But it’s the neurological impact of sustained aerobic fitness in young people that is especially compelling. A memorable years-long Swedish study published last year found that, among more than a million 18-year-old boys who joined the army, better fitness was correlated with higher I.Q.’s, even among identical twins. The fitter the twin, the higher his I.Q. The fittest of them were also more likely to go on to lucrative careers than the least fit, rendering them less likely, you would hope, to live in their parents’ basements. No correlation was found between muscular strength and I.Q. scores. There’s no evidence that exercise leads to a higher I.Q., but the researchers suspect that aerobic exercise, not strength training, produces specific growth factors and proteins that stimulate the brain, said Georg Kuhn, a professor at the University of Gothenburg and the senior author of the study.