NationStates uses a variety of national statistics to determine the effects of issues and WA decisions on every nation. Effects of these statistics are reflected (in part) in the third paragraph of the national profile (in the form of alarmingly descriptive manifestations of the new laws), in the daily World Census Rankings and in the "Scales" on the top of the main page.
The statistics begin with the nation creation process (see Spoiler below), and are affected by each subsequent decision. A number of factors come into play with how the statistics change, including some apparently irrelevant factors such as nation name or national animal.
Some of the statistics can be seen via the XML feed, but the majority of them are hidden to keep the game from becoming too predictable. Some players have made educated guesses and produced a variety of calculators, but it should be made clear that these are in fact guesses, and not NationStates official policy.
Nation Creation For Dummies gives a list of probable results from the initial questions and how they set up your nation. Be aware that this can take some of the random fun out of the game.