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Daily Dose

How do siblings shape your personality?

By Deborah Kotz
Globe Staff / October 24, 2011

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Research has long established that parents play an integral role in shaping our personalities, but scientists are now finding that our siblings may contribute just as much, or perhaps even more. In an intriguing new book called The Sibling Effect, science writer Jeffrey Kluger argues that brothers and sisters leave a firm imprint, helping to determine whether we’ll become free-spirited risk takers or careful studious types; whether we’ll be shy or the family entertainer; and whether we’ll be inclined to smoke, use alcohol, or take illegal drugs.

“Over the past 15 years or so,” Kluger writes, researchers “have begun studying brothers and sisters as never before -- teasing apart the genetic, sociological, and psychological threads of the sibling relationship.”

What they’ve found is that certain things clearly play a role in influencing our personalities, such as where we fall in the birth order, whether we grew up with a twin, whether we had half-siblings or step-siblings, or no siblings at all.

Firstborns and only-children, for example, have a 3-point higher IQ on average compared with those born second, according to 2007 Norwegian study cited in the book, and second children are about a point ahead of those born third. Those differences may seem subtle but they translate into a 15 to 20 point difference in SAT scores, which could explain why your older sibling got into Harvard or Penn while you had to settle for Dartmouth or Cornell. (Full disclosure: my older brother went to Penn, while I attended Cornell.)

“It’s how you were raised, not how you were born, that tends to explain differences between sibling IQs,” said Frank Sulloway, a University of California researcher and former MIT professor who performed landmark research on the impact of birth order. “The mechanism appears to be that parental investment isn’t equal for all offspring.”

Parents can devote 100 percent of their child-raising resources to the first child until they must divide those resources when the second child comes along and so on for each additional child. To compensate for the lack of parental attention, the youngest child may develop certain personality traits -- like humor, spontaneity, or gregariousness -- to shift the spotlight onto themselves, Sulloway told me.

They also tend to take more risks, since they have less to lose. (In previous centuries, older siblings were more likely to survive to adulthood, and even today parents are less likely to vaccinate their youngest children than their oldest ones.) Sulloway’s recent review study found that younger brothers are more likely to participate in dangerous sports like football and boxing than their older siblings, and among sets of brothers who play professional baseball, younger siblings steal far more bases, which Sulloway pointed out is “one of the riskiest things players do.”

And siblings -- especially those close in age and of the same gender -- can influence each other’s bad health habits. Younger brothers and sisters are four times as likely to take up smoking, for example, when an older sibling smokes. Drug and alcohol abuse follow a similar pattern.

While all of this is interesting food for thought, I’d like to know what I can do as a parent to foster strong sibling relationships among my three kids and make sure I don’t put any at a strong disadvantage.

I asked Kluger whether any of the hundreds of studies he reviewed for his book gave him insight into parenting his own two daughters. For instance, how much should parents intervene in their children’s fights?

“One study found that when parents get involved, it takes 50 percent longer for a conflict to get resolved,” he told me. “But that’s because parents are teaching kids basic conflict resolution skills.” For example, they can teach their children the importance of justifying their needs, such as the reason they want a particular toy at a particular time, or why they don’t want their sibling in their room.

“Parents can then remind kids that the resolution that [was] worked out yesterday can be applied to today’s conflict,” Kluger said. The goal, of course, being to get kids to settle fights on their own.

Playing favorites can also destroy sibling relationships: A University of Denver study that Kluger cited in his book found that not only do unfavored children tend to be more aggressive and hostile towards their favored siblings, they’re also more likely to develop anxiety, low self-esteem, and depression.

Kids can’t and shouldn’t be treated exactly equally, but researchers have found that parents need to make an effort to find a favorite aspect in each of their children. “I tend to go to my older daughter for reflective conversations and my younger one for fun, active things,” said Kluger. “Both are equally important and equally rich aspects of my life.”

Kluger remembers his mother valuing different aspects of his and his three brothers’ personalities and said that awareness helped strengthen the bonds between them. “If my older daughter sees me celebrating my younger daughter’s sense of humor, she may be inclined to do it too,” he added.

While the picture gets far more complicated when half-siblings and step-siblings are added to the mix, Kluger said he’s found through his own experience that close bonds can form between siblings even when they weren’t raised together. “I didn’t meet my half-brother and half-sister until I was an adult, yet the distinction between them and my full-blooded siblings has nearly completely vanished over the years in terms of the closeness of our relationship.”

He said it “defies all predictions” that researchers may have made, but he’s “all the better for it.”

Deborah Kotz can be reached at dkotz@globe.com. Follow her on Twitter @debkotz2.

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