There are three names - Jeff, his wife and an assistant. But in terms of who programs and writes and comes up it all - all Jeff
The programming component of software is actually normally the smallest part unless it has been coded in an inefficient manner, it's the artwork/graphics and design that take most of the time/space.
While I was complaining about the graphics a little, thats not to say I don't enjoy the games (I have a level 37 character in Geneforge 4 at the moment) and I am happy the graphics are improving slowly overtime. I hope he does a bit of work on the GUI too (I've always thought you should be able to press "g" or "i" a second time to be able to close the inventory screen).