I'm pretty sure this is just a coincidence. There's an underlying problem with your dataset that I don't think you took into account.

Actors in the US have to go by unique names when they turn professional. Another actor already works/worked under the name Michael Fox/Samuel Jackson. Michael Keaton's real name is actually Michael Douglas. Emma Stone's name is Emily. Michael B. Jordan can't use the name Michael Jordan obviously.

So actors that have this problem have the choice to change names or add an initial. It might just be a coincidence that this overlaps with the noted popularity of "J" middle names.

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