(MONEY Magazine) – Bill Clinton, President; Roger Clinton, ex-con. Alec Baldwin, Oscar-nominated actor; any of his brothers, not-so-famous actors. It seems that in all too many families, one sibling is considered a success while another lags. Academics, not to mention nagging relatives, have long debated the reasons. Is it birth order? Genetics? Childhood jealousies?

Try parental favoritism. That's the provocative theory offered by sociologist Dalton Conley in his new book, The Pecking Order: Which Siblings Succeed and Why (Pantheon, $24). On to the bullet points:

-- Each family creates its own pecking order (or status hierarchy, in sociology lingo), driven by parental attention as well as each child's natural ability and social influences. So some children get a larger share of family resources--and a better head start in life. Virginia Clinton, Conley notes, lacked money; she focused what she had on Bill, leaving Roger with little support. Sums up Conley: "Inequality starts at home."

-- This is more than couch fodder for therapists--it has national economic repercussions too. Analyzing U.S. Census data and long-term national social surveys, Conley found that only 25% of all income inequality is between families. The remaining 75% is accounted for by sibling differences within families.

-- Other influences on siblings' fortunes include divorce, bad health and simple luck. Yet Conley's research shows persistent pecking patterns. Middle children, caught between the costs and commitments of their older and younger sibs, are more likely to be held back in school. Daughters of mothers who work are just as likely to be as successful as their brothers, unlike daughters of stay-at-home moms, who lack career models. Family size matters too: The larger the litter, the more likely some will end up as favorites at others' expense.

-- The rules don't apply so much to the rich: Conley cites the Kennedy and Bush families, where all the children tend to succeed, despite setbacks along the way. "Those born to privilege enjoy a magic halo," he writes, "a buffer...that prevents them from falling too far."

-- How to nip the forces of sibling inequality? "Stop at just one," Conley wryly suggests. It's a notion that's not likely to gain much support outside of Maoist China. (In fact, Conley himself has two children.) Better yet, just try a little harder to make sure that your kids receive an equal share of your attention every day. --PENELOPE WANG