Yes, I already answered that. That is a function of the Image Size dialog, it has nothing to do with cropping.
You should rarely need to use the Image Size dialog; and if you do, constrain proportions must always be checked, always.
You began this thread by asking about the crop tool, and I'm sure you're right - this is definitely about cropping, NOT about Image Size.
So I need to know more details about the requirements for these headshots, so I can help you further.