Case studies are often used where there is a rare behaviour being investigated which does not arise often enough to warrant a larger study being conducted.
A case study allows data to be collected and analysed on something that psychologists have very little understanding of, and can therefore be the starting point for further, more in-depth research.
By only studying one individual, an isolated event or a small group of people it is very difficult to generalise any findings to the wider population, since results are likely to be so unique.
This creates issues with external validity as psychologists are unable to conclude with confidence that anyone beyond the 'case' will behave in the same way under similar circumstances, thus lowering population validity.
These unique insights can often be overlooked in situations where there is only the manipulation of one variable in order to measure its effect on another.
The case study of Genie (Rymer, 1993) allowed researchers to understand the long-term effects of failure to form an attachment which they could not do with a human participant unless it naturally occurred.