For its 2018 World Happiness Report, the Sustainable Development Solutions Network (SDSN) for the United Nations studied 156 countries and considered six variables: GDP per capita, healthy life expectancy, social support, perceived freedom to make life choices, perceptions of corruption and generosity. These were rated on a scale from 1 (worst) to 10 (best). We count down the 20 unhappiest and the 20 happiest nations.