Heritability is a statistic that describes the degree to which genetic differences between individuals cause differences in a property, such as shyness or height (Plomin, DeFries, McClearn, & McGuffin, 2001; Larsen & Buss, 2005).
The proportion of phenotypic variance that is attributable to genotypic variance. Phenotypic variance refers to individual differences; genotypic variance refers to individual differences in the total collection of genes possessed by each person.
Heritability can be expressed in numbers. Its numerical value range from 0.0 (genes do not contribute at all to phenotypic individual differences) to 1.0 (genes are the only reason for individual differences). For human behavior, almost all estimates of heritability are in the moderate range of .30 to .60. A heritability of .50 means that 50 percent of the observed phenotypic variation is attributable to genotypic variation. The larger the heritability, the lower the environmentality.
The most common misconception about heritability is that the concept can be applied to a single individual. It can’t. A heritability of .32 informs us that, on average, 32% of the individual differences that we observe in, say, shyness may in some way be attributable to genetic individual difference. It does NOT mean that 32% of any person's shyness is due to his/her genes and the other 68% is due to his/her environment. It is meaningful to say that individual differences in height are 91 percent heritable, but it makes no sense to say that a specific individual’s height is 91 percent heritable. It is like saying that the first 61 inches of her height are due to genes, and the other 7 inches are due to environment. For an individual, genes and environment are inextricably intertwined. Both play a role in determining height and they cannot be separated. Thus, heritability refers only to differences in a sample or population.
Larsen, R. J. & Buss, D. M. (2005). Personality Psychology: Domains of Knowledge about Human Nature. New York: McGraw-Hill.