A study finds that Harry Potter fans are less authoritarian, more tolerant of differences, and more politically active than non-fans.
The article doesn't discuss the methodology in too much detail but two points-
1) correlation does not mean causation (which the above article acknowledges)
2) Kids who read more in general (and hence have a greater likelihood of reading Harry Potter) may be less authoritarian, more tolerant of differences etc.
Moreover, it's possible that kids who relate more to the themes of equality, justice, tolerance underlying the seven-part series are more likely to recognize themselves as fans.
But, let's not nitpick too much, okay?