1. Demographic Transition 2

2. Crowded planet?
In the previous lecture, we discussed how much of the world has experienced a demographic transition – the birth rate decreased to meet the lower death rate produced by development. Much of our worries about sustainability are connected to population growth. How quickly will population change – are we course for an “S-shaped” growth pattern, in which population slowly reaches a stable level, or do we decades of accelerated growth ahead of us – in which case we might be headed for a “J-shaped” crash.

3. Population trends
It is possible to use demographics to forecast the future population. These tools aren’t perfect, but in the absence of system collapse or civilization wide disaster (such as a global pandemic, nuclear war or run-away climate change) changes in the direction of population growth are very slow, evolving over decades. The population of much the 21st century is largely already determined by current trends. Half of Yemen’s population is under the age of 18, while the median age in Greece is Knowing this, we would immediately predict higher rates of population growth in Yemen, relative to Greece.

4. Average Population Age
Younger populations do imply higher fertility rates. This is a map of the world that shows the average ages of different countries. Lighter colors mean younger ages, while the blue countries in Europe, Canada, and Japan, indicate average ages in the 40s. At 38, I’m an average age for someone living in the US or Australia – you can use this map to see if your age is higher or lower than your country’s average!

5. Average Population Age
Using this map, where would you predict fertility rates to be highest?

6. Average Population Age
You might have said countries in sub-Saharan Africa, Yemen, Sri Lanka, or Afghanistan, as these countries have very young populations, averaging under 20 years old.

7. World fertility rates Do we see any regional trends?
And you would be right! The “hot” colors on this map show fertility rates – yellow is 4 births per woman, with the redder and pinker countries having birth rates of 6 to 7. All the countries with low average ages have high fertility rates.

8. World fertility rates
Note also that the older countries have much lower fertility – in many places below births per woman is considered the replacement rate – so many countries are on track for lower future populations, or have already seen death rates exceed birth rates.

9. Population growth rates
In fact, we see this in many places. Note how Eastern Europe, Russia and Japan (light purple) have growth rates below 0 – that is, they are shrinking. Young, high birth rate countries (green and yellow) have high growth rates.

10. Fertility Factors CIA World Factbook
What determines birth rates? Can they be changed? This will be examined in more detail in the homework, but it should be said that it is not completely clear why countries experience lower birth rates when they develop. One important factor is wealth. Wealthy countries have lower birthrates. In this graph, you can see that high GDP per person countries (towards the right) have low fertility rates, at or below replacement (2.1 children per woman).

11. Fertility and Wealth CIA World Factbook
You may have noticed that Saudi Arabia is an outlier in this graph – fertility is more than twice as high as in comparable income countries. The social status of women also appears to be important in fertility.

12. Fertility and Marriage Age
Women's Age at First Marriage and Family Size in Selected Countries, 2000s
When women marry later, they have less children. US women marry, on average, when they are in their mid 20s, and the US has a replacement fertility rate. The top four countries shown have much lower marriage ages, and much higher fertility rates.

13. Fertility and Education
Women's Education and Family Size in Selected Countries, 2000s
Increased female education is also clearly related to fertility. In all countries, more education for women is associated with lower fertility. This graph suggests a clear relationship, although, as with all of these data sets, this may not be directly causal, and education, wealth and social factors are all interconnected. Women with higher levels of education are also more likely to work than to marry young.
Given that the worldwide trend is towards higher levels of education, wealth, and human rights, does this then imply that future population growth will slow? We will examine this proposition in the next lecture.