Ideally, you'd respond with the truth: You were looking for more responsibility, or wanted a job closer to home, or you were laid off, or whatever the real reason happens to be.
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But what if the real reason is that your boss was a raving lunatic or that you hated all your coworkers? If that's the case, you need to finesse your answer.
To understand how to approach this, it helps to know the secret about the "why did you leave your last job?" or "why are you leaving your current job?" interview question: It's okay if the real reason you're leaving is because of a crazy micromanaging boss, unpleasant coworkers, or a toxic culture. Hiring managers have all had those experiences and know those situations are out there. You just can't say it in an interview, because if you do, you raise the following questions in the interviewer's mind:
Given how many crazy bosses and toxic workplaces are out there, this might seem unfair. After all, why shouldn't you be able to tell the truth and have the interviewer give you the benefit of the doubt?
There are two reasons for this: First, while interviewers will certainly allow for the possibility that your account is completely correct and objective, they don't know you. They don't know how reasonable or objective or sane you are, so the situation raises enough of a question mark that they'll have to wonder and worry, and it won't help you to have those sorts of questions hanging over you. And once those questions are raised, it is very difficult to definitively put them to rest during the hiring process.
Second, rightly or wrongly, the interviewing convention is that you don't badmouth a previous employer. You just don't, no matter how deserved it might be. And interviewers are looking for evidence that you know what is and isn't appropriate to say in business situations.
So instead of "I'm leaving because my boss made me sick to my stomach every time I looked at him," try explaining that you are "leaving for new challenges," "excited about this particular opportunity," "taking the time to find something right," and so forth. Your interviewer may realize there could be more behind it, but will be glad that you're handling it appropriately, not boiling over with rage or badmouthing an employer.
Alison Green writes the popular Ask a Manager blog where she dispenses advice on career, job search, and management issues. She's also the author of Managing to Change the World: The Nonprofit Leader's Guide to Getting Results and former chief of staff of a successful nonprofit organization, where she oversaw day-to-day staff management, hiring, firing, and employee development. She now teaches other managers how to manage for results.