I know this sounds a lot like other questions which have already being asked, but it is actually slightly different. It seems to be generally considered that programmers are not good at performing the role of testing an application. For example:
Joel on Software – Top Five (Wrong) Reasons You Don't Have Testers (emphasis mine)
Don't even think of trying to tell college CS graduates that they can
come work for you, but "everyone has to do a stint in QA for a while
before moving on to code". I've seen a lot of this. Programmers do not
make good testers, and you'll lose a good programmer, who is a lot
harder to replace.
And in this question, one of the most popular answers says (again, my emphasis):
Developers can be testers, but they shouldn't be testers. Developers
tend to unintentionally/unconciously avoid to use the application in a
way that might break it. That's because they wrote it and mostly test
it in the way it should be used.
So the question is are programmers bad at testing? What evidence or arguments are there to support this conclusion? Are programmers only bad at testing their own code? Is there any evidence to suggest that programmers are actually good at testing?
What do I mean by "testing?" I do not mean unit testing or anything that is considered part of the methodology used by the software team to write software. I mean some kind of quality assurance method that is used after the code has been built and deployed to whatever that software team would call the "test environment."
Best Answer
The question seems to be asking specifically about System Testing, so that's what I'm referring to throughout this answer.
I think there's an important distinction to be made between being a bad person to choose to perform testing, and actually being bad at testing.
Why programmers are bad at testing:
Why programmers are good at testing:
Why programmers are bad testers: