Summary

Sometimes people agree that meat production harms animals, but they say that their own individual meat consumption won't make a difference compared with the scales of production decisions that meat companies make. In fact, it is true that any given individual is unlikely to make a difference through meat-consumption choices, but there's a small chance that he makes a huge difference, and the expected value works out such that avoiding eating one chicken or fish roughly translates to one less chicken or fish raised and killed, ignoring elasticity effects. Sometimes the social effects of being vegetarian are very significant as well.

Translations: German, Spanish, Portuguese

"My consumption doesn't make a difference"

Occasionally people say it's not important for them to personally become vegetarian because their individual consumption is so small that it wouldn't matter in the scheme of things. Let's explore this idea further.

Everyone in the debate agrees that, at some point, a substantial decrease in demand for meat ‑‑ for instance, conversion to vegetarianism by half of the US population ‑‑ would diminish the quantity supplied of factory-farmed animals.

"That's great," argues the opponent of vegetarianism, "but it has nothing to do with individual purchasing habits. The only decisions that will actually affect the amount of animal suffering are massive investment choices: whether to build a new factory farm or whether to put an addition on a colossal chicken house. One consumer cannot even change the bulk purchases that each individual grocery store or dining center makes, much less the collective demand that all of these retailers exert. So I can continue to eat meat without creating demand for the production of any more animals. A certain amount of meat will be produced whether I am vegetarian or not, so why shouldn't I at least subsist off those excess scraps, instead of letting them go to waste?" The following sections reply to this argument.

A hypothetical supermarket supply system

Suppose that a supermarket currently purchases three big cases per week of factory-farmed chickens, with each case containing 25 birds. The store does not purchase fractions of cases, so even if several surplus chickens remain each week, the supermarket will continue to buy three cases. This is what the anti-vegetarian means by "subsisting off of surplus animal products that would otherwise go to waste": the three cases are purchased anyway, so consuming one or two more chickens simply attenuates the surplus.

What would happen, though, if 25 customers decided to buy tempeh or beans instead of chickens? The purchasing agent who orders weekly cases of chickens would probably buy two cases instead of three. But any given consumer can't tell how far the store is from that cutoff point between three vs. two cases. The probability that any given chicken is the chicken that causes two cases instead of three to be purchased is 1/25. If you do avoid the chicken at the cutoff point, you prevent a whole case—25 chickens—from being ordered next week. Thus, the expected value of any given chicken is (1/25) * 25 = 1 chicken, just like common sense would suggest.

The same logic applies in reverse to the "subsistence from surplus" excuse. The critic of vegetarianism claims that the supermarket would buy three cases of chickens per week anyway, regardless of whether he himself buys one and lowers the store's surplus waste from, say, 8 to 7. But at some point—at some minimum level of surplus or deficit—the store's purchasing agent will choose to buy four cases instead of three. As before, any individual consumer has no way of knowing which number chicken he happens to be buying. The expected value is once again (1/25) * 25 = 1 chicken.

Of course, the exact expected values will fluctuate on account of the randomness of the purchasing agent's decisions (if, for instance, she would not buy one fewer case until 35 fewer consumers demanded chickens, even though each case includes only 25 chickens), but they should average out over the long run in such a way that forbearing the purchase of any given amount of an animal product will be expected to reduce bulk purchase of that amount of the product. Especially in the age of electronic tracking of purchases, we should expect these decisions to be made fairly precisely.

A firsthand account

In a comment on this argument, Chris Sea said:

This discussion is needlessly complicated. Anyone who has worked in a grocery store—however small—will tell you that they have a pretty good system in place to avoid throwing away excess meat. They sometimes mess up and end up throwing away fruits and veggies but very rarely throw away meat or dairy products. In one year of working at a grocery store, it happened once. There's a cash incentive to not order excess products and to not hold inventory for long periods. One person going vegan would be picked up. Maybe not by those doing the work, but the amount of meat ordered would be decreased.

