Stylometric analysis for case linkage of Japanese communications from criminals: Distinguishing originals from copycats

摘要

The effectiveness of stylometric analysis of Japanese communications from criminals for linking crimes through text mining was investigated. Forty letters of threats and blackmails in the 'Glico-Morinaga' case were analyzed as originals. These original letters were compared with four copycat letters in which the suspect in the 'Kuroko's Basketball' (KB) case imitated the writings in the 'Glico-Morinaga' (GM) case. We focused on four stylometric features in the writings: (a) bigram of characters, (b) bigram of parts-of-speech, (c) unigram of vocabularies, and (d) frequency of use of Kanji, Hiragana, and Katakana. We analyzed these stylometric features using correspondence analysis and hierarchical cluster analysis. The results showed that all the 'KB' texts gathered in the same crowd or cluster, which was separate from those including the 'GM' texts. Moreover, the 'GM' texts were not consistently distant from each other. This study suggested that stylometric analysis could distinguish between original and copycat communications, and appropriately link crimes on the basis of written information.