Yeah, none of the study results I've seen have been strong. I don't trust them all that much.

I prefer snake_case and believe it probably is mildly more readable than camelCase.

The underscore comes closer to space-delimited words like we use in English, typographically speaking. It's hard for me not to think that must be easier to read that way.

Obviously I can't prove it's better, but it both makes sense to me that it would be and fits with my subjective experience.