A while ago I saw a movie called Food Inc. They mentioned, among other things, that chickens are raised in cramped, dark areas. At no point did they have an expert come in and say how much chickens dislike that, or even if they dislike it at all.

It seems to me that there's too much focus on how animals are treated, and to little about how they want to be treated. Does anyone know? If so, where can I find stuff about it?

Also,from a theoretical perspective, I'd like to know how they figure it out.

I think animal happiness is often measured by checking levels of stress hormones in the blood. If animals are stressed they're probably not happy, and that shows up chemically pretty clearly.

Good way to determine preferences for different states is with revealed choice, the same way we economists do it with humans. Let animals choose different environments. If they prefer A>B then offer them compensation with food for choosing B over A and see how much they need to make them indifferent between A and B.

Rob, the methods you mention seem useful. Any ideas of where one might find such studies?

I think this is an important question which is too often neglected (with sub-ER standards on importance). Sadly, almost every utilitarian who chooses to become a vegan (or use logic of the larder arguments to justify their meat eating) don't take this kind of data into consideration when making their decision but instead base it on more or less anecdotal data.

A more indirect way of measuring would be to have an expert look for outward signs of stress.

This is indeed an important topic, not just for purposes of farm-animal welfare but also for assessing the balance of happiness and suffering in the wild.

RobertWilbin makes some good points: Those are indeed two of the standard aproaches. You can find details on those and lots more by, say, searching GoogleScholar for "animal welfare" and "animal welfare measurement" and similar terms.

As far as taking these considerations seriously with respect to dietary choices, I think you'd appreciate the Ham and Eggonomics book and blog.

I already posted this in the "Hedonism vs preferentialism" thread, but this is what Singer has to say about the subject in "Peter Singer Under Fire":

I have already mentioned, in my response to Frey, the work of Marion Stamp Dawkins, a professor in the department of zoology at the University of Oxford. Dawkins has tested how hard chickens will work to change their conditions--for example, by having to repeatedly peck at a button in order to move from a wire cage to a grass run. She "informed" the chickens about their choices, by familiarizing them with both sets of conditions. She found, not surprisingly, that they prefer the grass run. She also found that hens have a strong preference to lay their eggs in darkened, soft-floored nesting boxes, rather than in a bare wire cage.

The source for that info is to an article "The Scientific Basis for Assessing Suffering in Animals" by Marian Stamp Dawkins and it appears in the book "In Defense of Animals: The Second Wave".

The entire Dawkins article can be read on Google Books: The Scientific Basis for Assessing Suffering in Animals