 The second claim, regarding adaptation and set points, reflects well-known findings that many major life events, like being disabled in an accident or winning the lottery, appear strongly to impact happiness only for a relatively brief period, after which individuals may return to a level of happiness not very different from  before.[17]  As well, twin studies have found that subjective well-being is substantially heritable, with .50 being a commonly accepted figure. Consequently many researchers have posited that each individual has a characteristic “set point” level of happiness, toward which he tends to gravitate over time. Such claims have caused some consternation over whether the pursuit and promotion of happiness are largely futile enterprises (Lykken and Tellegen 1996; Millgram 2000). However, the dominant view now seems to be that the early claims about extreme adaptation and set points were exaggerated: while adaptation is a very real phenomenon, many factors—including disability—can have substantial, and lasting, effects on how happy people  are.[18]  This point was already apparent from the literature on correlates and causes of happiness, discussed below: if things like relationships and engaging work are important for happiness, then happiness is probably not simply a matter of personality or temperament. As well, the large cross-national differences in measured happiness are unlikely to be entirely an artifact of personality variables. Note that even highly heritable traits can be strongly susceptible to improvement. Better living conditions have raised the stature of men in the Netherlands by eight inches—going from short (five foot four) to tall (over six feet)—in the last 150 years (Fogel 2005). Yet height is considered much more heritable than happiness, with typical heritability estimates ranging from .60 to over .90 (e.g., Silventoinen, Sammalisto et al.  2003).[19]