"If genetics and DNA sequence were the only driver for biology, then essentially twins should have the same diseases," said Michael Skinner, corresponding author on the new study. "But they don't. So that means there has to be an environmental impact on the twins that is driving the development of disease." The researchers recruited 70 pairs of identical twins. Alongside measuring their body-mass index and surveying their exercise habits, each participant wore a fitness tracker for one week to objectively ascertain their physical activity levels.