Australian child custody law presumes that it is in the best interests of the child for each of the child's parents to have equal shared parental responsibility. (Australia's Family Law Amendment (Shared Parental Responsibility) Act 2006).
This presumption has reportedly created significant damage.
Judge Graham Mullane has reported to the Association of Family and Conciliation Courts that recent research by the Institute of Family Studies has shown that shared parenting has increased substantially in Australia but that the effect of many of these arrangements has been to subject infants or very young children to a pattern of separation from their primary care giver that may be seriously damaging to the child.
Many of the parents' relationships are poor and they have no capacity to co-parent. For many children, the arrangement involves a high level of inconsistency and instability, and repeated exposure to parental conflict.
Moreover I can certainly attest to the fact that the 2006 law has made child relocation cases very much harder to win in Australia -- or, depending on your point of view, much easier to defeat -- which has caused devastating results in some cases.