Jack -- good question.
IMHO as a psych professor (somewhat biased in favor of psychology!), the most relevant behavioral sciences fields for working on 'AI alignment with human values' would be the key branches of psychology that have actually studied human values, such as moral psychology, political psychology, psychology of religion, social psychology, and evolutionary psychology.
Then there are the behavioral sciences fields that study the diversity of human values across individuals (e.g. personality psychology, clinical psychology, intelligence research, behavior genetics), across cultures (e.g. cross-cultural psychology, anthropology, political science, sociology), and across history (e.g. intellectual, political, religious, social, sexual, and family history).
Also, I think fields such as behavioral game theory, evolutionary game theory, microeconomics, and decision theory are very useful for AI alignment work.
There's a bit of neuroscience that studies human values, preferences, and decision-making (e.g. affective neuroscience, cognitive neuroscience) that might be relevant to AI research. But, in my opinion, neuroscience hasn't discovered much that's relevant to AI research that wasn't already discovered by behavioral sciences. Neuroscience has mostly identified where certain kinds of processing happens in the brain, without really adding much to our understanding of what kinds of processing are happening, and why, and what the implications are for AI alignment. (Epistemic status of this claim: somewhat weak; I studied neuroscience fairly deeply decades ago in grad school, and have kept up to some degree with recent brain imaging work, but I'm not a neuroscience expert.)
The big advantage of neuroscience is that it has high status, cachet, and fundability, and sounds like 'hard science'. So, people who don't understand the behavioral sciences, and who might consider a field like moral psychology to be 'soft science' or 'pseudo-science', might take a neuroscience degree more seriously. Honestly though, if you study neuroscience at PhD level, I would bet that the stuff that proves most useful to AI alignment will be the psychology theories, methods, and findings rather than the neuroscience research.
So, you'd have to decide whether the status benefits of a neuroscience PhD (vs. a PhD in moral psychology, or in behavioral game theory, for example) out-weigh the time costs of having to learn an awful lot about the details of brain anatomy and physiology, brain imaging methods, and voxel-based imaging analysis -- most of which simply won't be very relevant to AI alignment.
Abby -- excellent advice. This is consistent with what I've seen in neuroscience, psychology, and PhD programs in general.