The paper is The Impostor Phenomenon in High Achieving Women: Dynamics and Therapeutic Intervention, by P. Clance and S. Imes (1978).
They interviewed about 150 women, from undergrads to professionals, in many different fields. The main findings are the women fall into two main groups when discussing origins of the phenomenon, and there are four general behaviors that perpetuate the feelings of inadequacy.
For origins, it's all about family dynamics. In one group, the women had a sibling that was labeled as the "intelligent" one, or the "academic". No matter how the woman performed in school or work, they could never live up to the other sibling (even if they receieved better grades or more awards). These women tend to work extremely long hours, and are over-prepared (and make sure everyone knows how hard they are working), in hopes to "prove them wrong". When they do not get the reaction they want from their family (or collegues), they think they must not be smart enough, and the cycle repeats.
The second group is the opposite: the woman was labeled as the superior child in everything - intellect, personality, talents, looks, etc.. Not only that, but everyone tells her how easy everything comes to her, and how she just has "natural" abilities. The issue arises when the woman is in school and realizes she must study to do well. Since she's been told her whole life that she is so smart and doesn't need to study, they begin to feel stupid. These women tend to hide the fact that they work hard, and often minimize their work and accomplishments.
Once the feeling of being an impostor, or fraud, begin, it tends to perpetuate itself. Four main behaviours they discuss are:
1. Overworking - a fear of being "discovered" as being stupid drives this. What happens is the woman will work extremely hard on a project. Of course, they will do an amazing job, they will get a good reaction, and the woman believes the only reason they succeeded was because of the amount of work they did. Therefore, they must continue this behaviour or else they will fail.
2. Intellectual inauthenticity - where the woman chooses to not reveal their true opinions or ideas. In addition, they tend to cater to the beliefs/ideas of those around them. For example, a student will cite multiple papers of their professor in their work (even if they don't agree with it); or a grad student will use their supervisors ideas in a proposal and downplay their own. In the end, when they succeed, they believe it was only because they kept their mouth shut and didn't share their true ideas or opinions. Again, because of the success, it perpetuates the behaviour.
3. Using charm - these women are looking for a mentor to "discover" how smart they are. They seek approval from an admired authority figure by using their charm, looks, friendliness, humor, sexuality and perceptiveness. They study the mentor to see what they can "give" to them (be that a friend, a pet-student, a sexual partner, etc.). In these situations, when the mentor indicates the woman is intelligent and special, the woman does not believe them because they must only think that because of their relationship. She then moves on to another mentor and the behaviour repeats again.
4. Issues with society - Women who display confidence in her abilities tend to be seen as "less feminine". So, in order avoid social rejection and to keep her femininity, she downplays her successes and intelligence. Of course, this behaviour does "work", and so the woman feels she must continue or she will encounter problems.
The authors discuss methods of therapy, which I won't get into here, but one thing they mention is that women need to become aware of the superstitious and magical aspects of their impostor beliefs. For example, it probably isn't necessary for someone to work 16 hours a day - but some women believe that's the only way they can be successful because that's what they've been doing all along. Or, they need to get away from automatically thinking "I will fail" (because then they won't be dissappointed if they do).
A quick note they had at the beginning of the paper is that they do not see this phenomenon nearly as much with men - and if they do, it's with men that are more in touch with their "feminine" qualities (i.e., emotions, sensitivity). I have a feeling, since this paper was written 30 years ago, that it is actually more common among men these days, since they are more in touch with those qualities than generations past.