At least in the American college setting, most people who double major pursue a much higher number of simultaneous courses (also described as 'how many credits you are taking this semester').

This means less time for fun and socializing and more time in class and studying.

Industriousness is displayed by the willingness to work harder than is expected of someone with a single major which is the dominant way to do undergrad. IQ would be involved in the ability to study for and take more exams in finals week(s).