Does Vegetarianism Make A Difference?
With respect to this strategic argument, we can set out a continuum, on which both of the extreme ends are obviously false. One extreme -- often endorsed, for the sake of hyperbolic impact, by critics of ethical consumerism -- says that personal choices in the market make no difference at all. If that were true, we'd all be drinking New Coke. On the other hand, only someone in the grip of extreme free market mythology would believe that personal choices in the market are entirely sufficient to bring about any desired social change. Given that the world is not made up of fully-informed rational egoists experiencing no transaction costs, collective action aimed at structural change is a critical part of any social movement.
The question, then, is: how much of a difference does (a particular instance of) ethical consumerism make? Does it make enough difference that it should be pursued at some cost (in, at the very least, foregone convenience and time spent thinking about it)? I think the left strategic criticism of ethical consumerism tends to undercount the amount of difference it can make by being misled by the term "ethical consumerism" into imagining that whatever impact it has is going to fall within the parameters suggested by the free market mythology. Within those parameters, the impact of ethical consumerism qua reduction in demand/profit is often quite small (though as mentioned above, not non-zero). But it also has other forms of impact. And vegetarianism in particular, I think, carries the potental for these other forms of impact farther than most ethical consumerism.
What would be the end result of the collective action proposed as a replacement for, or at least supplement to, ethical consumerism in the case of vegetarianism? It would be the elimination of the practice of raising and slaughtering animals for food. Were this to be achieved, we would all then have to be vegetarians. This would be a significant shift in how we organize our way of life. Food is such an intimate part of life that major changes in it necessitate major changes -- at the most basic level, acquiring new understandings of how to plan and cook healthy and tasty meals. Present-day ethical consumerist vegetarianism serves an important role in working through those issues of how to have a meatless life, and inducts people into what has been learned.
The change that vegetarianism requires also creates an inevitable sort of witnessing for the cause. Because food is central not just to how we live our individual lives, but to how we socially engage with others, it's hard to be unobtrusively a vegetarian (and even more so a vegan). Having people around modeling the end-product of a social change draws attention to the issue and accustoms others to see the animal rights position as at least reasonable and worth treating respectfully -- widening the Overton window -- aside from any explicit debates or conversions.
Contrast what I've said about vegetarianism with another example of ethical consumerism -- boycotting Wal-Mart. The end product of that struggle is to either drive Wal-Mart out of town or get it to reform its business practices. These things surely make a big difference to the intended beneficiaries, e.g. workers farther back in the production chain. But it doesn't make a huge qualitative difference in one's life whether you got your laundry detergent and jeans from Wal-Mart or you had to go all the way to K-Mart or Spag's* for them. Thus, the impact of this type of ethical consumerism is mostly limited to the market principles suggested by the word "consumerism."
So even if vegetarianism's direct impact on the size of the meat industry is negligible (which I don't think it is, but for the sake of argument), it is valuable in laying the groundwork both for starting the necessary collective action, and realizing the gains that that collective action would ultimately win. The left strategic argument is a useful corrective to people who -- seduced by the free market mythology -- imagine that their personal food choices constitute the be-all of activism. But by it does not rebut the idea of vegetarianism as one significant part of an animal rights agenda.
*I went to Wikipedia to get a link explaining Spag's, only to find that it apparently closed shortly after I left Worcester. This makes me sad, not because I thought the store itself was so great, but because it was symbolic of the spirit of being a real, loyal native of Worcester.