3 of 7 reduction of scope

Why do some parents have a favorite (or least favorite) child?

Playing favorites with one's children is clearly a moral grey area at best...so why do we do it? I'm a personality psychologist by training, and particularly well-exposed to heritability research through my alma mater, so I can appreciate the power of genetic variation and individual experience on personality. There's no use in pretending that all of even one's own children will be truly "created equal," let alone remain that way, except maybe on a very holistic, moral or spiritual level. I realize that parents have limited power over their children's behavior (some would say very limited), and that children affect the way their parents parent too. Still, there's got to be more to it. Culture plays a role, because some (arguably most?) societies prefer boys, but I've also read that girls get favored more often in Western culture, and that birth order matters on top of that. Sex of the parent may determine the favored sex, and I'd bet that the rarer sex among more than two siblings of different sexes would see some special treatment more often than not too. Still, I don't think I've fully answered my own question here, and I hope you'll agree. Besides these, what else might lead to favoritism?

Additional, optional questions to solicit further depth of information from anyone who happens to have personal experience relevant to these secondary issues:

  • Don't want to get too deep into the issue of consequences and morality, but I don't want to force a false premise on this question either, so is it ever okay? Even if not, are there relatively good reasons that are particularly worthy of sympathy or difficult to avoid as a parent?
  • We don't have to get into it if so, but is disfavor a separate issue? In psychological research on emotions and attitudes, negatives often prove not to be simple opposites of positives. "Black sheep," "rotten apples," etc. does not necessitate a favorite among more than two children, so...
    • Is a least-favorite a result of substantively different factors?
    • Is it very different to be the favorite not because you're exceptionally loved, but because your sibling is exceptionally unloved? What about when no one's #1, but someone's clearly last?
  • Are the dynamics of favoritism very different in large families with 3+ siblings?
    • Is it very different to be tied for first place when not all siblings are?
    • Is it consequential or even salient to be somewhere in between first and last, especially if not simply "exact middle"? E.g., is there gold medal envy even or especially for the silver medalist?
    • In very large families (say 5+ siblings), is favoritism less of a threat, or more inevitable?

This being a parenting site, and the general consensus being that favoritism is definitely no bueno, I'm trying to avoid letting this get too broad by soliciting horror stories or descriptive experiences of being the favorite. I'd prefer to focus on the issues of choosing favorites, or being chosen for particular reasons, not what it's like in general. In my tertiary questions, I've gotten into this a bit, but mostly to ask whether these special circumstances should not be conflated with ordinary favoritism, not to ask what it's like to be in these special circumstances. If you have to get into these issues, I won't mind personally, but it seems I've already gone overly wide in the OP, so please don't worry about what parts you can't answer, just tell me what you can. I.e., "Don't flag me, bro!"

Full disclosure: this post is inspired by an answered question on cogsci.SE that just came up today, and I'm doing my best to mind the sensitive and divisive issue of cross-posting policy. If you find this issue fascinating, come see what @what (that's the user's username!) had to say in response! So far we've got a list of psychological research results and references (from which I've derived some of the material for my intro here), and we'd encourage more answers of the same kind over there. This version of the question is intended to request references from other fields of literature, and since personal experiences are acceptable here, I'd like to provide a place to discuss your personal experiences with favoritism too, both as parents and as siblings yourselves (if you are). I've expanded the issue quite a bit into likely uncharted territory (as far as I expect large-scale published research literature has dared to venture, that is), so resorting to evidence by personal experience seems particularly justified with respect to some of my deeper, secondary questions.

Also, on a personal note, I'm not a parent yet myself, but I plan to be, and am leaning toward fathering more than one. I'm an only child, so I have no direct familial experience myself, and limited access to the intimate details of my friends' experiences with their parents and siblings. We're all just reaching the parenting age ourselves, so no one I know personally has had to grapple with favoritism toward their own children either, as far as I know...but one dear friend of mine had unplanned fraternal triplets (which made for a dynamite blog!) a couple years ago, and one's a girl, so I'd love to have some good time-worn advice and experienced perspectives to share with her. She's a personality psychologist too, and quite a studious parent in general, so I bet she'll find a good discussion fascinating. Maybe I can even sell her on parenting.SE and get her to come weigh in on this and other issues too! Trust me, you'd love her. Or don't, and go see her work in advance! </spam><apologies><shame>

Back here on parenting.SE, I've found this question to be of related interest:
How does having a second child change your life?