The authors begin with a discussion of the fallacy that "my individual purchases don't matter." They give a nice illustration:
Suppose that we take 5,280 [one foot] rulers and placed them in a straight line, end to end. This line of rulers would then be one mile long. It would appear as one long line, and if you could view the entire mile of rulers from above, you would not be able to see one single ruler. If you removed one ruler, the line would grow shorter; there is no doubt as to that. Viewed from above, removing one ruler would not appear to have any effect on the line—but again, it does.
Of course, the analogy to food purchases isn't quite accurate. In practice, as discussed here, an individual's purchasing choice is extremely unlikely to change the number of animals raised, because food is produced and sold in bulk units. However, in the event that a consumer does have an effect, that effect will be huge. Thus, in ignorance of whether a particular purchase is the one that "breaks the camel's back," the expected values of each purchase do add in the same way as the rulers. (To the extent that this reality may be de-motivating for potential vegetarians, perhaps it's better not to mention it too much?)
Pages 3-4 contain a nice discussion of the relevance of elasticities to the question of how an individual's purchases affect the quantity supplied by the market. The authors argue that the supply curves for beef, and to a lesser extent milk, are likely inelastic, while those for pork, and probably also chicken and eggs, are probably relatively elastic. More elastic supply means a bigger change in production when consumer behavior changes. Thus, for instance, abstaining from eating 1 kilogram of chicken has a bigger expected impact on the kilograms of chicken produced than abstaining from 1 kilogram of beef has on the kilograms of beef produced, other things being equal. As far as demand elasticity, the studies that the authors have done suggest a slightly bigger kilogram-for-kilogram impact of abstaining from chicken, pork, veal, and milk relative to beef or eggs. The total results--combining information about supply elasticities and demand elasticities--are shown in Figure 8.2 (see this document for the Chapter 8 figures), which I've reproduced below in sorted order:
If [someone] gives up Total Consumption of ... the Product Falls By ...
One Pound of Milk 0.56 lbs
One Pound of Beef 0.68 lbs
One Pound of Veal 0.69 lbs
One Pound of Pork 0.74 lbs
One Pound of Chicken 0.76 lbs
One Egg 0.91 egg.
Of course, this is not the end of the story. Other (often more important) factors to consider when deciding on dietary purchases include the quality of the lives of animals of different types, and the number of animals required to produce a given quantity of meat, counting both the animals themselves and their parents. Pages 5-6 explain Bailey's own views on the quality of life of various farm animals on a scale of -10 to 10 (see Figure 8.4).
In particular, Bailey thinks some farm animals have lives worth living. Looking at only the non-breeder animals, these are cows (+6), AWA-certified pork (+4), broiler chickens (+3), and cage-free hens (+2). On broiler chickens, Bailey says they "have a life worth living, but because of their leg problems and confined environment, do not fare as well as beef cattle" (p. 5). I'm more skeptical that broilers on average enjoy their lives, but even if they do, I would still be wary of giving them positive welfare because the painfulness of slaughter has to be considered. I personally wouldn't want to live even a mildly pleasant life for only 45 days if it meant that I would afterwards endure slaughter. This is probably true even if I were given electrical stunning and definitely true if I were one of the birds for which the stunning was not effective.
I assume Bailey has included the painfulness of death in his numbers, but it would be good to make this explicit. Otherwise, many readers will just imagine what a broiler chicken looks like during a typical moment of its life, multiply that by ~42 days of life, and conclude that the total experience is positive. An explicit mention of the relevance of lengths of lives would be helpful as well; indeed, this consideration makes Bailey's numbers seem a little odd. How can a beef cow, which lives for 402 days (see the "Beef" section here) have only twice the total happiness (+6 instead of +3) of a broiler chicken that lives 42 days? If Bailey's calculations do involve multiplication of his welfare numbers by lifespan, I missed that part of the text. In any event, doing straight multiplication in that way would still be misleading because it ignores the painfulness of death at the end of a life, unless stress during transport and slaughter has been implicitly incorporated into the per-day average.
Pages 6-7 discuss the impacts of farming on wild animals. In my view, this is the most important part of the calculation, especially if we give more than vanishingly small probability to insect sentience. Of course, if insects' short lives aren't worth living, then it's not clear that pesticide use in crop production represents a net harm (though whether it is or not, the chemicals could still potentially be made less painful).
As they did in Chapter 6, the book authors comment on the possibility that bigger wild animals also suffer enormously:
Animal rights groups tend to romanticize the life of animals in the wild, but anyone who has watched wildlife documentaries can attest to the cruelty of nature. We ask you, the reader, would you rather be a Wildebeest in Africa who must constantly roam for food, always in pursuit by lions and crocodiles, or would you rather be a cow in the U.S., or a hog in the U.S.? (pp. 6-7)
The authors continue with a discussion of the consideration, How many animals does it take to produce a given quantity of meat? This is the primary variable of interest in my own calculations of suffering per kilogram of meat, but the book authors do a more thorough job, by including numbers of parent animals that need to be raised, as well as the efficiency of production under various conditions. An example of the latter is that cage-free hens produce fewer eggs per week than caged hens.
In fact, this last point is rather important to the question of whether to purchase cage-free eggs (or, at least, whether to encourage others to do so). As the authors explain (p. 11), if you believe that both caged and cage-free hens suffer and that cage-free hens suffer at least ~2/3 times as much as caged hens, then for efficiency reasons, caged-hen eggs entail less total suffering than cage-free-hen eggs. However, Bailey's personal opinion (see Figure 8.4) is that cage-free hens have net happy lives, in which case cage-free eggs would clearly be preferable.
The chapter continues with interesting discussions of public attitudes toward factory farming as assessed by a nationwide telephone survey, as well as how one could compute willingness-to-pay for animal welfare by different consumers. The Appendix describes the mechanics of how elasticities can be used when assessing changes in the quantity of a good supplied, as well as the detailed calculations of how many animals need to be raised to produce a given amount of meat.
Toward the end of the main chapter, the authors make a disturbing comment, though perhaps not one that comes as a surprise:
There is evidence to believe that many Americans simply do not care very much about the well-being of farm animals. In our conversations with 300 individuals from three cities in the U.S., one-third told us that they would rather not know how farm animals are raised. They simply want to continue consuming their delicious, safe, and inexpensive food without worrying about whether the animals that provide that food suffer. (p. 17)
I hope to get a chance to read more of the book at some point; it contains lots of high-quality and thoughtful discussion. Thanks to the authors for writing it!