I agree with most of this, but I think it's not countering what I was trying to argue: With regards to a certain individual's reproductive fitness, the “precision” of that individual's cuteness criteria is not that important. The actual reproductive advantage is in caring for one's young, thus raising the probability of perpetuating one's genes further.
Thus, even a not-very-well calibrated “cuteness” factor might not be very important (in the sense of not causing much selective pressure) as long as something else causes the individual to actually care for zer young. In this case I (weakly) conjecture that the “something else” is a mechanism that “focuses” the “cuteness evaluation” on one's young.
As an analogy, consider a myopic species. Selective pressure might be expected to cause it to develop better vision, to help it avoid predators and find food. However, if the same species happens to have, e.g., good (not dog-like, only good enough) smell — which brings it close enough to food for its myopic vision to work, and keeps it far enough from predators to not need eyes for defense, the selective pressure can be very diminished.
Consider vision: it is an extremely old feature, so it had ample time to evolve. In fact, I'm told it evolved separately several times on Earth. All current vertebrates come from a common ancestor, which as far as I can determine had eyes. However, their vision acuity varies greatly, even in species that share a habitat. Better vision is always an advantage wherever there is light, but it's obvious from the world around us that the selective pressure exerted by that advantage is often not enough to cause evolution (sometimes, the reverse happens).
Hmm, I just had another thought, reading your comment about “[a] deep structure” blocking selection: It's not blocking as much as making irrelevant.
It may be that we're just wrong. It's possible that the “cuteness” factor was useful, as we think, for causing us to like babies and thus propagate our genes by caring for them. However, another trait with similar final consequences (but better in some way) just happened to evolve with better efficiency, maybe some new hormonal pathway or something related to primate brains, or anything I can't think of.
The “cuteness” factor might actually atrophy in such a case. If a species develops very good smell for finding prey, they might not actually need their eyes much. The selective pressure for better vision diminishes, and drift takes over. I'm told this happened with some races of dogs. In our case, it's obvious that no matter how cute we think some things are, we actually do take care of our children a lot (and a visible majority of people, at least at the start, seem to quickly become very attached to their babies).
“Cuteness acuity” might simply be irrelevant. It might have become so a long time ago, and become re-purposed for something else (beauty, art, whatever; the brain mixes things from many evolutionary eras).
Daniel Dennett has advanced the opinion that the evolutionary purpose of the cuteness response in humans is to make us respond positively to babies. This does seem plausible. Babies are pretty cute, after all. It's a tempting explanation.
Here is one of the cutest baby pictures I found on a Google search.
And this is a bunny.
Correct me if I'm wrong, but the bunny is about 75,119 times cuter than the baby.
Now, bunnies are not evolutionarily important for humans to like and want to nurture. In fact, bunnies are edible. By rights, my evolutionary response to the bunny should be "mmm, needs a sprig of rosemary and thirty minutes on a spit". But instead, that bunny - and not the baby or any other baby I've seen - strikes the epicenter of my cuteness response, and being more baby-like along any dimension would not improve the bunny. It would not look better bald. It would not be improved with little round humanlike ears. It would not be more precious with thumbs, easier to love if it had no tail, more adorable if it were enlarged to weigh about seven pounds.
If "awwww" is a response designed to make me love human babies and everything else that makes me go "awwww" is a mere side effect of that engineered reaction, it is drastically misaimed. Other responses for which we have similar evolutionary psychology explanations don't seem badly targeted in this way. If they miss their supposed objects at all, at least it's not in most people. (Furries, for instance, exist, but they're not a common variation on human sexual interest - the most generally applicable superstimuli for sexiness look like at-least-superficially healthy, mature humans with prominent human sexual characteristics.) We've invested enough energy into transforming our food landscape that we can happily eat virtual poison, but that's a departure from the ancestral environment - bunnies? All natural, every whisker.1
It is embarrassingly easy to come up with evolutionary psychology stories to explain little segments of data and have it sound good to a surface understanding of how evolution works. Why are babies cute? They have to be, so we'll take care of them. And then someone with a slightly better cause and effect understanding turns it right-side-up, as Dennett has, and then it sounds really clever. You can have this entire conversation without mentioning bunnies (or kittens or jerboas or any other adorable thing). But by excluding those items from a discussion that is, ostensibly, about cuteness, you do not have a hypothesis that actually fits all of the data - only the data that seems relevant to the answer that presents itself immediately.
Evo-psych explanations are tempting even when they're cheaply wrong, because the knowledge you need to construct ones that sound good to the educated is itself not cheap at all. You have to know lots of stuff about what "motivates" evolutionary changes, reject group selection, understand that the brain is just an organ, dispel the illusion of little XML tags attached to objects in the world calling them "cute" or "pretty" or anything else - but you also have to account for a decent proportion of the facts to not be steering completely left of reality.
Humans are frickin' complicated beasties. It's a hard, hard job to model us in a way that says anything useful without contradicting information we have about ourselves. But that's no excuse for abandoning the task. What causes the cuteness response? Why is that bunny so outrageously adorable? Why are babies, well, pretty cute? I don't know - but I'm pretty sure it's not the cheap reason, because evolution doesn't want me to nurture bunnies. Inasmuch as it wants me to react to bunnies, it wants me to eat them, or at least be motivated to keep them away from my salad fixings.
1It is possible that the bunny depicted is a domestic specimen, but it doesn't look like it to me. In any event, I chose it for being a really great example; there are many decidedly wild animals that are also cuter than cute human babies.