Bibliography (9):

1. Physical activity, fitness, glucose homeostasis, and brain morphology in twins

2. A longitudinal study on genetic and environmental influences on leisure time physical activity in the Finnish Twin Cohort

3. Physical activity in adulthood: genes and mortality

4. Lifespan and Skeletal Muscle Properties: The Effects of Genetic Background, Physical Activity and Aging

5. Genetic Influences on Exercise Participation in 37.051 Twin Pairs from Seven Countries

6. Evidence for shared genetics between physical activity, sedentary behavior and adiposity-related traits

7. Unique Environmental Effects on Physical Activity Participation: A Twin Study

8. Causal Inference and Observational Research: The Utility of Twins

9. Wikipedia Bibliography:

   1. Natural experiment