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Fundamental but virtually unexplored issues in human-computer interaction involve the roles of biases in software engineering tasks. In studies of naturalistic testing tasks, as well as ones which follow common laboratory models in this area, we have found ample evidence that testers have positive test bias. This bias is manifest as a tendency to execute about four times as many positive tests, designed to show that “the program works,” as tests which challenge the program. In our prior work, we have found that the expertise of the subjects, the completeness of the software specifications, and the presence/absence of program errors may reduce positive test bias. Skilled computer scientists invent specifications to test in the absence of actual specifications, but still exhibit positive test bias.