Note, that incomes are mostly gained from taxes. Their level can be set for the whole country in the special tab, called Tax Tab.
On the picture below taxes are at the maximum level. As you can see there are separate bars for middle class and lower class citizens.
Taxes rates can result in many things. If you will set them on low level then your incomes will be smaller but citizens would be more happy and much wealthier.
When you will set taxes rates as it is shown on the picture above, it will result in <br>social dissatisfaction and may lead to riots. The death rate in the lower class will be very high and the population will be divided. Instead of this your incomes will be very high. The most important thing is to find a golden mean.
After you will set the tax rate, look at the map located above. Green color means that the people are happy, orange that they are disappointed. Red color is an alert: your citizens are unhappy and there is a high risk of riots, revolt, etc.
Happiness level in every city can be checked by looking in the detailed information tab of each city. Note that lower class citizens are often more unhappy that the middle class. That's why you must be extremely careful when you are setting tax rates for most poor people.
Don't forget that tax rates can be always automatically set by the computer.