Measures of mental wellbeing are heavily relied upon to identify at-risk individuals. However, self-reported mental health metrics might be unduly affected by mis-reporting (perhaps stemming from stigma effects). In this paper we consider this phenomenon using data from the British Household Panel Survey (BHPS) and its successor, Understanding Society, the UK Household Longitudinal Study (UKHLS) over the period 1991 to 2016. In particular, in separate analyses of males and females we focus on the GHQ-12 measure, and specifically its sub-components, and how inaccurate reporting can adversely affect the distribution of the index. The results suggest that individuals typically over-report pyschological wellbeing and that reporting bias is greater for males. The results are then used to adjust the composite GHQ- 12 score to take such mis-reporting behaviours into account. To further illustrate the importance of this, we compare the effects of the adjusted and unadjusted GHQ-12 index when modelling a number of economic transitions. The results reveal that using the original GHQ-12 score generally leads to an underestimate of the effect of psychological distress on transitions into improved economic states, such as unemployment into employment.