People often want to know the extent to which a trait is genetic versus environmentally determined (e.g., "nature" versus "nurture"). This distinction is not nearly as clear cut as is usually assumed. Let's consider the obvious example of height in a population, a trait that's well known to be purely hereditary. Many causes of a population's height distribution are not hereditary. For example, a population's height is also determined by economic factors, like whether there was malnourishment...