Assumption: For any wrongdoing there is a fixed amount of responsibility to be spread around.

I call this the Reponsibility Pie Assumption, because it presumes that responsibility is like a pie: a fixed amount to be divided up and given out.

So, for example, it is sometimes thought that if a victim of rape is partly responsible for what happened (e.g. by wearing "provocative" clothing), then the rapist is less responsible.