Parental overvaluation of children was measured with a scale that asked moms and dads how much they agreed with statements such as "My child is a great example for other children to follow."

Both children and parents reported how much emotional warmth parents showed, with participants indicating how much they agreed with statements like "I let my child know I love him/her" or "My father/mother lets me know he/she loves me."

The children were then measured for levels of both narcissism and self-esteem.

The researchers found that parental overvaluation was associated with higher levels of child narcissism over time while parents who showed more emotional warmth tended to have children with higher self-esteem.

"Overvaluation predicted narcissism, not self-esteem, whereas warmth predicted self-esteem, not narcissism," Bushman said. " People with high self-esteem think they're as good as others, whereas narcissists think they're better than others."

Parental overvaluation, however, is not the only cause of narcissism in children, the researchers noted, adding that genetics and the temperamental traits of the children themselves also have an effect.

Bushman, who is a father of three children, said his research on narcissism "has changed my parenting style."

"When I first started doing this research in the 1990s, I used to think my children should be treated like they were extra- special. I'm careful not to do that now," he said.