5 anxiety attacks, 2 Bs, 1 nervous breakdown, and 1 16 year-old-sister move-in later...

--Monica Macaulay in Surviving Linguistics: A Guide for Graduate Students

If you’re in graduate school, it is highly likely that you were a smart kid. You probably got very good grades, and probably didn’t’ have to try very hard to be the best in your classes. Then…you get to graduate school and suddenly you’re not the smartest person in the class any more. You’re surrounded by smart people. This can be a shattering experience for entering grad students, as they try to find their place in a very different environment than they are used to. But it doesn’t end there. Many graduate students have a persistent feeling that they are hiding the shameful secret of how unutterably stupid they are. They feel like everybody is smart, and they are the only dumb one in class. Often students live in fear of being found out – a common nightmare is that some day the admissions committee is going to realize that they sent the wrong letter, and that the student was only admitted by accident.
One result of this feeling is that students become afraid to speak up in class for fear of revealing themselves as the frauds they are certain they are. Another result can be excessive perfectionism, which can lead to an inability to complete projects. These kinds of behaviors can be very damaging to one’s graduate and post-graduate career, and sadly are more common than you might think.
In fact, this is so common that it has a name: Imposter Syndrome. Most academics suffer from it at one point or another in their careers, and many never overcome it. (I won’t name names, but a well-known professor in another field told me that at age 73 he was still waiting to get over it!) … In fact, the psychological literature is filled with studies that describe students, professors, CEOs, nurse practitioners, and others as suffering from this syndrome. It is more prevalent in women and members of ethnic minority groups, but is certainly not confined to them.