In my experience, background in psychology is not required but as far as this particular paper goes... syllabus is toooooo much for 100 marks. Also, the syllabus FPSC has given is so much confusing that you would not understand what to cover and what to leave. If you opt for it, it will DEFINITELY take a time toll on your other subjects. And it took decades for FPSC to reach this, how soon do you think they are going to change it back to 200.
So my advice is choose another subject that has relatively lesser syllabus coverage because my one whole subject got left behind for the reason that i opted for psychology.