Social psychology 2nd ed. A history of modern psychology 5th ed. Once we're born, we start changing the world, a fact that is not always appreciated by our parents, but still one of the aspects of being human. As we grow, the way we change the world, and the world itself, shapes us. We learn and grow by receiving information from the environment, and also by modifying the information we receive and send it back.
Psychologists can only measure that which varies, hence, only variance can be detected.
Since e.g. social temperament varies among individuals, it is possible to find out how much of a trait is determined by genetic variance and how much is determined by environmental variance. A common misunderstanding is that it is possible to alter the environment but not the genetic contribution, therefore, heredity is a measure of how much it is possible to alter a trait. The variance of a trait is an aritmetic sum of the variance of the environmental and genetic variance where it is measured. Introduce any change at all that affects the outcome of the trait, and the heritability will change. If one changes the environment, then one will change the heritability if the nature of the changes have the capability to affect the result.