Heritability, as used professionally in genetics, has a very precise definition. It is that proportion of the observed variation in a particular phenotype, and in a particular study, that can be attributed to the contribution of genotype (inheritance).

The equation for heritability is derived from the equation Phenotype (P, which always equals 1) = Genotype (G) + Environment (E):

h² = Variance(G) / Variance(P)

Estimating heritability is not a simple process, however, since only P can be observed or measured directly. Measuring the genetic and environmental variation requires statistical methods, usually involving a complex technique called variance component estimation. In common with many statistical methods better estimates can be obtained with large volumes of data, but the method works better if data are available on closely related individuals - such as brothers, sisters, parents and children rather than more distantly related individuals.

In some populations it is possible to collect information in a controlled way. For example among farm animals it is easy to arrange for (say) a bull sire to produce offspring from a large number of cows. This allows a degree of experimental control. But such control is impossible among human populations. As a result studies of heritability in humans has often involved searches for identical twins that have been separated early in life and raised in different environments. Such individuals have identical genotypes and so provide the best possible data for separating the effects of genotype and environment.

There can be some apparent paradoxes in the application of this strict definition. For example, the ‘heritability’ of the same phenotype could be near zero in one study, and close to 100% in another if, in one study, a group of army recruits (unrelated to each other) are all given identical training and nutrition, and their muscular strength is later measured, then those differences that are observed after the (identical) training will be largely ‘heritable’ – however, in another study, whose purpose might be to assess the efficacy of various workout regimes, or various nutritional programs, a group of subjects is chosen who match each other as closely as possible in prior physical characteristics, and then some of these are put onto Program A, and others onto Program B. Differences between the groups after being on the programs are then attributed to the differences in the program. They are environmentally-induced differences. They are not ‘heritable’.

In the case of scholastic ability, how well you do in the final school exams depends on both what and how well you were taught, how hard you have studied, how ‘naturally’ smart you are, and of course, a fair bit on luck. How ‘heritable’ it is depends on who is being compared with who, and in which circumstances.

Much the same goes for intelligence tests. However, the conclusions from studies involving intelligence tests often conclude that intelligence is highly heritable, because the tests are contrived so that it is (supposed to be) impossible to study so as to improve performance in them, and also because what ‘heritability’ actually means gets distorted in the re-telling in the popular media.

In human genetics, much use is made of twin studies in the analysis of heritability – monozygous (‘identical’) twins are clones of each other, and have effectively identical genotypes, and similarities between sets of monozygous twins can be compared with those between dizygous (‘fraternal’, or non-identical) twins, who have only a ½ coefficient of relatedness to each other.

Heritability is often misunderstood when presented in the non-scientific media. Heritability only describes how much variation in the phenotype is attributable to variation in genotype and environment, and not how much the genotype and environment as a whole actually effects phenotype, for example, the amount of variation in the genotype for 'number of fingers' in humans is negligible, so it will show a very low heritability.