Four main factors, it seems: gender, personality, external circumstances and age. Women, by and large, are slightly happier than men. But they are also more susceptible to depression: a fifth to a quarter of women experience depression at some point in their lives, compared with around a tenth of men. Which suggests either that women are more likely to experience more extreme emotions, or that a few women are more miserable than men, while most are more cheerful.
Two personality traits shine through the complexity of economists' regression analyses: neuroticism and extroversion. Neurotic people—those who are prone to guilt, anger and anxiety—tend to be unhappy. This is more than a tautological observation about people's mood when asked about their feelings by pollsters or economists. Studies following people over many years have shown that neuroticism is a stable personality trait and a good predictor of levels of happiness. Neurotic people are not just prone to negative feelings: they also tend to have low emotional intelligence, which makes them bad at forming or managing relationships, and that in turn makes them unhappy.