Here’s a paragraph Rick Wash and I wrote for the USENIX paper, somewhat revised for later use, concerning spyware:
An installer program acts on behalf of the computer owner to install desired software. However, the installer program is also acting on behalf of its author, who may have different incentives than the computer owner. The author may surreptitiously include installation of undesired software such as spyware, zombies, or keystroke loggers. Rogue installation is a hidden action problem: the actions of one party (the installer) are not easy to observe. One typical design response is to require a bond that can be seized if unwanted behavior is discovered (an escrowed warranty, in essence), or a mechanism that screens unwanted behavior by providing incentives that induce legitimate installers to take actions distinguishable from those who are illegitimate.