"Adult adoptees who grew up with secrecy talk about feeling that something wasn't 'right' about them, about feeling both betrayed and relieved when they learned the truth. Others interpreted their parents' reluctance to talk to mean that adoption was taboo," Lois Melina wrote in Adoptive Families magazine.
Cora encourages the same process for all parents who aren't biologically linked to their child. "Err on being truthful," she said.
A big factor in the "when" is if the children are old enough to comprehend the implications or if they start raising questions about their identity, Cora said.
If the child is really young and determined to seek out a biological tie that isn't part of his or her current life, the parents who raised the child might encourage him or her to wait a couple of years to seek out roots -- and agree to help when the time comes.
Hogan said innately understanding where you come from is something children begin to want to understand. But the age at which it's discussed should be when they can steel themselves for rejection.
"It is the ultimate rejection if your biological parent doesn't have interest in you," she said.
Hogan suggests parents be honest in their explanations, acknowledging their own fears and doubts about telling the truth. This is especially pertinent if a child could have been abused, mistreated or neglected in the care of a biological parent.
"People think that they really want to know the truth (about their biological parents), but they've idealized that truth," Cora said.
The child, even as an adult, will discover and decide for him or herself what comes from nature and what's nurtured.
For Hansen, who had a rough relationship with his dad, he said he felt a release of issues he thought were innate.
But he was upset that his mother didn't tell him the truth while he dealt with his own anger issues, or when he and his wife asked about medical history before trying to have a baby. With a lot of processing -- therapy, that is -- his relationship with his mother is at a peaceful point, he said.
"She thought she was doing the right thing by not telling us, and in her view, he loved us, she loved us, they were our parents -- and none of the other stuff mattered," he said.
"Because of the way that I was raised, he's always going to be a part of who I am -- biological or not."