Attention Deficit / Hyperactivity Disorder and Gender

Attention-Deficit/Hyperactivity Disorder, more commonly known as ADD or ADHD, has become much more common recently. As of 1994 the DSM-IV stated that about 3%-5% of American school aged children have this disorder. While it is quite a few children, what I find to be even more interesting are the male-to-female ratios within the disorder. They range from 4:1 to 9:1. What I've looked into finding out is why there is such a gender difference. I wanted to know if it was because the way society raises girls, or are boys really at a higher risk for it.
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