Elsevier

The Lancet

Volume 368, Issue 9529, 1–7 July 2006, Pages 44-52
The Lancet

Articles
What is the contribution of smoking and socioeconomic position to ethnic inequalities in mortality in New Zealand?

https://doi.org/10.1016/S0140-6736(06)68813-2Get rights and content

Summary

Background

Mortality rates for Māori are twice those for non-Māori in New Zealand. We have assessed the contribution of tobacco smoking and socioeconomic position to these inequalities in 45–74-year-old census respondents during 1981–84 and 1996–99 (2·3 and 2·7 million person-years, respectively).

Methods

We used linked census and mortality cohort datasets with measures of socioeconomic position (household income, highest educational qualification, car access, labour-force status, and neighbourhood deprivation) and smoking (never, ex, current). We used direct standardisation to adjust for smoking and Poisson regression to adjust for socioeconomic position.

Findings

The apparent contribution of smoking to mortality differences between Māori and non-Māori non-Pacific people was greatest for women in 1996–99 (8% reduction in standardised rate difference); it had increased from 3% in 1981–84. The corresponding reductions in men were 5% in 1996–99 and −1% in 1981–84. The apparent contribution of socioeconomic factors to mortality differences between Māori and non-Māori non-Pacific was greatest for men (39% in 1981–84 and 37% in 1996–99) and increased over time for women (from 23% in 1981–84 to 32% in 1996–99).

Interpretation

Although small, the contribution of smoking to ethnic inequalities in mortality increased over time and might grow more during the next two decades if differences in smoking between ethnic groups continue to increase. Better measurement of socioeconomic position (eg, lifecourse measures, asset wealth) might increase the proportion of ethnic inequalities attributable to socioeconomic position, perhaps to about half. Action to redress socioeconomic gaps and control of tobacco use will both be important in reducing ethnic inequalities in health.

Introduction

There are large and increasing inequalities in mortality by ethnicity in New Zealand.1 Māori, the Indigenous population of New Zealand that migrated from Polynesia about 1000 years ago, have roughly twice the mortality rate of non-Māori non-Pacific people (largely of European extraction, arriving in New Zealand in a continuing migration since the early 1800s). Pacific people, mostly migrants from the Pacific Islands (eg, Tonga, Samoa) since World War II, have mortality rates between those of the other two ethnic groups. Trends in life expectancy since 1950 for Māori and non-Māori show convergence between Māori and non-Māori up to the 1970s, then divergence in the 1980s and 1990s, and lately a possible start of parallel tracking of life expectancy trends (figure 1).

There are many possible explanations for these mortality inequalities. Similar to the experience of other Indigenous populations, the colonisation of New Zealand since the 1800s severely affected the Māori with loss of land and other resources.2, 3, 4, 5 Socioeconomic resources are now unequally distributed: Māori and Pacific people have lower incomes, employment rates, and educational achievement than non-Māori non-Pacific people.6 Racism, whether structural (eg, as the result of colonisation) or interpersonal, also affects health.7, 8, 9 Access to, and quality of, health services contribute to ethnic inequalities. Even in a country such as New Zealand with a mostly universal health-care system, there is growing evidence that Māori have less access to, or lower quality of, life-saving treatments.10, 11, 12, 13 Proximal health risk factors such as smoking are also unevenly distributed by ethnic group.14 There are some genetic differences between groups with common ancestral origins,15 but their role in the total mortality disparity is debatable.

