Second-born children are more rebellious than their older siblings, according to new research from the US.
Saliva samples were taken from more than 360 youngsters between the ages of seven and 19 to measure testosterone levels.
They were also interviewed by the experts at the Pennsylvania State University, the University of Hawaii at Manoa and Purdue University and asked to keep a diary of their activities out of school.
A spokesman for the researchers told the Daily Mail: "Second-born children showed increases in traits like adventurousness and independence across adolescence, whereas in firstborns, these traits did not change much over time.
"These findings are consistent with the idea that firstborns conform more, while second-borns are more likely to rebel."
Interestingly, the study found that both boys and girls became more adventurous and independent if they played with female friends.
It also revealed that in most cases, boys start off with similar personality profiles to girls but become less warm and sensitive as they become teenagers.
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