Why is it flawed to ask how much of a particular behavior is due to genetics and how much is due to experience?
It is nonsensical to ask how much genetics and experience contribute to human behavior because they collectively supply the basis for behavior. In the text, I particularly liked the illustration of the heat and the water. How much does the water or the heat contribute to evaporation? The answer is that rather than contributing to measurable amounts both heat and water work in unison to achieve evaporation. In the same way nature and nurture work in unison to bring about human behavior. Likewise, water could not evaporate without heat and heat is useless, at least as far as evaporation goes, without water. Similarly, genetics without experience would be like having a map without the means to achieve the destination, and experience without genetics would be like having the ability to achieve the destination without the map to show you how to get there. As you can see, nature and nurture interact in non-quantitative ways to produce behavior.
Why is it appropriate to separate the contributions of genetics and experience when measuring the development of differences among individuals?
It is appropriate because when dealing with the development of differences we are not talking about quantifying contributory mechanisms, such as the heat or the water, we are talking about determining to what degree the heat and the water effect the evaporation in different situations. For instance, we could experiment with less water or more water in order to determine how much of an effect the amount of water has on the evaporation. Likewise, we could change the level of heat applied to the water in order to determine the effect on evaporation. In the same way, we could survey a set of identical twins, which have been brought up in different environments, in order to determine to what extent those environments have affected the development of differences in their lives. See, we are not trying to quantify the amount to which nature and nurture affect behavior, but rather how much genetics and experience affect different people in different situations. It is not a question of “how much” as much as “in what way” nature and nurture affect our behavior.
Pinel, J.J. (2007). Basics of biopsychology. Boston, MA: Allyn & Bacon.