Getty Museum head of Hades

An ancient head of the Greek god Hades that Getty Museum officials recently said they will return to Sicily after research uncovered evidence that it had been looted. (Associated Press )

The ethics for adding ancient works to American art museum collections became substantially more stringent five years ago when the Assn. of Art Museum Directors (AAMD) decided to set the bar higher -- prompted by complaints from Italy, Greece and other ancient lands that museums had long turned a blind eye to evidence that pieces they owned had been looted from archaeological sites.

On Wednesday, the AAMD, which has 217 member museums in North America, announced a few more subtle tweaks to those guidelines, including requiring a public explanation on the AAMD's website if a museum decides to acquire a piece despite gaps in its ownership record going back to the fall of 1970.