I found a bug (either a major one or a minor one, I'm not sure.)

[Kirov]

Try starting a game playing as Yang, use the scenario editor to make a pact with Deirdre, place solars and echelons plus energy bonuses around both players' starting bases to produce a lot of energy, and see what the commerce looks like.  If negative ECONOMY has no effect on commerce rating, then both should get the same commerce.

Ah, I think I see now where the problem is. You see, as you may have noticed, it's actually not that nice and easy to test such stuff in the editor. In the early game settings, if you get a commerce tech, it may have no effect on your treaty output simply because your bases are so small. And in late game it's also not that easy, because you need to manipulate bases to match the energy before the switch to negative ECON.

The thing is, the situation you describe doesn't conclude that negative ECON gives a commerce rating hit. Yang will have lower treaty output because the most important factor is your energy-per-base output. And Yang has less energy simply because he's got -1 energy per base. That doesn't mean that there is a separate commerce rating drop, apart from the fact that ECON-challenged factions are simply poorer. What you need to do is put one uranium on his base tile and then compare the results.

Still, it may not be conclusive if the game is early enough, as you may not see any difference.

I checked it in my one late game save and I'm 95% sure that no, switching from 0 ECON to -3 ECON doesn't mean you lose commerce rating. It would take me some time to verify it for 100%, but at this point I don't see anything that suggests thorough testing should be done.

[Yitzi]

Ah, I think I see now where the problem is. You see, as you may have noticed, it's actually not that nice and easy to test such stuff in the editor. In the early game settings, if you get a commerce tech, it may have no effect on your treaty output simply because your bases are so small.

With the scenario editor you can change the sizes of bases and surround them with solar collectors on energy bonuses and echelon mirrors to get plenty of energy.

And in late game it's also not that easy, because you need to manipulate bases to match the energy before the switch to negative ECON.

Not that hard; the difference between 0 and -2 is only 1 energy per base, so put uranium or geothermal on one square for equality.

The thing is, the situation you describe doesn't conclude that negative ECON gives a commerce rating hit. Yang will have lower treaty output because the most important factor is your energy-per-base output. And Yang has less energy simply because he's got -1 energy per base.

Except that the commerce is dependent on the sum of the energy of both bases equally, so it would affect Yang and his trade partner equally.
However, I checked again and it does seem to be the same; I think the reason it went wrong the first time is that I used the social-engineering trick to reset Yang's "commerce techs known", but did not do the same for his trading partner.

So it does seem that the only effects are commerce techs known, being governor, being Morgan, and a +3 or higher ECONOMY rating.

[Kirov]

Yeah, you're right, I prefer to tinker with actual savegames than scenario situations built from scratch. It saves a lot of time, but I made a particular mistake this time.

Taking that into account, I checked and yeah, I'm positively sure that negative ECON (of -3) doesn't confer a commerce rating hit. You tested -2, so I think we can call it a day. 

[Yitzi]

Yeah, you're right, I prefer to tinker with actual savegames than scenario situations built from scratch. It saves a lot of time

Not that much time for simple tests (it can generally be done within 5 minutes or so IIRC), if you remember the following shortcuts:
-shift-F2 to learn a tech, ctrl-F2 to modify techs known.
-shift-F1 to add a unit.
-When selecting a terrain change, ctrl-clicking on the same space more than once will (unless resource bonus is selected) apply it to ever-increasing squares.  Same goes for ctrl-right-clicking to remove something.