Imposter syndrome is a bias in thinking in which the individual believes all of their accomplishments to be the result of coincidence and luck instead of actual skill. This is often combined with the fear of being revealed as a "fraud" or "imposter" who does not deserve the things they have obtained in life.

Characteristics 
People experiencing the imposter syndrome feel they do not belong in their environment, because they think others know more and can do things better. This often happens in relation to studying or work, but can also apply to personal relationships with other people. In the case of relationships with other people, those with imposter syndrome worry that they are not good at what they do as well. They believe they are not a good partner, colleague or friend and that they have tricked others into liking them.

Imposter syndrome is also feeling colleagues and authorities value you more than they should. Praise (when someone says you did well) often results in being shocked and not believing in what is said about you. Another trait is thinking your achievements and success are effects of luck, mistakes or bias. That is why these do not convince people that they are intelligent and still cause them to believe they do not deserve to be where they are. These people often refer to themselves as ‘impostors’.

Impostors worry that someone else will discover their true nature. This would have negative effects, like losing a job or others’ trust. To avoid this, they try to prevent anyone from finding out.

Connections 
Those with impostor syndrome have a high chance of also being perfectionists. This means they wish to do everything without errors. They also very often procrastinate (put off things to do for later), because when they do, whether they succeed or fail, they can think it was because of good or bad luck.

The clinical symptoms associated with imposter syndrome include anxiety, fear of failure, depression, decreased mental health, lower self-esteem, feelings of guilt and humiliation and frustration. 

The imposter syndrome has also been found to be linked to some traits from the Big Five personality traits. Negativity, depression and anxiety are common in both neuroticism (one of the Big Five character traits) and imposter phenomenon. The two are then highly linked together. People with imposter syndrome have low conscientiousness (another Big Five character trait) because of low self self-esteem and low feeling of competence (being able to do things successfully).

Factors 
Some people are more likely to experience imposter syndrome. These include: 

 People with a brother or sister that the family thinks is the ‘intelligent one’. This can cause young people to do extra things, like work hard to get good grades, so that the family accepts them as smart ones too. However, thinking that the brother or sister is the intelligent one in the family can cause the person to worry they will never become as smart. This spreads to doubting their own intelligence and abilities.

 People whose parents thought their child was intellectually perfect. This can cause the person to feel pressure both as a child and adult to perform perfectly. When they do not do so well, they start to doubt whether they are smart at all because they think not performing perfectly at everything is a result of them being ‘dumb’.

 People belonging to ethnic, racial or gender minorities. A minority is when a group differs from the rest of society. This feeling of being different may cause them to feel worse than the others around. Some are worried they are the imposter because of affirmative action (treating a group better because it is often discriminated against), concentrating on them being different rather than their knowledge and skills.

 People whose job is associated with the opposite sex or when their job is mostly done by people of the opposite sex. Being one of the few men/women with this job can cause a feeling of being an impostor that does not fit in and perhaps should not be there.

Gender differences 
Some researchers say women experience feeling like an impostor more often than men, but others say they experience it equally frequently.

The ones that agree have found that women on average believe they are able to successfully do tasks less than men. When they do succeed they think it is because of external (not relating to them directly) causes, like luck. When they fail, women think it is because they do not have the necessary abilities. With men, it is the opposite case. When they succeed, they think it is because of internal (directly related to them) causes like being smart and their abilities. However, when they fail they blame it on bad luck or the task being too hard.

Other researchers have found that men are less likely than women to accept and deal with imposter syndrome. They feel the need to have a typical ‘man’s job’, like a mechanic or engineer, and not the ones assumed to be more of a ‘woman’s job’, like a nurse or teacher. Having an imposter syndrome, they avoid feeling different and without talent or abilities in the more woman-typical job. They would rather pick a job where they will not differ as much.

Management 
Many things have been suggested to be done to reduce the feelings of being an imposter. 

 Group settings - hearing others talk about being an imposter can help to become more aware and relate. Sometimes seeing successful people feel what you feel can help to realize that others may also truly think of you as intelligent.

 Changing from thinking about failure (“I will definitely fail”) to thinking about success (“I will succeed”).

 Notice and share with others how your tasks add value and contribute to something larger. This helps to find confidence.

 Mentor - a mentor is someone who can support and guide a person in need. They can inform and give advice regarding the imposter syndrome and other connected things.

 Constructive feedback. Constructive means encouraging. In this case, talking about the good parts as well as the worse ones in feedback (reaction to how a task was done). This is better than only pointing out the bad sides, which can lower self-esteem even more.

References 

Personality disorders
Psychology
Syndromes