Being a psychologist partially requires being a researcher, which means that we always have to come up with new ideas. The interesting thing about studying psychology is that the new questions never end, since we study the behaviour of individuals and there are many factors that change it.
Background is a basic factor that modifies behaviour. Even if we try to get results that describe the average, every person is different because of the experiences they’ve had and the cultural differences. Religion, gender, status and educational level are also closely connected to the way someone behaves.
Mental disorders can influence the validity of the results of a study. What if some of the participants are compulsive liars? The possibility of lying when it comes to their answers is pretty high, so the researcher can’t be actually sure if every single answer is the one it is supposed to be. On the other hand, it is impossible to test every single participant. It would be time consuming and would certainly cost lots of money, as well.
Motives can also be triggering. For instance, if the study includes giving money to the participants, they might go for the results they think are more likely to be the “correct” ones. Under these circumstances, the least a researcher can do is modifying the questions in a way that no motives are shown, unless it’s part of the study.
As I’ve said before, we work with human beings, so how sure can we be about them? We can’t predict, nor guarantee for the outcomes of our studies.