Justifying the enclosure of monetary incentives in mail survey cover letters

An experiment was conducted in which survey questionnaires were sent to two hundred randomly selected residents of a midwestern city. For half of the respondents, a one dollar bill ($1.00) was enclosed with the questionnaire whereas for the other half, only a quarter ($.25) was enclosed. Furthermore, the enclosure of the monetary incentive was justified in the cover letter as either an appreciative gesture or as a direct attempt to induce feelings of obligation to return the questionnaires. Subsequent response rates were assessed by condition. The overall response rate for the $1.00 condition was significantly higher than the response rate for the $.25 condition. Furthermore, the obligatory cover letter produced a significantly higher response rate than the appreciative cover letter, but only within the $1.00 conditions. Theoretical implications are discussed.