Real-time bidding tools are allowing advertisers to carve up Web users into distinct demographics, which are then used to determine the ads presented to any given person, Jeffrey Rosen writes. That's potentially troubling since it means ads and promotions can be targeted to people based on their perceived spending power. "The segmented profiles that advertisers ... use to define us may become more pervasive and significant than the identities we use to define ourselves," Rosen writes.

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