From the Documentation:
The welfare index is taking the lowest value of several other indexes:
The Health index
The Education Index
The Transportation index
The Supply index + 30
The Employment index + 40
The Social Security Index + 10
The Government Salary Index
A last factor in the welfare index is the game level of the player. The higher the game level, the higher the index will be.
The index can fluctuate between 50 (and is some cases even lower), and 130.