The first born signifies a momentous passage for a person, for most people, the passage that created their parenthood and opened their eyes and heart to emotions they could never comprehend before the birth of that human, no matter how well they thought they could.
I do not think that the bond is necessarily greater than with children born after, or that first borns are favored, so much as I imagine there is a certain sort of connection in the mental pathways to attaching those first felt emotions in a unique way with the first child. As well, every step of the way is an entering into the nearly unknown with the first born. The parent, under normal circumstances, has never been here before.
Before the second child is born it is hard, nearly impossible, to comprehend that one could ever have thoughts and feelings as strong as they do for the already existing child for anyone else ever. Almost a fear in some ways, that one will be unable to love like this again. Then that second child comes and the impossible becomes possible with ease as a second nature (again, under normal circumstances, I mean). The aware parent is able to see that this a completely different person, while the well-meaning but less aware parent that at times is overlooked, but in either case it is only a sliding scale of newness and no longer the unknown- the scale that measures how unknown the experience of this new human is based on how aware the parent strives to be that it is a separate person who will live a separate life and have separate behaviors and desires and nuances than the first, but still no matter what end of this scale of awareness the parent lands on it is still charting at least somewhat know territory.
With each successive birth after this the territory of parenthood becomes more and more well known, the parent likely will have no or little fear (for example) as to whether they will love their third or fourth or fifth et cetera child as deeply, they now know, taught by their second child, that they are capable of this inconceivable amount of love and devotion (in fact that is a unique lesson and experience betrothed upon them by the second-now knowing they have enough love and will not run out-, a gift second children may not be aware of having given that is unique as the gift of the unknown the first gave them) as well as a general loosening of the grip and a gaining of frazzledness that children after the second bring/give. The younger children bring with them delight in a different manor than that of rejoicing the accomplishments of the first, and too a lessoning of the lamentations of the failures. Younger children bring freedom, in a way, by default, the parent is too worn out but also more wise and can be and let their child be to a much greater extent.
As far as effect on the child goes, I believe first borns tend to feel a greater burden of responsibility, and my observations have led me to think that middle borns tend to feel more lost in the fray and overlooked, while youngest children I believe do enjoy a greater sense of freedom. All of these traits can play out into different levels of neurosis too. Sense of responsibility can become need to control, feeling overlooked can become chronic self-esteem issues and sense of freedom can become a lack of control. Of course I am not saying anything too profound here, and this is just surface skimming of the A typical model without considering any other factors affecting the dynamics.
When I wrote that I was the “default spokesperson” I was actually thinking of a specific memory- my grandpa’s memorial- when I felt compelled to stand and speak on behalf of my brother and sister and cousins, in front of hundreds of people and shaking, simply because it seemed my duty to spare the others and represent this population of the family that called him “grandpa”. There have been other times with my siblings when I simply had to take charge of their problems because they were in need and unable, where as my sister offers support and help but never takes control of any of my problems no matter how out of it I am (my brother I won’t ever know what his reactions would have been).
In my own family (the one I am a parent in) there is an interesting dynamic I noticed- we have four, but two "youngests" instead of two "middle" children. The first three are boys and the youngest is a girl. So our daughter behave like a youngest child, but so does our youngest son, as he is in fact the youngest of his "group". Only our second behaves in typical "middle child" ways and the oldest is the oldest, of course.
I don't know. These were my thoughts to those questions.