People also assume that happiness is a response to what happens in life, such as getting a great job or finding or losing a spouse. While obviously big events do have an impact, if you study people over time, they usually return to their preexisting level of happiness (or unhappiness) leveling off after joy and rising up after sorrow. This leads us to a core issue: can one’s long-term sense of well-being be significantly increased or are we limited by how far we can rise? Diener (2000) suggests that people have a happiness set point, similar to the phenomenon of a weight set point. Events may shift one’s level of emotional well-being, but other homeostatic forces pull one back over time. Fujita & Diener (2005) examined this with life satisfaction, and about one-fourth of the study participants made significant shifts over 17 years; 10 percent shifted very significantly. Some suggest that happiness might be hard-wired in, but further exploration challenges this notion. Studies of subjective well-being of identical twins show, however, that only about half of the variation of happiness seems to be based on genes (Lykken & Tel-legen, 1996). Longitudinal studies show that over a two-year period, happiness seems to have a stable set point. However, if you widen the timeline and study people over four years, there is far greater variation (Csikszentmihalyi & Hunter, 2003). These research programs on normal populations suggest that over the short run people do have a tendency toward a set point of happiness, but if you study them over time there can be significant shifts. Other authors suggest that since external factors (events, success, etc.) are not the key to understanding variation in a person’s happiness, we should focus instead on internal ones. In other words, it isn’t what happens to people; it’s how they construct and interpret those events (Schwarz & Strack, 1999). In light of the hedonic treadmill, for example, it might not be what you have that matters, but how mindfully you experience it