The international law standards in the United Nations Convention on the Rights of the Child and the United Nations Declaration on the Rights of Indigenous Peoples say children have a right to their indigenous culture. I think it's the obligation of adoptive parents as well as countries to ensure children know who they are, and through that knowledge, they are provided with an opportunity to reach out to members of their group and understand who they are on a better level.
On December 7th, 2010. See this statement in context.