These numbers indicate that total variance in regular smoking (σ2 = .248) is composed of 35 percent (A = .087/.248) additive genetic influences, 28 percent (C = .069/.248) shared environmental influences, and 37 percent (E = .092/.248) unique environmental influences. In other words, roughly one-third of the reason that individuals smoke regularly is due to genetic factors, and the remaining two-thirds is due to their social environments.
