I believe it is the parent with the higher AGI (adjusted gross income) that should be claiming the child. In practice, sometimes it's the first one to file and then a pain to get it reversed.

Should definitely ask the accountant HOWEVER I just spoke with my cousin-in-law who is our accountant about this topic (originally it was about people getting their social security number stolen and getting told it was already filed that year.)

He said it now depends on who has the most overnights and who pays more in terms of the upkeep of the kids.

Obviously it's probably more complicated than that but have her ask an accountant.

Even in a 50/50 arrangement its likely that one parent has more overnights. If both parents were to claim the children, the IRS would audit them both, and give the credit to the parent with more overnights, even if its just one night.

the thing about using overnights, is you can't really prove it. if one parent is not given primary physical custody, then it goes by whichever parent has the highest AGI.

It's whoever has the most overnights. There are 365 days a year so one parent has the kid more than the other. In a 50:50 case, it should rotate each year.