I have the following question:
The majority of fund studies has or have agreed that a common attribute for superior performance is due to managers' skill.
Like the words some and most, a premodifying phrase like the majority of is in and of itself neither singular nor plural. It has no number.
The noun it is modifying continues to function as the subject for purposes of agreement with the verb. This allows then for both possibilities:
Notice how adding the majority of doesn’t change anything in those two examples.
Therefore because studies is plural, so too is the majority of studies:
The majority of fund studies have agreed that a common attribute for superior performance is due to managers’ skill.
That’s because it just means most fund studies; the number does not change.