In Canada, there are two provisions of law that are common in many "manager 101" training courses that I can recall:

The Employment Equity Act is aimed at protecting women, visible minorities, people with disabilities and Aboriginals.

"5. Every employer shall implement employment equity by

(a) identifying and eliminating employment barriers against persons in designated groups that result from the employer’s employment systems, policies and practices that are not authorized by law" (1)

The Canadian Human Rights Act is aimed at preventing discrimination "practices based on race, national or ethnic origin, colour, religion, age, sex, sexual orientation,marital status, family status, disability orconviction for an offence for which a pardon has been granted."

"7. It is a discriminatory practice, directly or indirectly,

(b) in the course of employment, to differentiate adversely in relation to an employee,on a prohibited ground of discrimination." (2)

I'm no lawyer but I would tread lightly on "differentiating" by any of these statuses.

(1) http://laws-lois.justice.gc.ca/eng/acts/E-5.401/page-3.html#...

(2) http://laws-lois.justice.gc.ca/PDF/H-6.pdf

I am not a lawyer, but having worked in a corporate setting for years this is something that's drilled into us constantly.

Edit: A bit of searching leads me to the Civil Rights Act of 1964 (https://en.wikipedia.org/wiki/Civil_Rights_Act_of_1964). I'm not sure if the wording specifically prohibits asking about protected statuses, but any legal decisions about that would probably have their roots in that Act.