jase wanted to keep it to 24 genres to avoid change to the interface, and on that basis, the last time we reviewed the list t'was he himself who suggested:
"How about merging Spy and Mystery to just "Intrigue"? Not perfect, I know, but it gives us 1 more category to play with."
There's probably more in this discussion thread than you wanted to know about the history of genres (aka catagories aka tags) -
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