All of it - again not saying they're wrong, just that the overall mindset I've seen is "I believe animals have emotions, and that's what matters" or "we don't need proof, it's just obvious", not "I have collected evidence beyond anecdotes that animals have emotions". By knee-jerk, I mean people tend to believe first, and look at the evidence either later or never.
(Though to be fair, much of the animal-non-rights movement seems to be similarly non-scientific :P)
Have you had a look at The Cambridge Declaration on Consciousness?
We declare the following: “The absence of a neocortex does not appear to preclude an organism from experiencing affective states. Convergent evidence indicates that non-human animals have the neuroanatomical, neurochemical, and neurophysiological substrates of conscious states along with the capacity to exhibit intentional behaviors. Consequently, the weight of evidence indicates that humans are not unique in possessing the neurological substrates that generate consciousness. Non- human animals, including all mammals and birds, and many other creatures, including octopuses, also possess these neurological substrates.”
Perhaps (I think you could say similar things about the average supporter of any mainstream issue.) I support animal rights, but evidence came out that they don't feel, I would find that extremely comforting.
> (Though to be fair, much of the animal-non-rights movement seems to be similarly non-scientific :P)
Much of the "animal non-rights" movement is anti-free speech (e.g. ag-gag laws) (and is the recipient of lots of government hand-out money, tries to convince that government that peaceful protesters should be classified as 'terrorists' for pushing an agenda that is detrimental to their business, etc), so they should be opposed by more than just the animal rights people.
I wouldn't call it "knee-jerk". It's not a reaction to a new thing. It is the new thing. Animals not having rights has been the norm for a long time.