Q: Isn't child custodoy always decided on the basis of what is in the best interest of the child?
A: No. the first question in a transfer of custody is not what is in the best interest of the children, but whether there has been a change of circumstances since the original custody order was entered.
Look at it this way. After child custody is determined by a court order in divorce or paternity proceedings on the basis if the best interest if the child, the judge, in effect, states, "That's it. I determined the custody of the child once and that was done on the basis of waht is in the best interest of of the child. (And this holds true whether the judge's determination was based on a trial, or the parties' agreement.) Now, if you want custody changed, you are going to have to show me (the judge) there has been a change of circumstances."
Q: Does the law have a bias in favor of the custodial parent and against the non-custodial parent?
A: Yes. It is a strong bias. The law has a bias in favor of the child's life being stable.
Q: What facts would be sufficient to show a change of circumstances sufficient to bring about a change of custody?
A: Examples are: a significant drop in the child's school performance; significant health problems due to the custodial parent's neglect; the development of significant social/psychological problems by the child; substantial neglect of the child by the custodial parent, such as leaving the child home alone, neglecting the child so the child becomes injured; exposing the child to what the judge may consider to be immoral conduct, like having a live-in significant other.
Q: What could other causes be for a transfer of custody?
A: Serious abuse of a child, whether physical or sexual, may be grounds for a transfer of custody. The instability of the custodian is frequently a significant (but not sole) factor in allowing a transfer of custody. Imprisonment of a custodian is an obvious change of circumstances. Smoking by the custodial parent is now being raised in change of custody proceedings. In the Illinois appellate court opinion which addressed the smoking issue the custodian's smoking did not result in a change of custody because it was not proved that the children's asthamatic conditions were caused by the smoking and the mother testified that she no longer smoked in the residence.