It feels so natural to organize tags by hierarchy when using tags referring to concepts of the real world. For example tag: ‘psychology’. Then it has children tags like ‘experimental psychology’ and ‘psychological statistics’. They are organized like:
- psychology[list][*]experimental psychology
- psychological statistics
[/*:m][/list:u]
Then I tag some files whose content is more general as psychology, others more specific as experimental psychology and psychological statistics. But here comes the problem. If I want to take a look at all the files related to psychology, I click ‘psychology’ at the tag list, it shows only the files directly tagged by psychology instead of all files of psychology and its children tags which is more natural and logical.
So why is it designed this way? And is it possible to set Devonthink to show all the files?
All views except the Tags view show the actual group/tags hierarchy, therefore switching the view should enable this.
When I create a hierarchy of tags (or groups), the top level of that hierarchy doesn’t have documents assigned to it. Relevant documents would be tagged by one of the sub-tags in the hierarchy (or located within a sub-group of a group hierarchy).
I would have designed your tag hierarchy as:
psychology
- general psychology
- experimental psychology
- psychological statistics
If I wish to see all the items tagged under the psychology tag hierarchy, I would then (in Three Panes view, for example) Command-click to select all the sub-tags under that heading. The list of all documents so tagged will be displayed.
Select any document from that list and you will see its tags, which will be displayed in the Tags Bar to show the place in the hierarchy, e.g., ‘psychology’ ‘experimental psychology’ (as well as any other tags you may have assigned to that document).
Of course, if you had tagged items at the top of the hierarchy, e.g., ‘psychology’, you can also Command-click on it to add the top of the hierarchy to the list of selected tags. But in that case you haven’t used the concept of hierarchy properly. (Which isn’t to say that I don’t sometimes use the concept messily. I’m far from perfect.) 
If you do a search using the Advanced button of the full Search window (or create a smart group) using the following criteria:
All of the following are true:
Tag is psychology
Then all items tagged under the hierarchy of the ‘psychology’ tag will be listed.