1 Variation is Everywhere

Almost everything that can be measured or counted in humans shows variation around the mean value for the population. Figure 1.1

Figure 1.1: Variability in self reported weight in a sample of US twins.

shows the pattern of variation for self-reported weight (lb.) in a U.S. sample. The observation that individuals differ is almost universal and covers the entire spectrum of measurable traits, whether they be physical such as stature or weight, physiological such as heart rate and blood pressure, or psychological such as personality, abilities, mental health, or attitudes. The methods we shall describe are concerned with explaining why these differences occur.