Psychologists are guided by scientific principles. One of these principles is critical thinking, and the principles of critical thinking are:

1. Be Skeptical
2. Insist on evidence
3. Examine definition of terms
4. Examine assumptions or premises of arguments
5. Be cautious in drawing conclusions from evidence
6. Consider alternative interpretation of research evidence
7. Do not oversimplify
8. Do not overgeneralize
9. Apply critical thinking to all areas of life.

In short, critical thinking means to not take anything at face value. As scientists, it is important that Psychologists stick to the scientific principles, and to follow a scientific method. 
Scientific Method
Psychologists usually begin by formulating a research question, which may be translated into an Hypothesis. An hypothesis is a statement about the mental or behaviour processes. When this has been achieved, the Psychologists will then examine the question or test the hypothesis through controlled methods. They will draw conclusions based on the observations or findings. If the observations or findings do not back up the hypothesis, the original theory may be altered.
Psychologists are obligated to share enough details of their work so that others can replicate the work.
We must remember that although Psychology is a science, people are not as easily manipulated as chemicals or atoms. Some people will react differently in certain situations. As such, Psychologists must be careful to pick only people who closely match the general population that they are researching.
There are several ways of selecting people to study; Random Sampling, Stratified Sampling, Case study, Survey and Naturalistic observation.
Random Sampling is a sample of a population, and everyone is just as likely to be selected to participate.
Stratified Sampling is more specific, dealing with only the people that are being researched, for example rich people, or people with minority backgrounds.
Case Study is the study of one person, and a biography is drawn up through interviews, questionnaires and psychological tests.
Survey is similar to Random Sampling, where a large sample of people are given questions with regards to their attitude or behaviour.
Naturalistic Observation is watching organisms, in this case people, in their natural environment.

Psychology is a very grey subject. Though research may prove that certain types of people are more likely to behave in a certain way, there are exceptions to the norm. But like all scientists, Psychologists must adhere to the scientific principles.