people with incapacities on to IB and SB from other parts of the benefit system. Other possible explanations relate to changes in the structure and intensity of employment which may have caused employment opportunities for people with ill-health and disabilities to worsen in spite of the improvement in overall employment conditions. Analysis of the New Zealand Disability Survey suggests that the presence of a disability reduces an individual's probability of full-time employment to less than half that for a non-disabled individual with similar demographic characteristics, and that employment disadvantage is particularly notable for those experiencing psychological or psychiatric disabilities (Jensen et al 2005). Yet another set of possible explanations relate to changes in the way that qualifying incapacities were interpreted or administered. Determining whether an individual is sufficiently incapacitated to qualify for IB or SB is not straightforward (Lennan 2000). The eligibility criteria are subject to interpretation and a range of factors other than medical eligibility may influence the decisions of assessing doctors (White 2000). Given this, changes in the way in which doctors interpreted the medical criteria for entry could have increased the proportion of the population viewed as having qualifying incapacities. A final set of possible explanations relate to changes in the prevalence of incapacity. Growing numbers of people were coming on to SB and IB with a mental illness. The increase is consistent with trends in other developed countries. Comparisons of the prevalence of mental illnesses over time are difficult to make but rates may have been