From someone with a science background: There are a lot of expenses with that type of clinical trial.
In particular, if you’re going to assign someone to a group that is known to be unhealthy (brushing once a month) you need to pay for any dental or medical problems that arise from them not brushing.
Not in any way, no. Our brains can’t predict what we will be interested in or good at.
But we do have predispositions. If you are able to focus intently on things, or are creative, or are more athletic, which are reflected in neuronal connectivity, you might be more interested in more detail-oriented or creative or athletic hobbies.