As I understand it, Clark argues that two factors acted to change this state of affairs. First, technological change accelerated to the point where the the amount of resources that could be exploited was changing faster than the time scale on which birth and death rates responded, keeping the system permanently out of equilibrium. Second, people started practicing real fertility control, which acts to damp the population response to higher income levels (and of course there are natural limits on how much lifespan can increase solely on the basis of income.) [There's also a whole bunch of stuff about how these changes are due to cultural and biological evolution as a result of differential reproduction between the rich and the poor, but I don't want to talk about that just yet.]
Much of the first half of the course is devoted to explicating the model, and in classic counterintuitive economist style, Clark makes a big point about how in the Malthusian world, things that you ordinarily think of as good are bad, and vice versa. To take on example, if a horrible disease gets introduced into your society so that it increases the death rate by 10%, that leaves more resources for everyone else with the result that the people who don't die of plague have a higher overall standard of living. Here's Clark's Table 2.2 (page 37 of "A Farewall to Alms"):
|Malthusian "Virtues" and "Vices"|
|Harvest failures||Public granaries|
|Income inequality||Income equality|
Note the scare quotes around virtues and vices. Clark sort of equivocates between the view that societies with short life spans and high material standards of living are really better and the view that income levels are just one instrument. For instance, on page 36 he writes:
In summary table 2.2 shows Malthusian "virtues" and "vices." But virtue and vice here are measured with reference only to whether actions raised or lowered material income per person.
This sort of supports the "it's just an instrument" view but then on page 38 Clark writes:
The failure of settled agriculture to improve living conditions, and the possibility that living conditions fell with the arrival of agricultire, have led some economists, anthropologists, and archaeologists to puzzle over why mankind abandoned the superior hunter-gatherer lifestyle for inferior agrarian societies.
This argument isn't a new one. In fact, this specific form of the argument was famously made by Diamond's The Worst Mistake in the History of the Human Race. Clark comes back to the more general theme repeatedly, and suggests in a number of places during the class that people in fact would prefer to live in societies with a lot of disease and violence but correspondingly higher material living standards (though again, there is some ambiguity about the level to which he is actually endorsing this view.)
It seems to me that one ought to raise at least two objections to this general line of argument. First, it's not at all obvious that it's that useful to assess people's welfare by their income level (or as I once heard it put more crudely, by the number of calories they are able to consume per day.) [I wish I could remember where I read this.] First, to a great degree human sense of how happy they are is positional, so if everyone in society gets 100% richer, that doesn't make everyone 100% happier, it's just that instead of being jealous of the guy next to me at the stop light in his Audi RS4, I'm now jealous that he has a 911 Turbo; not much of a net win. Second, in the Malthusian model a lot of the "virtues" that result in a higher standard of living are things people find really unpleasant. In particular, random violence, crop failures, and disease (and uncertainty in general) is incredibly stressful (see Sapolsky's Why Zebras Don't Get Ulcers for a good primer on this.) It's not clear at all that people in general would rather trade much higher rates of catastrophic events for a somewhat higher level of expected welfare if you survive. Clark does offer some arguments that people make choices along these lines, for instance that people voluntarily joined the East India Company even though the risks were very high, but it's not clear that it's really that useful to use the risk/reward behaviors of 20-year old male adventure seekers as a stand-in for the entire society.
Even if we are to concede that people are individually happier in societies with high violence and disease rates (and hence lower populations) but high standards of living than they would be in societies with lower violence and disease rates (and hence higher populations) but correspondingly lower standards of living, that does not mean that those societies are truly more desirable. (See, for instance Parfit's "Only France Survives" in Reasons and Persons). Surely, most people in societies with a low standard of living would nevertheless prefer to be alive than dead, even if dead would mean that other people would live better, so it's difficult to say that the society with the lower population is better, especially when that equilibrium is obtained by high death rates (i.e., the killing of people who exist and have their own interests) rather than low birth rates (i.e., the nonexistence of people who might otherwise exist.)