The more friends a girl has and the more time she spends on Facebook, the more likely she is to become depressed, a US psychologist has warned.

Dr. Leonard Sax, author of "Girls on the Edge," said the risks were higher for girls because they usually only posted "good things and happy stuff."

"Girls post the happy things and they turn the camera on themselves so it's 'look here at what I'm doing,'" he said.

"Then they look at all the other girls' Facebook pages, look at them being happy and think, 'my life sucks, look at all the things those girls are doing and how much fun they're having.'"

Currently in Australia to speak on gender issues at an education conference, Sax said teenage girls, unlike adults, struggled to understand that people were intentionally making themselves look good.

"The problem is she is spending all this time on her presence on Facebook and not nurturing strong friendships because Facebook prioritizes acquaintances," he said.

"Many girls now say they don't have one or two best friends, they have 12, 15, 20. They are losing the skills to nurture close friendships."

Sax recommends parents limit time on social networking sites to 20 to 30 minutes a day and institute set times they would spend with their daughters.

"Parents need to know everything their children are doing on their laptop and they need to know that you know," he said.

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