Headey (2008) concluded that an internal locus of control and having "positive" personality traits (notably low neuroticism) are the two most significant factors affecting one's subjective well-being. Headey also found that adopting "non-zero sum" goals, those which enrich one's relationships with others and with society as a whole (i.e. family-oriented and altruistic goals), increase the level of subjective well-being. Conversely, attaching importance to zero-sum life goals (career success, wealth, and social status) will have a small but nevertheless statistically significant negative impact on people's overall subjective well-being (even though the size of a household's disposable income does have a small, positive impact on subjective well-being). Duration of one's education seems to have no direct bearing on life satisfaction. And, contradicting set point theory, Headey found no return to homeostasis after sustaining a disability or developing a chronic illness. These disabling events are permanent, and thus according to cognitive model of depression, may contribute to depressive thoughts and increase neuroticism (another factor found by Headey to diminish subjective well-being). Disability appears to be the single most important factor affecting human subjective well-being. The impact of disability on subjective well-being is almost twice as large as that of the second strongest factor affecting life satisfaction—the personality trait of neuroticism.