One sign a new condition may fall into this category is that it mainly affects teenage girls and young women. They are more likely than other demographics to indulge in ‘co-rumination’: repetitive discussion and speculation within a peer group. That can lead to internalising problems, and thence to anxiety, depression and self-harm. Girls are also often more empathetic than boys, and better at reading moods, which means emotions spread faster in a female peer group than in a male one. That is why self-harm and eating disorders can run through female friends, and why historical episodes of mass
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