I found this one through Hendry Lee’s blog:

Journal of Interactive Advertising has published an academic paper studying email and postal direct marketing. Here are the hypotheses drawn from this study:

• Consumers are likely to find spam more intrusive than direct mail.
• Ad intrusiveness caused by a) spam; b) postal direct mail negatively correlates with attitudes toward the advertising technique.
• The level of perceived loss of control by consumers will be greater for spam than direct mail.
• Perceived loss of control mediates the relationship between ad intrusiveness caused by spam and ad irritation.
• Consumers are likely to experience a higher level of advertising irritation from spam than direct mail communications.
• The degree of advertising irritation caused by a) spam; b) postal direct mail is negatively correlated with attitudes toward the advertising technique.

For methods of measurements, refer to the paper for details.