Sex ratio at birth
The current excess of approximately 105 male births for every 100 female births is a well established phenomena. It has remained largely unchanged for the past century. Virtually all countries for which complete data is available have sex ratios at birth of between 104 and 107.
Some researchers have suggested that the sex ratio of births may be influenced by the mother’s age, ethnicity, location or by whether she is living with the father. There is very little evidence of such effects in New Zealand, or in other countries.
The universality of the pattern would support the notion that the sex ratio at birth is biologically pre-programmed to compensate for higher male mortality during infancy, childhood and adolescence – nature’s way of trying to ensure that a sex ratio of 100 males per 100 females is reached during childbearing ages.
Historical patterns
Historically, childbirth took a significant toll on the females aged in their 20s and 30s. Nevertheless, male mortality (combined effects of accidents, suicide, conflict and illness) exceeded female mortality at other ages. Thus, throughout New Zealand’s history, life expectancy at birth has been greater for females than males. The pattern of slightly more male babies but greater female longevity would suggest a sex ratio close to parity. The fact that early colonial New Zealand had a large surplus of males among the settler populations, especially in the young adult ages, is largely attributable to the effects of migration. The risks associated with long-distance travel, and the primary industries to which early immigrants were attracted resulted in a disproportional number of male migrants. At the beginning of the twentieth century, male immigrants outnumbered female migrants by a margin of 2 to 1. As travel and communication became easier and cheaper there were radical shifts in nature of permanent and long-term migration.
Today, similar numbers of permanent or long-term migrants are male and female, with a slight net gain of females or net loss of males. Motives such as education and career advancement, which had traditionally fuelled internal migration, are increasingly fuelling international migration.
Changes in the sex ratio
The 1858 census recorded 131 males for every 100 females. Subsequent censuses saw the ratio of males to females draw closer to parity. With exceptions during the two World Wars, females first outnumbered males in the 1971 Census. The excess of females has increased steadily since then. This reflects the cumulative effects of lower fertility (males outnumber females at birth), improvements in longevity (particularly among females) and changes in immigration and emigration patterns, together with the propensity for more males than females in the young adult ages to be missed by censuses.
The United Nations estimates, females outnumber males by 6 percent in the "more developed regions" (Europe, North America, Australia, New Zealand and Japan). This compares a 3 percent excess in New Zealand. The difference is related to New Zealand’s younger population (median age of 35 years compared to 40 years in developed regions). The ratio of females to males in New Zealand rose from parity as the median age of New Zealand’s population also rose. The median age increased by more than 9 years, from 25.7 years in 1971 to 35.2 years in 2004. However although New Zealand’s population is projected to continue ageing in the near future, the sex ratio appears to have peaked - New Zealand males appear to be pegging back the female advantage in longevity (male life expectancy increased by 1.9 years between 1996 and 2001 compared to an increase of 1.4 years for females). This would suggest that, at least among the elderly, the excess of females will diminish in future.
Regional variations
Although nationally the 1971 Census recorded similar numbers of males and females, there was marked regional variation. In 1971, females outnumbered males in regions such as Auckland, Hawke’s Bay, Canterbury and Otago, whereas males outnumbered females in Northland, Bay of Plenty/Waikato, Marlborough, Westland and Southland. Primary industries (agriculture, fishing, mining, etc.) often provide vocational training for school leavers, but have attracted more males and fewer females. In contrast many of the service industries to which females have gravitated have, even if they are located in the rural community, sought qualifications that can only be obtained in a larger town or city.
Among the elderly, the presence of a partner often enables a couple to remain living in a rural community. The tendency for females to partner older males coupled with the greater longevity of females, means that often older females leave rural communities for the care and support services offered by a larger community, while males of a similar age still have the support of their younger wife.
Despite the fact that nationally the ratio of females to males has been increasing, the diversity in regional experience is actually decreasing. A number of factors may be a play here; for example, more gender equality in employment, increased ability to commute to work, increased availability of aged-care, etc. Latest figures indicate that the West Coast is the only region with more males than females, all other regions range from just above parity to a female excess of 5 percent. Within regions, more rural districts (e.g. South Taranaki, Ruapehu, Hurunui, Southland) are typically the areas which have an excess of males over females.
Age variations
Since the ratio of male to female babies is so consistent at birth, variations at older ages are due to to subsequent migration and mortality. Under the current constant low level of mortality, parity in male and female population size would not naturally be reached until around the age of 60 years. Latest population estimates (which adjust for such factors as census undercount and people temporarily overseas) indicate that parity is currently reached at age 25. There is a subsequent peak in the early thirties, when females outnumber males by approximately 9 percent, but it then declines to just above parity for people aged in their late fifties and early sixties before climbing again as the female advantage in longevity starts to become apparent at older ages. By the time people reach their late 80s, females outnumber males two to one.
The 1986 Census was the first to record an excess of females over males among the population aged in their thirties. The excess of females in this age-group has increased sharply in each subsequent census. Nevertheless, it is noteworthy that population aged in the 30s at the 1986 census would now be in their 50s, yet current data shows that at this age the male population is only slightly smaller than the female. Moreover, the observed sex ratio for those aged in their 50s is very close to the ratio expected with current mortality patterns Thus there is some evidence to suggest that if New Zealand is in fact losing males aged in the late-20s and 30s, these losses may not be permanent.