While playing the game, the happiness level of the citizens must be maintained.

Many different types of foods have been shown to improve our happiness levels.

Researchers have identified other factors that affect happiness levels far more than income does.

For example, happiness levels rise substantially with the number of close friends someone has.

The second group, on the other hand, returned at the end of the day with their happiness levels still high.

These needs can be used as a basis for evaluating the overall happiness level of individuals.

There is a significant correlation between feeling in control of one's own life and happiness levels.

Some studies attribute a large percentage of our happiness level to genetics.

But research shows winner's happiness levels drop and return to normal baseline rates within months to years.

When asked, spouses reported similar happiness levels to each other.