Comparing Populations

Populations can be compared in many ways, for example changing world and national populations across time, differences between national populations and differences between groups (for example, ethnic groups) within a national population.


World Population - Explanation

Watch the video by Dr Hans Rosling on world population, this formed part of the BBC programme The Joy of Stats. If the video does not work correctly you can download a version of the clip here.

The video clip is ©Wingspan Productions Ltd 2010

The world population reached one billion for the first time in the early 1800s then took over a century to reach two billion in the late 1920s. It took less and less time to reach three (just over 30 years), four (about 15 years), five (roughly 13 years) and six (about 12 years) billion. However, it took 13 years to reach seven billion in 2012 and is predicted to take a longer and longer time to reach eight (16-18 years) and nine billion (about 20 years) in roughly 2050.

Most estimates then have the world population being steady or in decline. China and India together account for over a third of the world's population while some small nations such as Monaco and Nauru each account for less than 0.0005%.



When talking about population it is necessary to consider both population growth, and population decline. Take a look at the graphs below. What causes the types of changes seen in these two graphs?


Once upon a time the world's population grew...

The growth peaked and then slowed down... down...

There are two building blocks to population change at the global level– births (fertility) and deaths (mortality). The changes in fertility and mortality seen today have been unfolding for many years, a process known as the demographic transition. Large countries (such as India and China) contribute more to world population growth than other countries.

The following pie chart shows the geographic distribution of the world's population in 2012.

What will cause the end of growth and decline?...

IMG = Internal Momentum Growth | IMD = Internal Momentum of Decline

The answer is changes to the structure of the population such as population ageing.

An ageing population is caused by:

  1. Increase in numbers of elderly (from increased life-expectancy and immigration of the elderly) – This is numerical ageing.
  2. Increase in proportion of the elderly to the young (from low / falling birth rates).

Globally, there are slightly more men than women (sex ratio of 1.01:1) but this is not the case in all countries. Some countries have a marked gender balance in favour of males (according to the UNFPA the United Arab Emirates, Bahrian, India, Samoa and China had sex ratios of 2.34:1, 1.64:1, 1.07:1, 1.06:1 and 1.08:1 respectively in 2012).

Other countries have a marked gender imbalance in favour of females (according to the UNFPA the Russian Federation, Ukraine, Latvia, Tuvalu, Tokelau and Curacao had sex ratios of 0.86:1, 0.85:1, 0.84:1, 0.89:1, 0.92:1 and 0.83:1 respectively in 2012).

You can find more information on Sex Ratio by Country with the 'Search' button below (external link to Wikipedia).

However, some countries in 2012 had a 1:1 ratio of males: females including the Dominican Republic, French Guiana, Honduras, Kiribati, Libya, Norway, Tonga and Zimbabwe. About 95% of female children born in a developed country today will make it to age 60, while 88 per cent of males will do so.

Although a number of countries have concerns about ageing populations (for example, in Europe, America and Australasia) overall the world's population is young with roughly a quarter aged under 15, and almost another two thirds aged between 15–64. However, this is expected to change in the next 30-40 years with the median age globally rising from about 30 years to 35 years.



Fertility is usually measured by the Total Fertility Rate.


The Total Fertility Rate (TFR) is a synthetic estimate of the average number of children a woman would expect to have across her lifetime if she were to experience all of the age-specific birth rates occurring in that year. The method for calculating the TFR is provided in the section titled National Populations. In most countries the TFR is declining.

An example of this decline is given in the link below, which illustrates the declining TFR in China.


New Zealand

The following graph shows that although the fertility rate in New Zealand is declining the total number of births may not.

New Zealand - TFR compared with birth cohort size



Changes in the death rate also influence population growth. The expected mortality rate of any given cohort (people born in the same time period) is measured by Life Expectancy, the number of additional years of expected life for all those born in a given year.


'Life expectancy' should not be confused with the 'average age at death' (average age of all those dying in a given year). Note that life expectancy changes over a lifetime. The following graph demonstrates the impact of life expectancy on the age of death.

Force of mortality graph

Graphs exploring the relationship between life expectancy and GDP and between fertility and infant mortality can be found on the Gapminder website,

Graph showing Percentage Surviving against Age


New Zealand

In New Zealand, the number of people surviving to at least age 60 has increased as life expectancy has increased.