Can incentives undermine intrinsic motivation to participate in epidemiologic surveys?

Response rates to surveys are decreasing. The purpose of this study was to evaluate the use of lottery tickets as incentives in an epidemiologic control group. A self-administered questionnaire was sent to parents in the municipality of Stockholm, Sweden, who were to be used as a control group in a study addressing stress in parents of children with cancer. A stratified random sample of 450 parents were randomized into three incentive groups: (a) no incentive; (b) a promised incentive of one lottery ticket to be received upon reply; (c) a promised incentive of one lottery ticket to be received upon reply and an additional lottery ticket upon reply within 1 week. The overall response rate across the three groups was 65.3%. The response rate was highest in the no incentive group (69.3%) and lowest in the one plus one lottery ticket group (62.0%). In a survival analysis, the difference between the two response curves was significant by the log-rank test (P = 0.04), with the no incentive group having a shorter time to response than the incentive group. Our findings suggest that the use of lottery tickets as incentives to increase participation in a mail questionnaire among parents may be less valuable or even harmful. Incentives may undermine motivation in studies in which the intrinsic motivation of the respondents is already high.