The appropriateness of lie scales commonly used in psychometric tools for health research with children was investigated. A convenience sample of 74 children was used to test the hypothesis that children at a less mature stage of cognitive development would score higher on a lie scale than would children at a more mature stage. Each child was tested using the Cartoon Conservation Scale to assess the Piagetian cognitive level and the nine-item lie scale for the “How I Think and Feel Test”. The findings supported the hypothesis; a two-way analysis of variance showed a significant main effect of cognitive level (F = 13.274; p < 0.001). A stepwise multiple regression indicated that age and cognitive level accounted for 53% of the variance in lie scale scores.