Younger siblings seem to have an immune advantage as early as 1 month of age, which may help explain where they get the energy to tease older siblings. Rebecca Schortinghuis/Getty Images hide caption

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Most parents have a favorite child, psychologists say, even if they try to be fair. Hero Images Inc./Corbis hide caption

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When Kids Think Parents Play Favorites, It Can Spell Trouble

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Beheading Barbie is the kind of aggression that can cause sibling distress. iStockphoto.com hide caption

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