Wow, that's pretty interesting.  Validis's writeup explains this structure as a reflection of political tendencies in the readers.  However, I can think of at least one other generative mechanism.  If Amazon's "buddy book" marketing is effective (and I'm pretty sure it is, because it regularly "gets" me), then the system tends to reproduce its own links.  If there was any polarization in preferences when the system was launched, then new buyers would have been steered toward these same choices (and given limited budgets, therefore away from others).  You could spin this out into friendship networks and back too: Discussing books with like-minded friends leads to recommendations for other books from a similar political perspective, which are then purchased and contribute to Amazon's data.  Amazon's buddy book system may actually cause political polarization, and would appear to at least represent a source of intertia in that phenomenon.

Steven R. (Steve) Corman
Professor, Hugh Downs School of Human Communication
Arizona State University

Vice-Chair, Organizational Communication Division
International Communication Association

-----Original Message-----
From: Steve Borgatti [mailto:[log in to unmask]]
Sent: Tuesday, March 04, 2003 12:53 PM
To: [log in to unmask]
Subject: A window in the American souls -- politics and book
co-purchasing patterns

This page by Valdis has been making the rounds. It's pretty interesting ...


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