Before you read this, however, I recommend you read our piece on the Missing Heritability Problem, which provides important context for interpreting this information. Newer, genome-wide association studies tend to get substantially lower estimates for heritability than the twin study figures listed below, and it is still a debate in the field which approach to measuring heritability is more accurate.

Focusing on twin studies, let’s examine estimated heritabilities across five categories: body, mental health, physical health, cognitive/mind-related traits, and personality (including the Big Five personality traits).

You can find high-resolution versions of these charts here:

• Heritability of body-related traits
• Heritability of mental health-related traits
• Heritability of physical health-related traits
• Heritability of cognitive/mind-related traits
• Heritability of personality-related traits
• Heritability of all traits in one chart

Note that here, we are looking at h^2, which represents the “narrow sense heritability”. It measures the proportion of phenotypic variance in a population that is attributable to additive genetic variance. This measure focuses specifically on the additive effects of alleles, excluding other genetic effects like dominance or epistasis. In particular,

h^2 = (additive genetic variance) / (total phenotypic variance).

All of the h^2 numbers in this article come from (or are derived from) papers (or from combining papers). In most cases, these h^2 numbers were calculated by comparing the similarity of traits between monozygotic / MZ (identical) and dizygotic / DZ (fraternal) twins to isolate the additive genetic component. The intuition for how such studies estimate heritability is that the more similar identical twins are in that trait relative to fraternal twins, the more heritable the trait generally is.

Note that h^2 is different than H^2 (broad sense heritability, sometimes written h_b^2 or h_{b}^2), which measures the proportion of phenotypic variance attributable to all genetic factors, including additive, dominance, and epistatic (gene-gene interaction). In particular,

H^2 = (total genetic variance including additive, dominance, and epistatic variances) / (total phenotypic variance).

H^2 will always be greater than or equal to h^2. The two numbers will be similar in cases where the additive genetic variance constitutes the majority of the total genetic variance.

For traits heavily influenced by dominance or gene-gene interactions, H^2 will be significantly larger than h^2. Examples include traits influenced by a few genes with strong dominance effects or complex traits with significant gene-gene interactions.

Note that h^2 values should be considered rough approximations only. Calculating them can be complex and estimates can vary for a variety of reasons, including:

• Differing populations: sometimes h^2 will differ in different human populations due to differences in genetics or differences in environmental influences
• Equivalent environment assumption: if identical twins and fraternal twins don’t “share their environment” to the same extent, that can lead to issues in calculation. If identical twins share more similar environments than fraternal twins, heritability may be overestimated.
• Gene-environment correlations: if there is a correlation between genetics and the environment experienced by individuals, this can bias estimates.
• Differences in the environment: heritability estimates are merely estimates of the percent of phenotypic variation that is accounted for by genetics in a particular environment. If the environment changes, this number can be expected to change. For instance, if everyone’s environment became more identical, heritability would go up because the environment then can’t account for as much variation.
• Simplifying assumptions: additionally, sometimes additional assumptions are made when estimating heritabilities. For example, Falconer’s formula says that h^2 = 2 (r_MZ – r_DZ) where r_MZ is the correlation coefficient for monozygotic (identical) twins and r_DZ is the correlation coefficient for dizygotic (fraternal) twins. But Falconer’s formula makes some assumptions, for instance, that the correlation in traits between twins is not significantly influenced by shared environmental factors, that the effect of genes on the trait is independent of environmental factors, and that assortative mating doesn’t occur (i.e., parents of the twins are not more similar to each other in terms of the trait than would be expected by chance).

Heritability figures will sometimes change as the environment changes. For instance, in an extreme situation where every person has an identical environment, the only variation is genetic. Hence, the more constrained an environment is the greater the heritability.

A common misconception is that zero heritability would imply that genes are not involved. Suppose, though, that in a specific population, everyone has a gene causing trait X (which the environment then modifies). The heritability of X for that population will be found to be 0, even though the trait is caused by a gene!

Now, let’s look at the heritability of all traits in one bar chart:

And here is a list of heritabilities in table form:

Sources:

It’s common to try to explain things as either due to nature OR nurture.

Or, at best, we say: some percentage of the variation in outcome is due to genes, and some percentage is due to the environment. It’s important to remember, though, that outcomes can be a complex interaction between the two.

Consider this:

Our genetically-influenced traits impact what environments we seek out and find ourselves in.

For instance, risk-taking seems to have moderate heritability, and it influences career and life choices that change our environment.

A risk-taker is more likely to seek out riskier environments. And this choice of environment might itself depend on the opportunity set (i.e., current environment). In one environment, risk-taking might mean a greater likelihood of entrepreneurship; in another, it might mean crypto trading, and in a third, drug dealing.

Genetic propensities and the environment we are in can become intertwined.

Consider depression:

Depression seems to have a heritability in the 20%-60% range.

Some are unusually susceptible to it, others less so. Yet whether someone has a depressive episode or not may depend a lot on what environment they happen to be in.

With meaningful work, supportive friends, a robust exercise routine, and effective self-regulation strategies, even with a high propensity toward depression, you may be able to avoid it. On the other hand, if you’re unemployed, lonely, and without helpful routines, even with a low propensity toward depression, you may still become depressed.

So while it can be useful to think about the heritability of a trait, it is important to remember that heritable traits can influence what environment we end up in and that the interplay between genes, environment, and our habits/skills/strategies are what leads to outcomes.

So while it’s common to think in terms of:

(1) nature OR nurture

…or, at best… 

(2) an additive model: outcome = w * nature + (1-w) * nurture…

…Sometimes, a better model has an interaction term:

outcome = w1 * nature + w2* nurture + w3 * nature * nurture.

This was first written on April 28, 2021, and first appeared on this site on December 9, 2022.