Some Fermi estimates; feel free to disagree with specific numbers and provide your own.
Let's take an average human life as a unit of value; i.e. the value of human's life is 1.
How large part of "a value of human's life" is "having lunch, in general, as opposed to only having a breakfast and a dinner every day of your life"? Let's say it's somewhere between 1/10 and 1/100, because there are many other things humans value, such as not being in pain, or having sex, or having status, or whatever.
If we estimate an average human life to be about 10 000 or 20 000 days, then "having this specific lunch" is between 1/10 000 and 1/20 000 of "having lunch, in general".
But the choice is actually not between having a lunch and not having a lunch, but between having a chicken lunch or having a vegan lunch. Let's say the taste of chicken provides between 1/4 and 1/10 of the value of a lunch.
Putting these numbers together, a value of "having a chicken for a specific lunch" is about 1 / 1 000 000 of a value of a human life.
As a quick check, imagine that you are both in a vegan country, where chickens are simply not available for lunch. Would you sell 1% of your remaining lifespan (less than 1 year) to the Devil in return for having a chicken for lunch each day of your life? I guess many people would, probably even more than 1%; and the revealed preferences (e.g. people dying as a result of salmonella) seem to match this.
So, it seems like ethically it is right to eat chicken if and only if a value of a human life is greater than value of 1 000 000 chicken's lives. Which, according to many people, it is.
Possible methodological problems:
1) Scope insensitivity: maybe people say that 1 000 000 chickens are less worth than humans simply because they cannot imagine what "1 000 000" actually means; they only imagine about dozen chickens when making the emotional judgement. On the other hand, there are people who as a part of their profession kill large numbers of chicken, so they would have a near-mode idea of what it means. How many people would be willing to do such profession, though?
2) How much is the desire to eat chicken a result of cultural brainwashing? Do people in countries where vegetarianism is normal agree that having a chicken instead would increase the value of their lunch by 10%? That is, how much is "wanting to eat a chicken" actually wanting to eat "a chicken", as opposed to simply wanting to eat "the same thing as yesterday".
Putting these numbers together, a value of "having a chicken for a specific lunch" is about 1 / 1 000 000 of a value of a human life.
I'd estimate that as ((amount you're willing to pay for a chicken lunch) - (amount you're willing to pay for a vegan lunch))/(statistical value of life). But that's in the same ballpark.
If it's worth saying, but not worth its own post, then it goes here.
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