[yaddayadda]

The authors did acknowledge the difficulty of finding twins with different fitness levels:

>The researchers were looking for young adult identical twins in their early- to mid-20s whose exercise habits had substantially diverged after they had left their childhood homes. These twins were not easy to find. Most of the pairs had maintained remarkably similar exercise routines, despite living apart.

And they also highlight that while the genetics and environment play a huge part they don't preclude change:

>More subtly, the findings also point out that genetics and environment “do not have to be” destiny when it comes to exercise habits, Dr. Kujala said. For these particular twins, whether their genes and childhoods nudged them toward exercising regularly or slumping on the couch, one of the pair overcame that legacy and did the opposite (for better and worse).