Certain personality characteristics such as self-esteem, life satisfaction, and optimism are fundamental components of positive mental health status and well-being. There is consistent evidence that these traits tend to be substantially correlated in individuals. However, no previous studies have investigated the origin of such correlation. This research used the twin method to unravel the genetic and environmental architecture of self-esteem, life satisfaction, and optimism, along with their mutual interplay. The sample was derived from the population-based Italian Twin Register, and included 428 twin pairs, aged 23-24 years. Multivariate genetic modeling showed that genes influencing self-esteem, life satisfaction, and optimism are largely overlapping. Furthermore, results indicated that the environmental components of the traits may overlap only modestly, and suggested that a sizeable amount of variance in the traits may be explained by environmental effects specific to each of them.