Nonfavored children are often emotionally scarred. Sulloway said they can be more anxious, neurotic and susceptible to depression. Sometimes a nonfavored child decides he can't compete and, to get attention, does everything wrong. For others, the slights, whether perceived or real, push him to prove he is just as good.
But they are forever imprinted with the idea that they were shortchanged. "It's part of what you carry with you," said Dr. Fred Rothbaum, professor of child development at Tufts University. "You're naturally more inclined to look for that in a new relationship, particularly a very close relationship, because you've been there, you've experienced that."
For the parents' part, the dynamic can be conscious or unconscious, subtle or extreme. Some parents are simply re-creating the relationships they grew up with themselves. If they weren't a favorite child, they may unknowingly favor the son or daughter in their place or work hard to make sure no child feels overlooked. Finding the qualities that make each child special and focusing on them helps.
But part of the phenomenon is a natural response. It's easier to get along with people who have similar likes and dislikes, or an easy temperament. An athletic father may have difficulty connecting to a brainy son. A mother may favor a daughter who reminds her of her favorite aunt.
"It's human nature that we respond differently to different people and offspring," Sulloway said. "Parents work hard to overcome those preferences, but it's not possible to completely overcome them. Some of them are just innocent mismatches."
Other experts say children sometimes want, and need, to be treated differently. One child, for instance, may be independent, less willing to give hugs or sit on a parent's lap, while another may be shy or want more of an emotional connection.
"It might look like the parent is `favoring' the second child, because he or she gives them more attention, but in fact the parent might be correctly reading the child's signals that the child needs more," said Dr. Karen Rosen, associate professor of psychology at Boston College. For her, the concept of favoritism is not a static feeling about one person, but a more fluid concept.
"Maybe it's more an appreciation," Rosen said, "or valuing a shared quality."
While middle children most consistently report feeling like the least favored, as the lives of parents and children change, the favorite can and often does change over time.
There is little research on only children, but, Sulloway said, because there are no siblings, only children are likely to get the same benefits an oldest child typically receives - basically, greater investment by the parents.
But in their own way, people with siblings are lucky, too.
Even though favoritism colors these relationships and drives sibling rivalries, brothers and sisters learn a lot from each other. They see shared experiences from someone else's point of view and become more sensitive to others. And the longest-lasting relationships of people's lives are with their siblings.
But many siblings let perceived or real favoritism get in the way of their relationships with each other and with other people. Howard suggests adult children talk to their parents about it. She said some may discover there were reasons why siblings were treated differently, such as illness or other circumstances.
"It's a loss if people can't sort of accept the fact that their parents did treat them differently, or talk it out," Howard said. "Having that conversation with your parents can be a healing process."
Those who don't work it out, studies show, will unconsciously do the last thing they ever imagined themselves doing: They'll pass that conflict down to their sons and daughters.