The place of population change in explaining geographical inequalities in health in New Zealand.

We appreciate Harper’s comments on the role of population change in explaining rising geographical inequalities in health in New Zealand. 1 As we note in our original paper, we agree that selective migration patterns may well be an important explanation as to why regional health status in New Zealand has become more geographically polarized during the 1980s and 1990s. 2 Selective patterns of migration between the regions of New Zealand and the high levels of immigration into the country, and emigration from it, are likely to lead to a high level of population sorting between areas and to have strengthened the widening life expectancy gap during the 1980s and 1990s. Given that New Zealand experiences almost the highest rate of combined immigration and emigration (population turnover) in the world, a if selective migration were to have an explanatory role anywhere it would be in helping to understand changing health patterns within this country. Although we agree that selective migration is likely to be an important factor in explaining our results, there are five areas of interpretation where we disagree with Harper. First, Harper contends that growing geographical inequalities in health observed using the Slope Index of Inequality (SII) may be due to differential levels of population growth between regions of the country (i.e. if areas with the highest or lowest life expectancies grow at a faster rate then the gap will increase even if the life expectancy in those areas remains the same). Harper gives an example of where he thinks the problem lies: