Researchers, at the Pennsylvania State University, the University of Hawaii at Manoa and Purdue University, interviewed children and their parents to assess their personalities.
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They also took saliva samples to measure testosterone levels and asked the children to keep a diary of their activities outside school.
The study – published in the journal Child Development – found personality traits developed differently in first and second-born children.
‘Second-born children showed increases in traits like adventurousness and independence across adolescence, whereas in first-borns, these traits did not change much over time,’ a spokesman for the researchers said.
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