邊隼史准教授は、奈良女子大学の全国古墳データベース2025年3月版を使い、墳丘の長さや高さから体積を推定。対象には前方後円墳4545基などが含まれ、3世紀半ばから7世紀に築かれた集団の動員力を、数理的に比べた。
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.
結果は、上位の巨大な前方後円墳ほど数が急速に減る、累積指数がほぼ1のべき乗則を示した。これは、売上高の大きい企業が少数に限られ、上位に大きな割合が集まる現代企業の分布と似た形だ。中間層は、対数を取ると釣り鐘型になる対数正規分布に近かった。文字による行政記録や貨幣経済、税制が整った社会で知られてきたパターンが、制度の定着前にも見えたことになる。
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.
時代や地域で古墳の規模そのものは変わったが、中央値で正規化して規模差を補正すると、多くの分布は共通の曲線に重なった。ただし、政治的中心だった近畿や、準中心的な吉備を含む山陽の一部では指数が1を下回り、少数の巨大古墳への資源集中がより強かった。全国に共通する構造と、中心地に固有の偏りが同時に浮かんだ。
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.
それで何が変わるのか。 古墳の体積を手がかりにすれば、文字による行政記録や貨幣経済が定着する前の時代でも、集団間の動員力の偏りを全国規模で比較できる。考古学の個別遺跡の観察に、企業分布などと同じ統計的な見方を重ねられるため、古代社会の組織化を別の角度から検討する道が開く。
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.
もっとも、示されたのはあくまで可能性だ。体積は人手と資源のおおよその目安であり、研究成果は渡邊准教授のプレプリント論文として報告されている。今回の分析だけで、古代の資源動員の仕組みや因果関係まで確定したわけではない。
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.