A first-year student is greeted upon arrival at University College in Utrecht, Netherlands, on August 16, 2010. Robin Utrecht/AFP/Getty Images hide caption

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A gene known as DRD2 affects the brain's dopamine system and is known to be associated with aggressive behavior. iStockphoto.com hide caption

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Laura Molina, 9, shows the mask she created expressing the feeling of "sadness." Her mother is being treated for breast cancer at the Lyndon B. Johnson public hospital in Houston. Carrie Feibel/KUHF hide caption

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Yvonne Condes helps her son Alec get ready for baseball practice. David Gilkey/NPR hide caption

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Teacher Denise Severing leads a math lesson at a Head Start school in Woodbourne, New York. John Moore/Getty Images hide caption

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