Though an industrious spirit has driven mankind since the Fall, scientists see great variance among people in the way of old-fashioned “get-up-and-go.” Whereas some Americans are Tough Mudders, the great majority of others tend to live less active lives — both physically and mentally.

Recent research on rats by investigators at the University of Missouri finds that an inclination toward physical activity may be at least partially genetic. And now new findings from that team suggest a correlation between laziness and a less mature brain. "What we're finding is that physical activity is linked to the maturing of neural networks in the brain," study leader Frank Booth told The Huffington Post this week.

He and his colleagues offer two possible explanations for the association, both of which appear equally plausible. Either physical activity causes the brain to grow in some ways, or specific genetic combinations encourage both physical activity and the greater maturation of some brain areas. In the study, investigators separated a group of laboratory rats into two groups of 26, which included all those identified as either the best or worst of runners. After 10 generations of breeding, the two rat groups had developed distinct neurological differences, Booth and his colleagues found.

The brains of the more active rats — as opposed to the “couch potatoes” — contained more mature neuron cells, indicating a quicker development of neural pathways throughout the brain. The study builds on recent genetic work suggesting that people may be genetically predisposed to anxiety, obesity, a tendency to procrastinate, and even a taste for dietary fat.

Yet the complex interplay between nature and the environment suggests a fair amount of opportunity for redirecting aspects of our lives, including the will to work out. In the study, rats bred toward laziness for 10 generations were nevertheless able to gain — or regain — greater levels of physical activity when given an exercise wheel early in life.

Eerily, the rat study seems not terribly different from human populations today. "Most complex behaviors are never determined 100 percent by genetics or 100 percent by environment," Booth said. "How the brain is used may make pathways in the brain that might be more fixed for life."

The researchers say they hope the findings encourage more adults to lead healthier, more active lifestyles.

Source: Booth, Frank, Laye, M.J. Lack of adequate appreciation of physical exercise's complexitites can pre-empt appropriate design and interpretation in scientific discovery. Journal of Physiology. 2014.