The point of MD/PhD is for people who want a research career so I would focus on what you’re interested in (broadly) from a research/PhD perspective first and then how that relates to whatever specialty/field you’re interested in second v. the other way around. You can honestly make a connection between nearly any field from a PhD/basic science standpoint and psychiatry but at this juncture that will be somewhat dependent on resources, active research, and PIs’ individual interests in your program. In some fields the connection is more obvious (e.g., neuroscience, cognitive sciences, pharmacology, epidemiology, etc.) but you can find relevant topics even in biochemistry and microbiology. My med school had/has biochem PIs doing research on serotonin neurotransmission and in lipid metabolism and neurocognitive disorders, and virologists researching association and mechanism of certain viruses with the development of subsequent depression, neurocog disorders, etc.
Tangent alert:
As per the logistical reasons above, a PhD in clinical psychology would be impractical though a PhD in cognitive science or some flavor of experimental (i.e., not applied) psychology would not be unreasonable - if it is something you’re genuinely interested in and not just as a means to attempt to bolster your app for residency. This isn’t terribly common and dependent on the school but I am aware of people who have done concurrent MD/PhDs like this. Clinical psychology came into being peri-WWII at a time when psychology was basically a research and philosophical discipline aimed at objectively observing and understanding human behavior, with the US military putting a large amount of money into developing systematic means of objectively quantifying and qualifying abnormal (“pathological”) psychology v. “normal” psychology and later developing ways of systematically and objectively addressing/treating “abnormal” psychology. This differed from psychiatry which at the time was dominated by psychoanalytic theory and thus more subjective, qualitative, and theoretical. Based on its philosophical origins, as clinical psychology developed into an applied discipline it differed from psychiatry in that it relied(relies) very heavily on statistics, used(uses) psychometrics, and focused treatment on things which can be somewhat more objectively assessed and measured (e.g., cognition and behavior). Hence things like cognitive and intelligence testing, personality testing, CBT, and DBT coming out of psychology. Over time, the distinction between clinical psychology and psychiatry has blurred some, but clinical psychology is distinct from psychiatry in the heavy amount of statistics, research methods, and research design in their training which medicine/psychiatry scarcely touches, learning to administer and interpret psychometrics (which in general requires a much greater statistics knowledge base than what is taught in med school and residency), and more robust therapy training in clinical psychology; particularly with therapy modalities that can be more objectively studied and measured. While I genuinely find psychometrics really valuable, there’s no real benefit to being able to administer and interpret them yourself as a psychiatrist outside of possibly forensic psychiatry. With the statistics and research skills learned, these will be covered in a more research oriented experimental psychology/cognitive science program as well, and an MD doesn’t really add anything to this if it’s the path you want to go. If you opt to go a more clinical route, your stats and research skill sets (particularly in regard to psychology) are going to atrophy. While clinical psychologists do generally receive more and much better therapy training (particularly from a theory standpoint) compared to psychiatry residency, if this is something you’re interested in you can start pursuing additional training in therapy outside of your program and residency and continue this after residency, so there’s no real added benefit to pursuing doctoral level clinical psych training before med school and/or residency. So this, plus the logistical issues mentioned in my previous post is why a MD/PhD in clinical psychology really doesn’t make sense - there’s some redundancy, the areas where the fields are different don’t add significant value to each other by being trained in both, and the aspects that do add value can be gained more efficiently through other means (e.g., PhD in experimental/cognitive psychology if wanting a research career, pursuing additional therapy training during/after residency if wanting more robust psychotherapy skills).
