Happiness

§ If he wants to be successful in Civilization IV, even the most ruthless dictator must give some thought to his people's happiness. A happy citizen is a productive citizen. An unhappy citizen is a useless drag on his City, producing nothing but still eating as much as anyone else.
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Effects of Unhappiness

§ Each citizen in a city has a job to do. Many are employed working the land around the city, and some few may be Specialists, performing various other critical work. When a citizen is "unhappy" he or she refuses to work, thus reducing the city's laborers or specialists by one. This slacker still consumes two food each turn, however, and a city with too many unhappy citizens faces loss of productivity, commerce, a slowdown of the generation of great people, and perhaps Starvation.
§ Rather than allowing a city to grow until it is unhappy, it is often a good idea to use specialists or the "Avoid Growth" governor command to halt a city at the size just before it would become unhappy. When more happiness is added later, the city can then go back to growing again.
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Determining Happiness

§ Each city has its own individual happiness level. There are many factors which can cause unhappiness in a city, with overpopulation and war being two of the most common (see below for a full list). Each source of unhappiness generates one or more "unhappy faces" (see Cities). There are also many factors which produce happiness in a city, with religion and access to certain resources being two of the most important. Each of these generates one or more "happy faces."
§ To determine a city's happiness, compare the number of happy faces to unhappy faces. If a city has as many or more happy faces as it has unhappy ones, all of the citizens in that city are happy. If the city has more unhappy faces than happy ones, a number of citizens equal to the difference will be unhappy. (Thus, if a city has five happy faces and seven unhappy faces, two of the city's citizens will refuse to work.)
§ If a city has at least one unhappy citizen, an unhappy face will appear next to the city's name on the main map.
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Causes of Unhappiness

§ Overpopulation: Every citizen in a city generates one unhappy face. The bigger the population, the more unhappy everybody is.
§ War Weariness: As a war continues, more and more citizens become unhappy about it. This is especially true in cities that you have recently captured from the civilization you're fighting.
§ Cultural Inferiority: A city will become unhappy if it is being "culturally dominated" by a nearby foreign city (see Culture and Borders).
§ Undefended: A city becomes unhappy if you have no military units in that city. The type of unit does not matter, so make sure to have at least one unit in every city.
§ Slavery and the Draft: Citizens are unhappy if you work them to death (see the Slavery civic). They don't much like it either if you forcibly draft them into the military (see the Nationhood civic).
§ Emancipation: Citizens become unhappy if other civilizations have adopted the Emancipation civic and you have not. The more civilizations that have adopted emancipation, the more unhappy become the citizens of the remaining holdouts.
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Causes of Happiness

§ There are plenty of them. Here are a few:
§ Resources: Many Resources produce happiness: Fur and Gold, to name two.
§ Buildings: Temples, Colosseums, and Theatres are some early buildings which produce happiness. The Jail removes some unhappiness caused by war weariness.
§ Wonders: The Globe Theatre removes all unhappiness in the city where it is built. Rock-n-Roll produces the Hit Singles resource, which generates one happiness.
§ Civics: The Police State civic negates all war weariness. Free Speech produces happiness (in combination with Broadcast Towers).
§ Culture Button: After discovering the Drama technology, you can devote a percentage of your civilization's commerce to the Culture Button, which increases both culture and happiness. Certain buildings like Theatres and Colosseums increase the happiness effect from the Culture Button even further.
§ And More: As you play, you will discover many other factors that can increase or decrease a city's happiness.