A person that suffers from it will refuse to write code most of the time. Instead, they will spend hours searching for software packages they could have written themselves in minutes. They will do anything to eventually be able to say that the software they glue together was “not invented here”.
It is caused by deep feelings of self-distrust. Sufferers also have self-harming tendencies such as never storing the code they use in their own repositories or elsewhere, so that the disappearance of their software dependencies can cause the downfall of their own projects.
A strange side effect of Not Invented Here Syndrome is saying that people who do write code themselves suffer from Not Invented Here Syndrome. The cause of this denial is unknown and is studied by many psychologists.