After following over a thousand sets of twins for 10 years, Lykken and Tellegen (1996) concluded that almost 50% of our happiness levels are determined by genetics. Headey and Wearing (1989) suggested that our position on the spectrum of the stable personality traits (neuroticism, extraversion, and openness to experience) accounts for how we experience and perceive life events, and indirectly contributes to our happiness levels. Research on happiness has spanned decades and crossed cultures in order to test the true limits of our hedonic set point.