Apples and oranges, Sarastro. Fertility is not a Trait at all, but rather an Attribute. The following code illustrates this:

;------------------------------------------
Trigger family_generalmarries_barren_1
WhenToTest CharacterMarriesPrincess

Condition SpouseAttribute Fertility < 0

Affects WifeIsBarren 1 Chance 100

Attributes appear with the word "Attribute" preceding them in the Condition line, where a trait appears with "Trait" in front of it always. Similarly, Traits are changed with Affects, while Attributes get Effects used on them. I assume this is a tag for the parser, to indicate what kind of variable it is that we're talking about... which coincidentally would only be needed if the different types actually behaved differently.

To elaborate: When you notice your characters having WifeIsBarren, it's not that it's a - trait. It's a bad trait and impacts the character negatively, but it will have positive trait points: it's just that the trigger which generates it checks an Attribute that had a negative value. Similarly, Fertile/Infertile would never be negative since they are always given positive points (note the trigger above) no matter whether the woman's Fertility was positive, negative, or zero.

So I'm gonna keep rolling along with the "Traits can't be negative" thing since I've seen evidence of it from several traits, and none to the contrary. If anyone finds any that don't behave like that though, let me know of course. Until then, use my file!

Also since I just noticed it... no wonder I've never seen a man with WifeIsFertile: it's set only to apply to "admirals," where WifeIsBarren correctly affects "family." I guess only sailors could procreate like rabits back in those days.