Examples to support my contention:-
1. Where there is a dispute between the parents as to the interests of the child the views of the child are most often sought by the courts. If, as is almost inevitable, the child has been told - contact with your parents is "your right" (- meaning your right to choose to continue with the disputed contact or not) then this leads to many children rejecting one of their parents because they feel they have to choose.
Probably no one can say for certain whether the rejection of a particular parent is indeed in that child's interests. However, it is the feeling that is conveyed to the child that they should choose between their parents to the extent of completely rejecting one of them that is the most important factor here surely. If the premise used to base our family law on tends to encourage rejection of a parent (and if adults, let alone the child themselves cannot know for certain the overall +/- effects on the child) then the premise itself can be said to be having effects detrimental to children.
2. Telling a child "your best interests are all that matters" makes the child consider only their own interests. Is it necessary or indeed right to tell any child to consider only their own interests? Should a child not be thinking about the interests of their parents as well as their own?
If self interest is "all" in our society then raising children to think only of themselves may be okay (everyone is doing it so why be different or left out). This affects the fabric of society and indeed the other children or adults who come into contact with the self centred child. They will know or come to know that they think only of themselves and react accordingly.
3. The prospects for a parent who is fighting for contact with their child winning their case is bound up with the premise used as the base for family law. If parents are being advised that they cannot succeed or maintain their relationship with their child by action through the courts then most will give up on that legal advice (and if they don't they may well have it proved to them in court).
The point being that all this challenges the parent's ideas that they will/can be there/involved in their childrens lives. Once the idea you either can't win as a parent or oppose a hostile ex. successfully, takes hold, this threatens the parents desire to be a caring, nuturing parent.
I hope I have brought out the most important examples of where I believe the very premise used for family law has effects upon people (parents and children) that may not be immediately apparent or have been thought about when the law was framed.
I believe experts in human behaviour, like Desmond Morris can be used to give advice as to the likely effects of divorce law or child custody law. I've read in one of his books human nature has been shaped over so many millennia that arguing for changes in the law to reflect contemporary society will always be in conflict with man's true behaviour which has been shaped over such a huge period.
Graham