Heritability Estimates

What Heritability Means

Heritability, in quantitative genetics, refers to the proportion of observed variance in a trait among individuals in a population that is due to genetic differences. It is typically expressed as a coefficient, ranging from 0 to 1. A heritability of 0 indicates that all phenotypic variation in the population is due to environmental factors, while a heritability of 1 indicates that all phenotypic variation is due to genetic factors. It is crucial to distinguish between two main types: broad-sense heritability (H²) and narrow-sense heritability (h²).

Broad-sense heritability (H²) includes all genetic effects, such as additive genetic effects, dominance effects (interactions between alleles at the same locus), and epistatic effects (interactions between alleles at different loci). Narrow-sense heritability (h²), which is more commonly reported in evolutionary and behavioral genetics, specifically refers to the proportion of phenotypic variance due to additive genetic effects. Additive genetic variance is particularly important because it is the component that responds predictably to natural selection, as it represents the average effect of individual alleles passed from parents to offspring.

Heritability is a population-specific statistic, not an individual one. It does not indicate the degree to which a trait in a particular individual is caused by genes or environment. For any individual, both genes and environment are 100% necessary for the development of any trait. For example, if the heritability of height is 0.8, it does not mean that 80% of an individual's height is due to their genes. Instead, it means that 80% of the variation in height observed across the population is attributable to genetic differences among those individuals.

Methods of Estimation

Heritability is estimated by comparing the phenotypic similarity of individuals with known degrees of genetic relatedness. Classic methods include twin studies and adoption studies.

Twin studies compare monozygotic (MZ, identical) twins, who share nearly 100% of their segregating genes, with dizygotic (DZ, fraternal) twins, who share, on average, 50% of their segregating genes (like other full siblings). If MZ twins are more phenotypically similar for a trait than DZ twins, this difference in similarity can be attributed to genetic factors, assuming the equal environments assumption (EEA)—that MZ and DZ twins experience equally similar environments relevant to the trait. This assumption has been a point of contention, with critics like Joseph Rodgers and David Rowe examining its validity across various traits. However, meta-analyses often support the EEA for many traits, finding that differential treatment of MZ twins is often a response to their greater similarity, rather than a cause of it.

Adoption studies compare adopted children to their biological parents (who share genes but not environment) and their adoptive parents (who share environment but not genes). If adopted children resemble their biological parents more than their adoptive parents, this suggests a genetic influence. Combining twin and adoption designs (e.g., studies of twins reared apart) offers powerful insights by separating genetic and environmental influences more cleanly.

More recently, genomic methods, such as Genome-Wide Complex Trait Analysis (GCTA) and its variants, estimate heritability by analyzing genetic similarities across large samples of unrelated individuals using single nucleotide polymorphism (SNP) data. These methods estimate the proportion of phenotypic variance explained by common genetic variants, often yielding lower heritability estimates than twin studies, a discrepancy sometimes referred to as the “missing heritability” problem. This gap may be due to rare variants, structural variants, or epistatic interactions not captured by common SNPs, or to limitations in the statistical models themselves.

Gene-Environment Interplay and Misinterpretations

Heritability estimates are often misunderstood as fixed or immutable properties of a trait. This is incorrect. Heritability is specific to a particular population at a particular time and in a particular range of environments. If the environmental variance changes, or if the genetic variance changes (e.g., due to migration or selective breeding), heritability will change. For instance, the heritability of height in a population might decrease if nutritional disparities are reduced, allowing more individuals to reach their genetic potential.

Furthermore, heritability does not imply genetic determinism. Even highly heritable traits can be modified by environmental interventions. For example, phenylketonuria (PKU) is a genetic disorder with 100% heritability in an untreated population; however, dietary intervention can largely prevent its severe symptoms. This illustrates that heritability speaks to differences within a population, not the potential for modification.

Gene-environment interaction (GxE) and gene-environment correlation (rGE) are critical concepts that complicate the interpretation of heritability. GxE occurs when the effect of a genotype on a phenotype depends on the environment, or vice versa. For example, a particular genotype might increase risk for depression only in stressful environments. rGE describes situations where individuals' genetic predispositions influence the environments they experience. This can take three forms: passive (parents provide both genes and environment), evocative (an individual's genetic traits evoke responses from the environment), and active (individuals select environments compatible with their genetic predispositions). These interactions mean that genetic and environmental influences are not always independent and can be difficult to disentangle fully, often leading to a portion of the variance being attributed to one factor when it is, in fact, a product of their interplay.

Critics like Evan Balaban and Jonathan Marks emphasize that simplistic interpretations of heritability can obscure the complex developmental processes through which genes and environments jointly construct phenotypes. They argue that focusing solely on variance partitioning can distract from understanding the causal pathways involved in trait development.

Implications for Evolutionary Psychology

For evolutionary psychology, heritability is a necessary but not sufficient condition for a trait to evolve by natural selection. If there is no additive genetic variance for a trait (i.e., h² = 0), then selection cannot act on that trait directly. However, many traits central to evolutionary psychology, such as aspects of personality, cognitive abilities, and even susceptibility to certain psychological disorders, show moderate to high heritability in human populations (e.g., Bouchard & McGue, 2003; Plomin et al., 2016). This indicates that genetic variation contributes to individual differences in these traits, making them potential targets for evolutionary processes.

High heritability for a trait does not, however, mean that the trait itself is an adaptation. Many traits with high heritability, such as individual differences in disease susceptibility, are not adaptations but rather reflect standing genetic variation or pleiotropic effects of genes selected for other functions. Conversely, a trait that is a universal human adaptation, and thus shows little or no variation within the species, might have a heritability close to zero (e.g., the presence of a nose), because there are no genetic differences to explain variation. In such cases, the trait's evolutionary significance is not captured by heritability estimates.

Understanding heritability estimates requires careful attention to their statistical nature, their population specificity, and the complex interplay between genetic and environmental factors. They provide a valuable tool for quantifying the genetic contribution to phenotypic variation but must be interpreted within a nuanced framework that acknowledges gene-environment interactions and the limitations of variance partitioning. The ongoing challenge is to move beyond simply estimating heritability to identifying the specific genes and environmental factors involved, and understanding their developmental pathways. This shift is increasingly facilitated by molecular genetic techniques that can pinpoint specific genetic variants and their functional consequences. While heritability estimates remain foundational, the field is progressing towards a more mechanistic understanding of gene-environment systems. T. D. Spector's work on twin studies and epigenetics further highlights the dynamic nature of these interactions, showing how environmental factors can modify gene expression without altering the underlying DNA sequence, thus influencing trait variation in ways not always captured by traditional heritability models.