Twin studies have estimated that approximately 20-30% of the variation in human lifespan can be related to genetics, with the rest due to individual behaviors and environmental factors which can be modified. Although over 200 gene variants have been associated with longevity according to a US-Belgian-UK research database of human genetic variants, these explain only a small fraction of the heritability. A 2012 study found that even modest amounts of leisure time and physical exercise can extend life expectancy by as much as 4.5 years.