The genetics of twin studies have a bias showing more heredity than in reality, owing to a statistical artifact. The twin studies for heredity are based on comparing the correlation between 2 identical twins minus that between 2 fraternal ones (assumed to be sharing half their genes). The use of fraternal twins as control is assumed to extract the “environmental” factors. Problem: Correlation is conditional and psychologists think it is unconditional. We show how the math is entirely different. The core error is that genes and environment are not separable and additive.