Some people use exercise to unwind, burn off excess energy, or process whatever else is happening in their lives. What do these lucky (and hardworking) people have that others don't? Should people blame anything for all the hours they spend sprawled over the couch besides their own lack of initiative?

It turns out that at least some of our motivation to work out is inherited. Scientist have yet to figure out the complete picture, but evidence from studies in mice and people suggests that our genes influence how much we crave physical activity.

When scientists separated mice into groups based on how much they opted to run on a treadmill, their progeny were shaped by this preference over the next few generations. After 16 generations, the descendants of the more vigorous mice were running almost twice as much every day as the others. The brains of these exercise-addicted mice had enlarged dopamine systems, which are involved in our experience of pleasure. The regions in their brains important for motivation and reward were also larger.

In people, too, genes may contribute to what makes a person work hard or procrastinate. A study of 37,000 twin pairs from seven countries found that genetic effects play an important role in explaining individual differences in exercise behavior.

Other studies have pointed to a link between our level of exercise craving and mutations in one gene that codes for a type of dopamine receptor.

Variations in another gene seem to have a hand in how good you feel while exercising and how much exertion you feel like you're putting in, as well as your feelings of intrinsic motivation and desire to keep on working out.

This doesn't mean that the less motivated among us can't enjoy working out. They might just never get to feel like this: