Another possible explanation: maybe popular kids have mild ADHD or some other psychiatric disorder that makes them daring and extroverted and more likely to socialize or play sports.

Then, when they get older, those problems become more severe. For example, lots of kids with ADHD later develop bipolar disorder.

I don’t deny the points raised in the article but other explanations are possible , too.

And lots of smart kids expetience difficulties when they finally take a course they struggle in.