I am going to take a much more common sense approach.
As many of you may know, I have been looking for my next full time role for some months. During this time I have been on a number of interviews and have both studied and advised on the proper way to prepare yourself and how to behave. You also know (if you have been following me at all) that I am quite active on all of the most popular social networks.
Moreover, as a senior manager for the last twenty years or more I have frequently been on the other side of the desk conducting the interview and doing the candidate evaluations. My focus is always on how well one will fit the roll. The question is whether you have the necessary technical skills, the required interpersonal skills and are you sufficiently motivated. What you do evenings and weekends is your business. It is inconceivable to me that anyone would probe your behavior outside of the workplace.
Such requests are ludicrous. And so is asking an applicant for access to their social network account.
No HR professional or hiring manager would make requests like these. If they did most candidates would be outraged and flatly refuse. That is exactly how you should deal with this password request should it happen to you during an interview. The answer is more than a simple no. It should be, "How dare you."
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