There is little to add to the excellent and thorough post of #4. It is hard to imagine that many aspects of psychology cannot be considered scientific when certain clearly scientific advancements have been made. One interesting one published fairly recently in Time magazine is the implantation of actual wiring in the brain which can avert seizures. Other scientific studies have revealed that a certain area in the front of the brain is larger in the brains of serial killers. This discovery may well lead to significant actions in the prevention of crime.
To what extent is psychology a science?
To what extent is psychology a science?
I'll expand a little upon what Krishna said. Psychology really has two fields within it. One is clinical psychology that is devoted to treating day to day problems with "mentation" (thought processes) and affect processes. Broadly speaking, clinical psychology addresses on one end of the spectrum individuals with chronic depression or bipolar disorder and at the other end of the spectrum individuals struggling to recover from physical and emotional trauma: one is apparently an internal miswiring of chemical or electrical pathways in the brain while the other is the result of external assault of some sort.
The other field is research, or cognitive, psychology. This field is devoted to experimentation and examination, through such means as fMRI and CAT scans among other things, of the processes of memory, perception, cognitive processes, emotional responses. It is the field through which such things as the intricacies of the workings of the left brain, right brain, frontal lobe, and the pathology of depression (Against Depression, Peter D. Kramer Ph.D.) have been discovered. It is also through this field that research is being done on such things as synesthesia.
It is easy for most people to see how the second field of cognitive psychology is to a full extent a valid science. However it is harder for some people to see how the first field of clinical psychology is to a full extent a valid science since it is founded upon theories, like Adler's and Maslow's, instead of upon the data of verifiable, repeatable experimentation and measurement. Yet both fields do comprise psychology and the second field proves that psychology is a science, even though its two parts may not be equally advanced and reliable in method.
To me, the answer is that psychology is not very much of a science. I think that anything that studies human beings' mentality (as opposed to physical bodies) cannot possibly be a hard science.
The main reason is that psychology cannot predict the behavior of any individual person. This is unlike the "hard sciences." In chemistry, for example, one can predict with great accuracy what will happen if two chemicals are combined with one another. In psychology, this is impossible. We may be able to know that most people will react in a certain way in a given situation. However, we can never know how any particular person will react. Because of this, I do not think that psychology is truly a science.
I disagree with the views presented in the previous answer that psychology is not a hard science. I particularly disagree with the reason given in support of this view that:
... psychology cannot predict the behavior of any individual person.
Greater the difficulty of understanding the the nature of behaviour of a subject, greater - not less - is the need and scope for scientific study of that subject. It is not possible to predict the motion of an individual particle of a fluid enclosed in a container. That does not make the concept of Brownian motion, that describes the nature of such motion, less scientific.
Psychology is very much of a science because it applies scientific methods to study, understand, explain, and predict mental processes and behaviour of individuals. Psychologists conduct systematic research and analysis to do this. Their research includes methodical observation of the reality as it exist. In addition they also perform experiments, as is done in many other branches of science.