I believe the answer is: <span>immature development of his prefrontal cortex.
Prefrontal cortex of our brain granted us the ability to make logical decision to properly address the situation at hand. From brooks' action, we can see that the fear that experienced by brook is illogical and baseless since there is no clear justification of how he feels the threat. </span>
The Answer you seek my fellow student is none other than FRANZ BOAS HIMSELF! yes the one the only FRANZ BOAS i know right? you can't believe it's too amazing to be true BUT IT IS!