Parenthood doesn't come with vows. Unlike marriage, where we promise to love, honor and cherish, we take no oath to treat our children equally. We may say we love them all the same -- a white lie right up there with Santa Claus and the tooth fairy -- but experts say many parents gravitate toward one child.

Guilt silences most moms and dads, but Laura Figueroa is open about her preference for her 7-year-old son. The Berkeley mother says his easygoing manner complements her Type A personality. Meanwhile, her eldest and firstborn daughter shares her fiery style. And that causes them to butt heads.

"I don't love him more than her, but I get along better with him," says Figueroa, 35. "When I tell people he's my favorite kid, they raise their eyebrows and think I'm crazy."

Playing favorites is taboo, but family experts say even when parents zealously try not to have a favorite, there are reasons why mom's eyes twinkle extra brightly for her son, and dad turns to mush around his little girl. Gender, birth order and personality contribute, but the biggest reason behind parental investment, as biologists call it, is evolutionary.

"Historically, we expect parents to invest more in offspring that offer attributes of good health and potential for reproduction," says psychologist and Darwin expert Frank Sulloway of UC Berkeley. "Good looks are one of those attributes, especially for daughters, though no parent would admit to that."

Birth order is easier to admit. Firstborns are often favored because they have no rivals and benefit from their parents' wealth and resources. The baby has the advantage of being last -- and feeds off mom's instinct to nurture her youngest. Where does that put the middle? As the least likely to be a favorite.

"When No. 2 comes along, all he gets is half in terms of resources," Sulloway says. "The middle child will always come out on the short end of the stick because he'll never be the only one in the house."

Experts say damage to the unfavored child is dependent on the degree of favoritism. Some may wrestle with low self-esteem, and go through life wondering why they weren't as worthy of parental doting.

But parents who are conscious of their preference may go out of their way to nurture the other child.

"Of course I'm concerned about my relationship with my daughter, so I try really hard to work on it so it is the best it can be," says Figueroa, who carves out special time to bond over mani-pedis. "I try to connect with her emotions and remember the things I like about her. She's creative, outgoing and friendly."

As an adult, the unfavored child may wind up on top. Unfavored kids tend to be more likely to take healthy risks and establish relationships outside of the home, according to sibling expert Catherine Salmon of the University of Redlands.

"They're also less likely to be asked to do a lot of elderly parent care, and some would consider that a plus," Salmon says.

In addition to birth order, gender is a powerful indicator of favoritism. In a 2003 survey published in the journal Human Nature, half of subjects told Salmon that there was a favorite in their family. Ultimately, the most likely favorite for mom was her eldest son; for dad, his baby girl.

"There's no evidence on why, but I think men tend to have an 'I am the protector' attitude toward the youngest female," Salmon says. "For moms, it's likely a combination of favoring the firstborn and the fact that firstborn boys often seem to be obedient toward their moms," Salmon says.

Sometimes, parents give preferential treatment to the child most like themselves.

Both in looks and disposition, Jane Costa, the second of four girls, is a replica of her mother. When she was 8 years old, her mom slipped her an extra slice of pie after dinner with a look that said: "Zip it. No one else is getting seconds."

"That was my first inkling that I was special," says Costa, now 65 and living in Walnut Creek.

By the time she was 11, Costa was not only her mother's favorite but also her confidant. There was little the two didn't share. Costa learned to sew at her mother's knee. They both loved skating, and watched the competitions on television together. Costa sat with her mother as she paid bills and helped with chores when the other kids scattered.

"I was like a reincarnation of her," Costa says.

Looking back, she says she feels badly that her mother played favorites. But, it made sense under the circumstances. "When you have so many to take care of, and your husband's an alcoholic, you probably just go to the child who's more agreeable and amenable," she says. "And that was me."

Sherry Simmons of Oakland says her mother also favored the compliant child -- but it wasn't her.

"Quite frankly, I was a pain in the (butt)," says Simmons, 64. "I would test and push the limit on everything. But (my brother) Gregg was quiet and perfect. She would forget my name, but she'd call him 'heaven sent.' "

On more than one occasion, Simmons confronted her mother about the favoritism. Each time her mother insisted that she loved them both the same. "But the only good thing she could find to say about me was that I had lovely hands," Simmons recalls. To preserve those hands, she wore gloves to bed into her 30s.

Does all this favoritism damage the sibling relationship? In most cases, experts say it doesn't.

Simmons admits she was jealous of her brother in their teenage years, but it was fleeting, she says. Today, they are best friends. And when she teases him about being the golden child, he just laughs.

So do Al Kelly and his brother, Dave. It was 30 years ago that Al, who lives in Berkeley, figured out Dave was the favorite.

It was dinner time. Mom had told Al to dip into the icebox and fix himself a plate of leftover string beans, meatloaf and mashed potatoes. Dave arrived home soon after. When Al offered to make him the same plate, mom rushed to his side in the kitchen.

"Don't give him those string beans," she whispered to Al.

"Why?"

"They've been in the icebox too long," she responded.

"Wait a minute," Al said. "You let me suck down a whole plate of those beans!"

Al, now 57, can still see the look on her face.

"The little minx was tongue-tied," he says, laughing. "She had this expression like she'd just been caught. And I never let her forget it."

how parents pick favorites
Dad isn't horrible because he seemed to like your brother a little more than you. Family studies experts say favoritism, or parental investment, is often based on factors we can't control. Here are some:
Birth order. Firstborns are often favored because they have no rivals and benefit from all the family resources. Last-borns are favored because they are the baby. Middle kids are the least likely favorite.
Health. From a biological perspective, we have evolved to favor the biggest, healthiest and most attractive offspring because she is the most likely to successfully reproduce.
Gender. Research suggests that parents often favor the opposite-gender child. Think of the mother enamored with her strong, eldest son, or the father who turns to mush around his little girl.
Personality. Parents are only human. Because every offspring is an individual and we resonate differently with each person, it's almost impossible to not feel a tug toward one child.
-- Frank Sulloway and Catherine Salmon