Impacting big farms

The same expected-value logic applies also to the rest of the demand chain: At some critical mass of fewer cases ordered by stores, distributors will purchase fewer chicken wings from farms, and that reduced demand from farms will, at some point, constrict production. By the end, the probability that any given consumer will impact animal production is miniscule, but the benefits if he does are immense. Thus, the expected value of refraining from the purchase of any given amount of an animal product is roughly equivalent to preventing the production of the portion of an animal that the product represents. (This ignores some considerations about the price elasticity of meat, but these don't substantially change the conclusion.)

It is entirely possible (perhaps even likely) that a vegetarian may go through her entire life and never, by failing to purchase factory-farmed animal products, have actually prevented any animal suffering by lowering production. But because she has no way of knowing when the special purchase that does set off the chain of significant demand reduction will be, she has to act as if every purchase does count. And for all she knows, she may be a consumer who has more than her share of impact on demand for factory-farmed products.

Different foods matter differently

The actual expected amounts of suffering prevented vary widely by the type of animal product avoided. Avoiding purchase of a chicken prevents roughly six weeks of direct suffering by a broiler hen. Avoiding purchase of a dozen eggs directly prevents a little more than 12 days of suffering by a battery-cage hen (not counting the suffering of ground-up male chicks). Avoiding purchase of a half-gallon (1.89-liter) carton of milk prevents between 2.3 and 2.8 hours of suffering for a milk cow, ignoring the effect on her calf (this assumes a cow produces 20-30 liters of milk in a day). For more, see "How Much Direct Suffering is Caused by Various Animal Foods?"

Special situations

While the mechanism described above for the impact of vegetarianism applies to most cases, there are some situations in which the demand-reduction benefits of abstaining from animal products are less likely to materialize. The most notable example might be a party or picnic to which—despite one's best efforts—people have brought hot dogs and hamburgers. The purchasing decisions of a supermarket may be imprecise and sometimes arbitrary, but they will eventually be affected if demand changes by a great enough amount. The same cannot be said of those who purchase items for a picnic. In general, the purchaser will buy some overestimated amount of food beforehand, regardless of how many people actually consume those comestibles at the event. And whereas a store that purchases far too much of a product will keep electronic records and change its behavior the next time, people buying picnic food probably will not, except maybe in a vague way through their mental models of consumption quantities. So it is somewhat less likely that one's decision to eat or refrain from eating a factory-farmed-animal product at an informal social gathering will make a difference to the amount of food that the organizer purchases the next time. (Perhaps the best way to limit the harm done by a picnic is to ask the purchaser ahead of time to buy less meat.)

I should add, however, that to the extent non-vegetarians take home and actually consume left-overs from a social event, they may thereby reduce the amount of food they buy in the future, including meat products. In fact, this effect may be sometimes true even for non-meat leftovers. The lesson is, then: If you have leftovers, give them to the person who's most likely to buy meat for his own meals. The situation may differ if, by taking the leftovers yourself, you save money that you can then donate to prevent suffering in other ways.

Social effects are important

Even if consuming a chicken sandwich at a picnic would assuredly not change the direct amount of meat purchased, there might still be other good reasons for not doing so.

People may not realize the extensive reasoning presented above and thence may view a vegetarian's consumption of the chicken sandwich as contradictory. This misunderstanding might diminish the respect that others have for the vegetarian in particular and for the cause of vegetarianism in general. Perhaps the vegetarian would unintentionally be showing how hard it is to maintain his diet, thereby discouraging others from trying.

Avoiding animal products can raise the topic of factory farming. This, indeed, is one of the most important consequences of being a vegetarian in any context. Vegetarianism is a convenient meme in that it brings itself up for discussion whenever social eating comes into play. Good discussions—combined with the literature exchange or further research that might follow—have the potential to permanently change other people's eating habits. In fact, Peter Singer himself, who has arguably done more for animals than any other human in history, was inspired to think about the topic of animal suffering over lunch conversations with a vegetarian friend at the University of Oxford.

"Expected Utility, Contributory Causation, and Vegetarianism" by Gaverick Matheny is an excellent article that makes a number of the same points discussed above.

"Questions of Priority and Interspecies Comparison of Happiness" by Oscar Horta includes good numbers on the extent of harm caused by factory farming. Several footnotes link to this site.

Postscript: What about humane meat?

Much has been written debating whether humane meat is ethically acceptable. I won't rehash most of the arguments here but will just make a few points: