a study is only a science (as argued by the philosopher 
 Popper) if it can be falsified.
That is, theories are established based on empirical evidence, and any case against it blows it apart.
A classic example of falsification is classical electromagnetism and the 
Ultraviolet catastrophe. following this, classical EM theories were completely scrapped and Quantum Mechanics emerged.
according to my philosophy teacher, psychology does not satisfy this "falsifiability" condition. when evidence is shown against contemporary theories, these theories adapt to include these anomalies, rather than to create a new theory that explains these anomalies, much like the rather controversial "Creationism" claiming to a science.
but then Kuhn (another philosopher) argued that scientist also follow their own predefined paradigm, that non-falsifiability doesnt really qualify something to be a non-science, an example is Chemistry before oxygen was discovered, when fire was believed to be caused by this thing called "phlogiston", but when the ashes were weighed it was found to have a higher mass (we now know thats oxides), so the phlogiston theory was changed to accomodate this by saying "in some cases phlogiston can have negative mass". Yet no one disputed that at this stage, chemistry is not a science.
In this realm of philosophy, there are no answers... =S