Doing the right thing is about more than having a job that does good, or giving a lot to charity. There are certain cases where how you choose to spend your money makes a big difference, particularly when it comes to eating meat and animal products.

Last week, I spoke with Will MacAskill, the author of Doing Good Better, an Oxford philosopher, and the co-founder of the groups 80,000 Hours and Giving What We Can, about why choosing not to consume eggs or chicken can make a really significant difference. For more, be sure to check out the rest of our interview, where MacAskill explains how to find the career where you can do the most good for the world.

Fair trade doesn't work. But veganism does.

Will MacAskill: Trying to do good via your purchasing decisions isn't that great a way of having a big impact. Attempts to incorporate making an impact into purchasing decisions, like fair trade, seem to have gone quite badly. Even when there are good opportunities to have an impact through your purchasing decisions, they're often quite limited, and you could do more good by, for example, increasing your donations to the most effective charities.

Dylan Matthews: So ethical consumption in general doesn't do much good. Is that true of becoming a vegetarian or vegan as well?

Will MacAskill: Within ethical consumption, the case for cutting out at least some forms of meat is by far the strongest, compared to other things. If you crunch the numbers on amount of harm done per meal, or per calorie consumed, then by far the strongest argument is to cut out chicken, then (non-free range) eggs, then pork. The argument for cutting out beef, and especially the argument for cutting out milk, is much, much weaker. Chickens suffer the most of all the animals, they're in the worst conditions, and you kill more chickens in the typical American diet than you do beef cows or dairy cows, simply because those animals are so much larger. "You're not saving animal lives — you're preventing animals from coming into existence" In terms of how much of an impact that is, my view is that at least cutting out the worst forms of animal consumption — so especially chicken, eggs, and pork — is a fairly trivial cost to yourself, yet pretty big impact. Every egg you cut out removes one day of chicken suffering from the world, on average, and that includes if you take into account elasticities. That seems like a pretty good option.

Your eating choices really do matter

Dylan Matthews: What's the actual mechanism there? What am I doing when I cut out an egg? Is it just that the farms become smaller because demand is shrinking?

Will MacAskill: Right. The chicken's already dead, so that's not where you're having an impact. Instead, you're just slightly decreasing demand for chicken. That means that shops are going to buy, on average, a bit less. They're going to stock a bit less chicken, so they'll buy less chicken from farmers, and in the future they'll produce fewer chickens. You're not saving animal lives — you're preventing animals from coming into existence. You may think it's such a small change that it wouldn't make any difference. Most of the time you don't make a difference. That's true. But occasionally you make a large difference, which makes up for it. Maybe the supermarket makes decisions on how many chickens it's going to buy in 1,000 chicken breast chunks or something. So if it sold above a certain number, then it would increase stock, and if it sold below a certain number, it would decrease stock. Most of the time, you won't make a difference, but if you take it over the threshold, then it won't decide to buy 1,000 more chickens. So maybe 1 in 1,000 times you'll make a difference to 1,000 chickens. "If you're eating a fish that would otherwise have gone on to eat 10 more fish, maybe it's a net benefit for you to eat that fish" Economists have actually studied this, and assessed how much of a difference you make. If you don't buy one egg, you decrease the supply by 0.91. If you reduce the amount of beef you buy by 1 pound, you reduce the supply of beef by 0.68 pounds.

Is it ever permissible to eat animals?

Dylan Matthews: Is it ethical to eat animals in a situation where you wouldn't be increasing demand? If there's leftover chicken at a dinner party and it's going in the garbage otherwise, is that okay to eat?

Will MacAskill: I think in those cases it's plausible, at least, that it's not wrong. It might be kind of weird. There are lots of things that are morally very weird. Suppose your grandmother dies, who you love very much and were looking after, and then after she dies you chuck her in the garbage. Where's the harm there? It's maybe not wrong at all, but it's very morally weird.

Dylan Matthews: It's certainly upsetting.

Will MacAskill: It's upsetting, it indicates you're quite a weird character psychologically, and so on. That's the explanation for why that might be wrong; it stems more from what it signals about your character. Psychologically it would be an odd thing to genuinely care for the suffering of animals and then also eat their flesh. I think we'd all find it very weird to eat the flesh of humans even if they're already dead and no one was going to care. It'd still be very strange. The second thing, and this is why I don't eat meat in those circumstances, is the worry about it being a slippery slope. If I were to eat meat in those circumstances, I start to get the taste for meat again, maybe that means I'm going to renege on my commitment to being vegetarian in the future and then start eating it more in other cases. I think that's actually a pretty reasonable consequentialist argument. I do know some vegans who would eat meat where it's just been left at a restaurant or something. It's also permissible to eat mussels and oysters. There are some pretty good arguments that they don't feel pain, and are really just little slabs of meat on a plate. They've got more in common with plants than animals. I know a lot of vegans who eat bivalves. The other article I really want to write is the ethics of eating animals that eat other animals. So if you're eating a fish that would otherwise have gone on to eat 10 more fish, maybe it's a net benefit for you to eat that fish. You're saving more fish in doing so. Obviously there are fish that would've eaten that but would've died of starvation, but I think it's plausible that some cases where you can eat carnivorous animals and it's a net benefit for animal welfare.

Dylan Matthews: That's fascinating. It doesn't really help you with chickens, pigs, or cows.