For one, twin studies suggest there is a large genetic component to happiness ratings, suggesting our level of happiness is at least partly pre-wired. In other words, the happiness ratings of identical twins, who share 100 percent of their genes, are more similar than those of fraternal twins, who share only 50 percent of their genes. Secondly, external conditions often have little to do with happiness levels. For example, demographic characteristics, such as age, gender, and income, have been shown to have weak correlations with life satisfaction. Even physical attractiveness has little to do with happiness. Moreover, research on people suffering negative life events, such as the loss of a spouse or a disabling accident, pointed to significant recovery in life satisfaction over time despite an initial drop in happiness ratings. Likewise, a famous 1978 study of lottery winners by Brickman and colleagues showed little difference in happiness ratings between lottery winners and a comparison group.