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The Ageing of the New Zealand Population, 1881-2051 - WP 03/27

3  Proximate determinants of changes in age structure

People enter or leave a population through birth, death, and migration. The age structure of a population is thus an outcome of trends in fertility, mortality, and migration rates.

3.1  Fertility

The birth rate for the total New Zealand population has been drifting downwards since the 1880s, with an interruption during the baby boom of the 1940s, 1950s, and 1960s (Figure 4). In the long run, fertility decline leads to a lower ratio of young people to old people, and hence an “older” age structure.

Figure 4 – Births per thousand population, 1860-2000
Births per thousand population, 1860-2000
Source: 1860-1994 (Thorns and Sedgwick 1997: Table 2.4); 1995-2000 (Statistics New Zealand 2001a: Table 2.08).

Note – Before 1 September 1995, the ethnicity of birth was determined by the ‘degree-of-blood’ of the child; from that date is was determined by self-identification of the mother. The apparent increase in Maori fertility during the 1990s is probably an artefact of this change.

However, the short-run effects of a fertility decline differ from the long-run effects. A fertility decline leads immediately to an fall in youth dependency, but the rise in old age dependency does not occur until the large pre-decline cohorts reach old age. During the intervening period the total dependency ratio is temporarily below trend: there is a temporary reduction in the number of dependents per working-age person. This reduction in dependency can give the economy a transitory boost, and has been referred to as a “demographic bonus” (Bloom and Williamson 1998).

The short-run effects on the age structure of the decline in fertility after the early 1960s are clearly visible in Figure 2. Youth dependency declined immediately, but the effects on old-age dependency have still not appeared; in the intervening period, total dependency has been reduced.

Fertility trends for the total population disguise quite different trends among Maori. As Figure 4 shows, Maori fertility rates did not begin declining until the 1960s, 80 years later than the population as a whole. The apparent increase in fertility during the 1990s is probably just an artefact of definitions used. Before 1 September 1995, the ethnicity of a birth was determined by the “degree-of-blood” of the child; from 1 September it was determined by self-identified status of the mother. Section 4.1 discusses how the contrasting birth rates are reflected in contrasting age structures.

3.2  Mortality

Death rates fell across all age groups during the twentieth century, as Figure 5 shows. Over the period 1901-1961, infant mortality declined substantially, while old-age mortality declined more modestly; after 1961 infant mortality continued to fall, but the largest absolute declines were among old people. Because they were concentrated especially among young people, the early mortality declines actually acted to reduce the average age of the population. They reduced it directly, by raising the number of surviving children, and indirectly, by raising the proportion of people surviving to the reproductive ages and having children themselves. Later mortality declines, in contrast, increased the average age of the population. They raised the number of surviving older people, with little effect on the number of births, since the proportion of people surviving to the reproductive changes was already very high (Preston, Heuveline and Guillot 2001: 159-60).

Figure 5 – Deaths per thousand population, by age-group, 1901, 1961, and 2001
Deaths per thousand population, by age-group, 1901, 1961, and 2001
Source: Statistics New Zealand (2002b: Table 5.15).

Note – 1901 estimates do not include Maori. All estimates are for males and females combined.

Mortality trends, like fertility trends, have been different for Maori than for the population as a whole. Figure 6 shows some representative life expectancy data. The “SNZ” series are official Statistics New Zealand estimates. The “NZCMS” series are from a recent publication of the New Zealand Cohort Mortality Study. These series correct for the fact that the ethnicity of Maori and Pacific people was often not recorded on death certificates during the 1980s and 1990s, leading to an underestimate of mortality among these groups[2]. As is apparent, Maori female life expectancy was much lower than total female life expectancy in the mid-1950s, but improved much faster over the next 25 years, reducing the difference considerably. From the early 1980s to the late 1990s, Maori female life expectancy improved little, while total female life expectancy improved sharply, so that the difference again widened. A similar pattern occurred with males (Figure 14). Life expectancies for Pacific people appear to be higher than for Maori, but lower than for the population as a whole. As will be seen in Section 4.1, the lower life expectancies reinforce the effects of the higher birth rates, to give Maori and Pacific Peoples younger age structures than other New Zealanders.

Figure 6 – Life expectancy at birth, females
Life expectancy at birth, females
Source: SNZ Total and Maori: Statistics New Zealand (2000: 102) and Statistics New Zealand (2002a: Table 4.10); NZCMS Maori, Pacific, and Non-Maori Non-Pacific: Ajwani et al (2003: Appendix Table 24).

Note – NZCMS refers to “New Zealand Cohort Mortality Study”. The figure shows the ‘prioritised’ NZCMS series. Data for males are shown in the Appendix.

Notes

  • [2]The estimates shown define as “Maori” anyone who included “Maori” among the one or more ethnicities they reported on their census forms; the estimates define as “Pacific” anyone who included “Pacific” among the ethnicities they reported, but did not include “Maori”. Another series was calculated in the New Zealand Cohort Mortality Study, defining people as “Maori” only if they reported Maori as their sole ethnicity, and defining people as “Pacific” only if they reported Pacific as their sole identity. Estimated life expectancies for Maori and Pacific people in these series were lower than in the “prioritised” series shown here.
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