Adapted from an email I sent to a friend who’s applying to PhD programs in psychology:
When your prospective advisor reads your personal statement, they’re ultimately trying to evaluate whether you’ll be a successful and productive member of their lab. Before you start writing a single word of your personal statement, take some time to think deeply about the kinds of questions you’d have for an applicant if you were a lab director looking for a new student. The art of writing is telling an engaging and eloquent story about your trajectory through academia. The science of writing is addressing how your trajectory has prepared you to be an outstanding PhD student. Neither is sufficient; you need both!
In case you’re feeling stuck, here are examples of questions that you might want to answer in your personal statement:
What got you interested in psychology to begin with? What steps have you taken to address these interests? How have your interests evolved over time as a function of the experiences you've had?
What academic/research experiences have you had, and how do they qualify you to be a PhD student?
How deeply were you involved in research projects? How independently can you work? What skills did you learn? How comfortable are you working with data? To what extent do you understand both the theoretical reasons for doing the research, and how the design/results of the research address those theoretical questions?
Of all the programs/advisors you could have applied to, why this one? What does this school have that other schools don't? How deeply have you thought about your potential fit here? Are there specific research directions you might like to pursue with this particular advisor?