The research, by Julie Fitness, associate professor of psychology at Macquarie University, shows 69 per cent of her sample of 70 could identify the family "favourite" and 80 per cent could identify the "black sheep".
"Parents say they treat their children equally. But when you ask people they say 'Of course there was a favourite.' They take it for granted," theage.com.au quoted her, as saying.
Dr Fitness said the middle child was almost never considered the favourite. The favourites were usually the oldest or the youngest, or the only boy or girl in a family dominated by one sex; or the child who shared a parent's interests and outlook.
"People say, 'Mum always liked her best because she looked like her or shared her interests. Or 'My father didn't take to me because I wasn't sporty like him,' " Dr Fitness said.
She said it was often easier for parents to like the child who was like them. They might love their children but not necessarily like all of them or relate well to different temperaments. Parents felt guilty and tried to disguise their preferences.
She said adults who considered themselves the black sheep placed themselves on a continuum from feeling not loved or part of the family to being just a little bit different and getting the "raw end of the stick" more often than was fair.
For some black sheep the consequences could be lasting, serious and sad.
"The family is the primary social unit and if you feel you are not accepted or loved by your parents where does that leave you in this tough world?" she said.
Dr Fitness said it could be tough for parents, too. But accepting a child's difference, and not blaming, was a start to understanding. And having involvement with an extended family was also beneficial. Those respondents who had most involvement with extended family were the least likely to say there had been a favourite or black sheep.
The research will be presented at a conference held by the Children's Family Research Centre at Macquarie University starting today.