Heritability is a statistic used in breeding and genetics works that estimates how much of the genetic diversity of a phenotypic trait in a population is due to genetic differences in that population. Other causes of measured variation in a trait are characterized as environmental factors, including measurement error. In human studies of heritability these are often apportioned into factors from ""shared environment"" and ""non-shared environment"" based on whether they tend to result in persons brought up in the same household more or less similar to persons who were not. Some humans in a population are taller than others; heritability attempts to identify how much genetics play a role in part of the population being taller. Heritability is estimated by comparing individual phenotypic variation among differently related individuals in a population. Heritability is an important concept in quantitative genetics, particularly in selective breeding and behavior genetics (for instance, twin studies), but is less widely used in population genetics.Geoffrey Miller, an evolutionary psychologist, has said, writing about sexual selection and biological fitness, ""The concept of heritability applies only to traits that differ between individuals. If a trait exists in precisely the same form across all individuals, it may be inherited, but it cannot be heritable.""