'Sibling Effect:' Mom Has a Favorite and Birth Order Counts

Sept. 30, 2011— -- At age 4, Bruce was small enough to stuff into his family's Baltimore basement fuse box – an old-style cabinet filled with "high-voltage, old style unscrewable fuses." His three brothers convinced him it was a space capsule, "just like Alan Shepherd's."

His older sibling, Jeffrey Kluger, now 57, admits it was an act of mischief, but also protection for the most vulnerable child, as they hid from their volatile father and, though they didn't know it then, their prescription drug-addicted mother.

Brotherly love -- and its accompanying intense rivalry -- is a topic close to Kluger's heart. In his latest book, "The Sibling Effect," Time's senior science writer takes a scientific -- and personal -- look at what he calls the "last unexplored frontier of family relationships."

"It was only years later that I would go a little cold, thinking about the deadly danger we courted on those mornings, squeezing a small child and high voltage so close to each other," he writes.

The four Kluger boys – five years between oldest and youngest -- propped each other up emotionally throughout the multiple divorces of their unstable parents. Their mother was married twice, bringing two stepsisters into the mix.

Their father remarried and had fraternal twins, giving them two half siblings.

"One of the points I make in the book that when the parents, who should be the anchors of the family, come unmoored, the kids do what people do in a fox hole," he told ABCnews.com. "We pull together."

But they are also fierce competitors and partners in crime. "My brothers and I played boisterously, fought frequently, and broke things constantly," writes Kluger.

Siblings are the ones, of all relatives, who share the longest stretch of time on earth, are more important than parents, children and even friends.

"Our spouses and children arrive comparatively too late in our lives; our parents leave us too early," according to Kluger, who quotes family sociologist Katherine Conger of the University of California, Davis. "Our brothers and sisters are with us for the whole journey."

The book, a memoir woven with relevant scientific research, explores every imaginable aspect of the sibling relationship: favoritism, birth order and even why brothers and sisters are not sexually attracted to each other.

Not only do our siblings smell too familiar to have sex with them, explains Kluger, they are a product of the "kibbutz effect," where growing up in close quarters tends to kill any physical attraction.

Siblings are by nature -- and he provides many examples from the animal world -- genetically programmed to be a "team of rivals."

Kluger confirms earlier research that shows first-borns and "singletons" do, indeed reign supreme. Both groups are smarter and more economically and emotionally successful than the middle child or the baby.

But he also debunks some long held myths. For starters, all parents have favorites.

"One message I have for parents is that they should quit feeling guilty about having a favorite," he writes. "I like to say that 99 percent of all parents do have a favorite child and the other one percent are lying through their teeth."

But that, he writes, is not always a good thing: Although less-favored children have lower self-esteem, the favorite can have a harder go of it in the real world. Favorites can be thin-skinned when it comes to criticism and "unprepared for having to prove themselves," said Kluger.