He said that couples, gay and straight, who wanted fertility treatment were often worried about the effect on a child of knowing that they could track down their missing biological parent.

'If you look at the studies into paternity, even conservative figures show that between eight and 15 percent of children haven't been fathered by the man who thinks he's the biological parent.

The 42% of Americans who have at least one step-relative typically feel a stronger sense of obligation to their biological parent, child or sibling than to their step-relatives, the Pew survey says.