It's been a while since I last opened the Genetics Kit, but you can sort the genes (Look under the Gene Editor tab) by type and you can look for the instincts there. You'll find the other 'express' genes, IIRC, under Creature Reactions or something similar.
OK. If you dig around the Creature: Instinct genes, you'll be able to find genes that make express part of the instincts. They'll be named things like "Shout and swear if angry".
Mute this gene by clicking "Edit gene" at the bottom of the program window. A window for the gene will pop up. Next, you want to check a little check box at the top that says "Do not express(carry)". This will make it so that the gene is 'silenced' and will the creature won't yell so much.
Next, look under Creature: Stimulus. There are genes here about expressing, too. "Swearing lets off steam" is one of those genes. Just like the instinct gene, you'll need to 'silence' this gene and all similar genes as well.
Those are really the only two genes I could find though (browsing the Harlequin gene). I think the best way to keep them from talking quite so much is to punish them when they are talkative. Instead of making shouting help alleviate anger, for example you could change "Anger" to Boredom and move the slider right so that it creates boredom.
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