Researchers from the University of Southampton say family ceremonies such as birthdays and Christmas create tension. Parents admitted having different levels of love and higher commitment to their natural children compared with their step-children.
The findings, published by the Economic and Social Research Council, found presents at Christmas were particularly powerful expressions of blood ties. Parents who did not live at home and grandparents were not expected to treat all the children in the house equally.
"Grandparents often gave step-grandchildren `token' presents," Dr Allen said. "But they would often give their biological grandchildren more substantial gifts." The research also found that relationships with step-children were perceived as being largely dependent on the stability of the marital relationship.
The research involved interviews with 80 people who were involved in step-family life, either through marriage or blood ties.