The Question of Cultural Fit
Especially among startups, cultural fit—whether a job applicant's personality fits in with the employees already working at the company—has become a major factor in the hiring process over the past few years. The ideal is a good one: Teams are delicate creatures, and companies do better when they hire new employees who will work well with those existing teams.
But cultural fit can be problematic. Because of the emphasis on fitting in, some companies use cultural fit as an excuse to focus on new hires who are very similar to existing employees, resulting in a lack of diversity. A religious adherence to a constructed ideal of what a good cultural fit might look like is a sign that anyone with major differences will struggle within that organization. While hiring managers usually focus more on personality and related characteristics, such filters can often wind up removing people from different cultural backgrounds, with families, or of a different gender.
Asking potential employees to work on a probationary basis in order to ensure culture fit is becoming more common. Such a request isn't a warning sign of a problematic culture, but you should consider it as an opportunity to ask more questions. You want to be sure you understand what you're expected to do during such a probationary period—do you have to accomplish certain milestones to fit into the culture? Subjecting you to some sort of “double secret probation” can be a signal that you should keep looking, rather than take weeks off your job hunt to try to fit in with a particular culture.