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Is asking such questions (smoking pot, questions about your social life) in a job interview legal in the US?I suppose you can ask any question you like in a job interview.
It isn't that certain questions are illegal; it's that there are certain bases on which employers may not discriminate - so called protected classes of individuals[1]. If you ask someone a question about whether they are married or pregnant, you might create the perception that you are discriminating on those bases. Not delving into those areas in interviews is merely a prudent HR policy to avoid the appearance of impropriety; actually discriminating on those grounds, however, is illegal.
Job interviews are not the same as security clearance checks (which some govt/govt contractor jobs require). I don't know what if anything is "out of bounds" in the context of those...
[1]: http://www.eeoc.gov/laws/types/index.cfm
EDIT: For a bit more information on the types of questions you might run into during a clearance check, here's a PDF of the form you fill out, SF86[2]. I don't know how closely they hew to this in the in-person interviews but I've heard anecdotally that the investigators primarily clarify and confirm responses you gave on the clearance form. Note that it is 127 pages long. The first 60 or so pages are basic information about you, your relatives, your marital status, people who know you well, education, personal military history and employment. Here are some of the more interesting sections along with the page number they start on:
- Foreign Contacts (p62)
- Foreign Activities (p66)
- Foreign Business, Professional Activities, and Foreign Government Contacts (p75)
- Foreign Travel (p83)
- Psychological and Emotional Health (p87)
- Police Record (p89)
- Illegal Use of Drugs and Drug Activity (p96)
- Use of Alcohol (p103)
- Financial Record (p109)
- Use of Information Technology Systems (p116)
- Association Record (p119) features the question: "Are you now or have you EVER been a member of an organization dedicated to terrorism, either with an awareness of the organization's dedication to that end, or which the specific intent to further such activities?" along with a form which you can helpfully use to provide the name and street address of the organization as well as any contributions you made:
[2]: http://www.opm.gov/Forms/pdf_fill/SF86.pdf