Richard A. Gardner was a man who studied a lot about children and families. He was especially interested in how parents and children could have good relationships even if their parents were not married or living together.
He found that sometimes, when parents split up, one parent might say bad things about the other parent to their child. This can make the child feel really confused and upset because they love both of their parents, but now they are being told that one of them is bad.
Dr. Gardner called this "Parental Alienation Syndrome", and it happens when one parent makes the child believe that the other parent doesn't love or care for them.
He helped people understand that this is not a good thing for children, because they should be able to have a good relationship with both of their parents.
Dr. Gardner taught people that it is important for parents to work together to make sure their child feels loved and supported by both of them, even if they are not living together.