Psychometric symptom validity assessment is becoming increasingly part and parcel of psychological and neuropsychological assessments. An unresolved and rarely addressed issue concerns the differentiation between factitious and malingered symptom presentations: present-day symptom validity tests can assess whether an examinee presents with noncredible symptomatology, but not why an examinee does so. We explored this issue by developing the Symptom and Disposition Interview (SDI); a symptom validity test that incorporates strategies intended to gauge internal incentives associated with factitious disorder. The merits of the SDI were explored and compared to a traditional symptom validity test (the Structured Inventory of Malingered Symptomatology) in two analogue studies, each with factitious and malingering conditions (n = 24–30 per condition) and a clinical control group (n = 34, n = 40). Overall, the results were positive: The SDI was as effective in detecting feigned symptom presentations as a traditional symptom validity test and superior in differentiating factitious from malingered symptom presentations. We conclude that the SDI is not ready for clinical use, but that psychometric approaches to the assessment of factitious symptomatology, such as the SDI, appear sufficiently promising to warrant future research.