Introduction
In the past, international comparisons have been treated as a useful starting point for studies of the social determinants of health, but that is all In fact, they have rather more to offer. For a start, national populations provide a natural unit of observation to tackle some of the biggest and most important questions in public health. These are questions such as:
• Why are mortality rates falling so consistently in most parts of the world, in virtually all age groups?
• What distinguishes the exceptions to this general pattern?
• Who or what deserves credit for the improvements (and the blame for the exceptional declines in life expectancy)?
Comparisons between countries also serve a unique bench-marking function, indicating what could, plausibly, be achieved if the performance of comparable countries was emulated.
Ecological studies have often been a source of hypotheses about the causes of disease, exploiting the fact that variations in behaviour and environment between countries tend to be greater than variations within countries. These hypotheses have typically involved risk factors for disease measured at the level of individuals. However, the causes of disease that are 'social’ are, by definition, characteristics of groups rather than individuals and in some instances, are characteristics of whole countries. (Examples include taxation policies and occupational health legislation.) Here, international comparisons provide the only means of studying directly the association between social circumstances and health outcomes. The same approach may be taken to analyse interventions that are implemented nation-wide, such as the introduction of co-payments in health care (Schoen 2000) or the relation between distributive income policies and health inequalities (Kunst 1997).
Australia and New Zealand are obvious candidates for comparison: despite extreme variations in geography, the societies have much in common. Donald Home suggested that ‘in one sense no country except New Zealand can be compared with Australia: these are the only two “Western” nations that are strategically part of Asia’ (Home 1966). They share a history of British colonial settlement, and have inherited language, culture and public institutions from a common stock. However, there are also important differences between the two in population make-up, natural resources and political arrangements. As the connections with Britain have loosened, the contrasts between Australia and New Zealand have become more marked.