While most good bosses try to be fair and balanced with their direct reports, it's only human to prefer the company and work styles of some team members over others, and employees are keenly aware of those preferences. They see favorites and non-favorites, ingroups and outgroups -- and when those divisions fester, they can destroy team culture and performance. Ginka Toegel, professor at IMD Business School, explains why even well-intentioned managers succumb to favoritism, how workers on both sides are affected, and what we can do to both avoid and rectify the problem. Toegel is the coauthor of the HBR article "Stop Playing Favorites."