I am in the process of doing my dissertation and having problems finding research material that work with my project. Most information is on police officers, but not on the personalities of private investigators or detectives.
Although I can't direct you to any sources on Detective personality types if you have any specific questions I'll help you out, and can access Detectives for you with any wider queries you have without issue.
As an aside my publications on Terrorism and Organised Crime In Scotland, and Islamic State contain detectives opinions on specific issues - don't know if these will be of any assistance.
I would look at the associations involved with Investigators, which have many renowned investigators in their files. This may lead to a survey type questionaire in regards to your goal.
The following research is not on the personalities of private investigators, but we do talk about the descion making processes of PSIR officers and mention the dearth of research on private invetigators: Fiftal Alarid, L., & Montemayor, C. D. (2010). Attorney perspectives and decisions on the presentence investigation report: A research note. Criminal Justice Policy Review, 21(1), 119-133.
Thank you so much Carlos because my instructor tells me I need to tighten up my justification on why personalities of investigators should be researched. Your information is much appreciated.