BEIJING—When Michael Phelps was a kid, his primary school teacher told his mother he would never amount to anything because he was unable to focus.

When Phelps won the first of his 14 Olympic gold medals, in Athens in 2004, he remembered those words as he stood on the podium and listened to the “Stars and Stripes.”

Despite being diagnosed with Attention Deficit Hyperactivity Disorder (ADHD) at the age of 9, Phelps went to prove that teacher spectacularly wrong.

“He was a very energetic little guy, always all over the place, ‘Why are we doing this? When are we doing this? What are we doing next?’” his mother Debbie told Reuters in an interview.

“Naughty isn’t a word that I would use, he was playful, inventive,” she added. “Definitely athletics channelled a lot of that energy.”

In his autobiography, he talks of being a “pool rat, running around, sneaking up behind people, stealing their snacks and goggles, tapping them on the shoulder and running away and just causing general havoc”.

She says her son’s extraordinary focus on swimming is common to many hyperactive children, unable to sit still for more than a few minutes at school but capable of devoting themselves entirely to something they love doing.

He took medication to address the symptoms of ADHD, as do around 1 in 25 children in the United States, but only on schooldays and not at weekends or holidays.