We used linked census-mortality data that create cohort studies of the entire New Zealand population to assess the contribution of tobacco smoking and socioeconomic position to ethnic inequalities in mortality. To address the issue of the contribution of smoking to these inequalities is challenging, for many reasons. First, smoking is a proximal or downstream risk factor for health that is strongly patterned by socioeconomic and other factors. For example, higher socioeconomic groups and non-marginalised populations tend to adopt smoking earlier, but also drop smoking earlier, than lower socioeconomic groups and marginalised populations.16 However, in New Zealand, tobacco was a common trade commodity between Māori and non-Māori in the 1800s, resulting in widespread uptake of smoking among Māori.17 Second, the tobacco epidemic is dynamic, with varying smoking prevalence over time, by sex, and by ethnic group. No long-term trend data on tobacco smoking by ethnicity are available, but from 1981 to 1996 the prevalence of smoking in the age-group 15–79 years fell from 50% to 38% in Māori men and from 52% to 45% in Māori women. The relative decreases in non-Māori non-Pacific people were greater: 33% to 23% in men and 28% to 20% in women. Consequently, the contribution of smoking to ethnic inequalities will also vary over time and by sex. Third, assessment from observational data of how much of a given exposure-outcome association (eg, ethnicity-mortality association) is mediated by intermediary variables such as smoking is difficult19, 20, 21 because the intermediary variables are correlated with other known and unknown variables that also have independent associations with the exposure and the outcome. Thus, statistical adjustment for the potential intermediary variable might also adjust for other causal (or confounding) mechanisms. However, in the absence of a longitudinal study with many repeated measures, carefully thought out and staged analyses are still able to provide information about the contributions of various causal mechanisms to social inequalities in health.22 Fourth, the relative strength of the association of smoking with mortality varies with ethnic group in New Zealand. We reported that the relative risk of mortality for current versus never smokers was greater for non-Māori non-Pacific people than for Māori in 1996–99 (2·22 [95% CI 2·12–2·33] vs 1·51 [1·35–1·69] in men and 2·20 [2·09–2·33] vs 1·45 [1·27–1·66] in women).23 This heterogeneity of the relative risk points to the importance of other competing risk factors for poor health for Māori, such that the relative contribution of smoking is partly overshadowed. But it also presents methodological challenges to assessment of the contribution of smoking to ethnic inequalities in mortality.

Assessment and interpretation of the contribution of socioeconomic position to ethnic inequalities is also challenging.24, 25 To account for ethnic inequalities by controlling for socioeconomic factors does not make ethnic inequalities in health acceptable. Rather, the unequal (and unfair) distribution of socioeconomic resources between ethnic groups in the first place is part of the reason for ethnic disparities; redressing socioeconomic inequalities is therefore a strategy to reduce ethnic inequalities in health. Second, socioeconomic position is a multifaceted construct. No study can claim to measure all facets of socioeconomic position fully and accurately over the lifecourse. Therefore, the true contribution of socioeconomic position to ethnic inequalities is likely to be greater than that identified empirically.26

Section snippets

Dataset

1981 and 1996 census records were each anonymously and probabilistically linked to 3 years of subsequent mortality data,27, 28 creating two separate cohort studies of the New Zealand population followed up for 3 years. 73% of eligible mortality records for the age-group 45–74 years were linked for the 1981–84 cohort, and 81% for the 1996–99 cohort.29 We defined this age-group because the New Zealand Census-Mortality Study does not attempt to link people aged 75 years and older on census night,

Results

Table 2 shows the distribution of person-years and deaths and age-standardised mortality rates within smoking by ethnicity groups, for census respondents with complete sex, age, ethnicity, and smoking data. The number of deaths of people of Pacific ethnicity in some cells is small (eg, 21 for current women smokers during 1981–84), so findings for Pacific people should be interpreted with caution. Mortality rates of current smokers increased slightly from 1981–84 to 1996–99 in Māori and Pacific

Discussion

What is the contribution of smoking to ethnic inequalities in mortality in New Zealand? In terms of Māori to non-Māori non-Pacific mortality gaps during the 1980s and 1990s, these analyses suggest a contribution somewhere between very little and around 10%. But there are two important patterns. First, the contribution to the gap was greater for women than for men in both cohorts. Second, the contribution increased over time in both men and women, reaching about 5% and 8%, respectively, in

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