Double bounded dichotomous choice (DBDC) contingent valuation offers increased efficiency of willingness to pay (WTP) estimates compared with the single bounded format. However, evidence suggests DBDC generates anomalous respondent behaviour. This paper provides the first investigation and explanation of these anomalies in health. Results suggest the incentives for truthful preference revelation are altered in the presence of a follow up question. This result is found using both regression techniques and analysis of raw responses. Although findings suggest 'very certain' respondents exhibit less anomalous behaviour inconsistencies remain across bounds. The results of this study question the use of iterative valuation formats.