Good topics for psychology papers vary based on the specific field of study. For example, a student focusing on addiction could write a paper on what substance is the most addictive or what substance is the most difficult to treat.
A student that is studying neurological psychology and focusing on Alzheimer's or dementia could write about the causes of these diseases and the ways to treat them. Theories regarding forgetfulness and improving memory could also be researched. Another option is to write about the changes in the brain as a person ages and how a person is affected by them.