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I’m Jonathan Tomkin from the University of Illinois. From the reading we know that in 1900, life expectancy at birth was 47 years in the United States, and between 45 and 50 years in Europe, Japan and Australia. This changed quickly over the course of a century. Uh, US life expectancy at birth shot up to 68 years by 1950, and is now 77 years in 2000. This increase has been due to many factors: uh, increased sanitation, better standards of living, better medical care. Uh, note the standard of care available in this 19th-century painting, um, which would be very different from what we’d experience in a modern hospital today. This increase in life expectancy — or, we could equivalently say decrease in death rates — has produced population booms, but these increases have stopped in many countries around the world today. Indeed, some countries, such as Japan and Germany, the red and blue arrows, are seeing death rates exceeding birth rates, and so experiencing flat or declining populations. Other countries, such as Uganda and Nigeria, green and yellow lines, have rapid contemporary growth rates. Why is there a difference between these two sets of countries? One way to think about this is by using the concept of the demographic transition. Population numbers change when there is an imbalance between the death rate, that is the number of deaths per person per year, and the birth rate, the number of births per person per year. Consider how birth and death rates today might be different from birth rates and death rates in the past. Here’s a diagram of birth and death rates versus time. Just take a minute to think about, uh, what sort of relationship we might expect, um, maybe 200 or 300 years ago, between birth and death rates. You might say that they’re fairly close to being in balance because populations weren’t changing very quickly, um, and indeed you might also think that they might be different from today because the higher life expectancies that we experience today translates into lower death rates, that is less people are dying per person per year than they were in the past. We’re living longer; it’s an equivalent statement. Through most of human history the population has been relatively stable in a given location. High birth rates, which I’ve marked with a green line, are balanced with high death rates, the red line. Uh, they might fluctuate from year to year. You might have a good crop or a bad drought or a war or a disease or a little baby boom, but overall there hasn’t been much change, and because there is a balance between these two forces, we see the population stay relatively stable. Life expectancies were lower than today, but on average more children were born, and this kept the population rate stable. People had their children earlier and they had larger families. As living conditions improved, which first occurred around the start of the 17th century in Europe, the death rate plunged, probably due to factors such as sanitation, higher standards of living, and, uh, belatedly, medical care improved as well. The birth rate did not change at first. People were accustomed to having large families, so there is a mismatch in the second stage of the demographic transition between the number of people being born and the number of people who are dying. Uh, there’s a big population increase, and I’ve marked that here with this blue zone. So if we think about this, we can see there are less people dying but the same number of people being born, the population must go up.