There's no denying it. In any household with more than one child, kids seem to naturally compete for their mother's love and attention. And mothers swear they love every child equally.

But just maybe Mom does really love you best. Or is it just wishful thinking that you're her favorite?

"Mothers worry about that issue of 'Am I closer to one than I am to the other?' " says Cate Dooley, a psychologist with the Wellesley Centers for Women in Wellesley, Mass. "Mothers really need to let themselves off the hook. You're going to have different relationships with each child. It's OK."

Research is shedding some light on what happens when a parent — particularly the mother — gives more time or attention or privileges to one of the children. Past studies have found that less-favored siblings may suffer emotionally, with decreased self-esteem and behavioral problems in childhood, while children who were even slightly favored report higher well-being as adults.

It could be a result of gender (favoring the same-sex or opposite-sex child), birth order (the oldest or the baby) or how easy or difficult a child's temperament may be, but a parent's differential treatment — real or perceived — has far-reaching effects, including fueling sibling rivalry.

Such questions are particularly pertinent for baby boomers — these unresolved feelings are playing out now in the care of aging parents. Experts agree that the feeling that Mom always liked a sibling better can affect lifelong psychological well-being.

"Ask any family and they'll tell you who was the favorite one," says Jacqueline Plumez, a psychologist in Larchmont, N.Y. "People are very shaped by their family situations and how they were treated. You can be 80 years old and still hurt by it and the parent is long, long dead."

In her new book, "Mom Still Likes You Best," author Jane Isay of New York City says favoritism is a "recipe for the next generation to not like each other."

SHE LIKED HIM BETTER

Laurel Kennedy, author of the new book "The Daughter Trap" and founder of a Chicago consulting firm for baby boomers called Age Lessons, interviewed 216 women and found that even though none of her questions asked directly about a parent favoring one child over another, about two-thirds of the women said there was a favored child, and most said it was the mother who did that.

"Out of the blue they'd say, 'She always liked my brother better, and he got to go to summer camp in 1968 and I didn't,' " Kennedy says.

Experts say because no two people are the same, they relate differently to others. "It does not mean the parent loves or likes one child more. It has to do with who each of them is separately," says clinical psychologist Laurie Kramer of the University of Illinois at Urbana-Champaign.

Plumez, who interviewed adoptive and biological parents for an adoption book in 1982, found that "what matters most is whether your temperaments are simpatico." She explains: "Two people who want to be in control are always going to be butting heads. Two people who are shy and withdrawn might get along well, unless the shy parent doesn't like that aspect of themselves and they try to push the naturally introverted child to be more extroverted."

Jodie Casagrande, 38, of Jamaica Plain, Mass., the mother of three sons, ages 6 1/4, 4 and 18 months, says her middle son is most like her. "His personality is just like mine and we're always butting heads," she says.

Although her trio is showing signs of sibling rivalry, she says, "it's not like it's all the time. I just noticed each of them getting jealous if I'm giving one of them attention in front of the other. I'll be sitting in the playroom and Roman will sit on my lap, then Matteo will want to sit on me and Lucca will fold his arms in and pout."

Kramer says it "seems as though most children are basically preprogrammed to watch how they're being treated in relation to their siblings."

SOCIAL FAIRNESS

Mark Feinberg, a senior research associate at Pennsylvania State University in University Park, says this sensitivity may not be instinctual, however.

"I don't think we have an ingrained, hard-wired sibling jealousy part of our brain, but I think we're highly sensitive to fairness in social situations, and the family is the basic social situation for many years of our formative development," he says.

Another Penn State researcher, Susan McHale, studied immigrant Mexican-American families. Her findings suggest that sibling rivalry is very "culture bound" and more related to the culture of individualism in the U.S. than the more "communally oriented" Mexican-American culture

And McHale says favoritism is less an issue in larger families, where there's a coalition of siblings rather than an alliance between parent and offspring.

Mother of 10 Brenda O'Shea of Munster, Ind., says those family formations are at work in her own household. Her six boys and four girls range in age from 22 months to 27 years.

"We try very consciously not to compare grades, abilities, talents, any of that sort of thing. We don't encourage competition between the kids," she says. "We find they encourage each other."

O'Shea, 49, a psychiatrist who stopped practicing 16 years ago to stay home with the kids, is attuned to emotional health.

Kramer says: "Families don't tend to talk about these issues. They don't explain it and kids are left to their own imagination: 'I'm not as good as my brother. She likes my sister better.' "

EXPLAIN DIFFERENCES

Her research has found that when parents explain why there may be some different treatment of a sibling and when the child considers it fair, there aren't damaging consequences for the kids or their relationships with parents or siblings.

"Kids analyze these situations and say, "Is it because he's older or because I'm a girl, or because moms always treat the older ones better or because they spend more time together, or because my sister is very lonely and needs more attention?' " Kramer says.

She adds: "Oftentimes, when they complain that way, a parent is likely to be a little bit defensive and say, 'No, I don't' or 'I love you all the same.' Those answers are never very satisfying to young children,"

Mother of three Sue Wilson, 56, of Stillwater, Minn., says she avoided favoritism by starting what she called Child of the Day. That day's special child would have the favored spot, but the next day another would. Her children, now 29, 27 and 24, rotated the benefits (such as who sat shotgun in the car, who got an extra cookie or any other special privilege that day) well into high school. "It stopped every argument," says Wilson. "I'd just say 'Who's Child of the Day?' "