The following are protected groups: http://en.wikipedia.org/wiki/Protected_class
Edit: See also http://en.wikipedia.org/wiki/Title_VII#Title_VII
I'll also state the obvious here - I'm not a lawyer and you shouldn't trust legal advice you receive on the internet.
I guess a legal alternative is to headhunt rather than advertise positions. In the tech industry, I suspect this is a lot easier (thanks to open source, blogs, etc) than elsewhere. Surely there aren't laws against a company hunting down people they want to hire, rather than hanging out their shingle and waiting for people to submit résumés.
Interesting metric. What happens if the difference comes about because you are more fair than your industry's average?
Say you have more female employees. Could a man sue you?
And how do you tell that `more fair' from `less fair'?
The main reason the idea was introduced was because, during the civil-rights era, a bunch of companies and even state/local governments abolished their official "no blacks" policies, but nonetheless unofficially still didn't hire blacks (or, if they had low-level black workers, had a de facto no-blacks-in-management policy). So the 1964 Civil Rights Act is intended to ban that sort of unwritten-rules discrimination. The background of Jim Crow laws was a motivator as well, since many didn't officially impose any restrictions on the basis of race, but used proxies that happened to correlate well with race; and the Civil Rights Act was intended to do away with all those roundabout methods, too.