Heritability

What makes you, you?

Why are your eyes brown, but your friend's eyes blue? Why are you more prone to anxiety, while your friend is always calm and collected? Scientists quantify how much genetics affect these differences by determining heritability.

• What is heritability?
• How is heritability influential in psychology?
• What is the heritability coefficient?
• Why are twin studies important in showing heritability?

Heritability Definition in Psychology

As you know, the observable characteristics that we possess are called phenotypes. Both our genes and environment interact to determine these characteristics. Genotype refers to the genetic makeup of a person. Within a population, the variations in phenotype can be vast. So how do you measure these differences?

You use heritability.

Here are some examples of phenotypes that often have a high heritability:

• Eye color
• Skin color
• Height
• Dominant hand
• Dimples

Hair texture often has high heritability, freepik.com

What Does Heritability Not Tell Us?

Now that we know what heritability is, let's briefly discuss what heritability is not. Heritability doesn't measure how much of a person's phenotype is influenced by genes and the environment. Heritability only measures the effect of genes on the variations that we observe.

Furthermore, heritability does not specify what genes affect the phenotypic variations. Heritability, for example, does not determine that there is just one gene that affects the difference in IQ in the previous example. Instead, many genotypes are at play in the heritability coefficient.

Twin and Adoption Studies in Heritability

One of the most effective approaches to exploring heritability is twin and adoption studies. When looking at a person's genetic makeup, we know that each parent passes down 50 percent of their genes to their offspring. Therefore, siblings share 50 percent of their genes with each other. However, identical twins receive the same genes from their parents, meaning they share 100 percent of their genetic makeup.

Twin Studies

While identical twins share 100 percent of their genetic makeup, the number of copies of those genes might differ. Also, some placental abnormalities during prenatal development can impact how much nutrients each twin receives, potentially affecting phenotypic presentation. However, identical twin studies allow researchers to "control" heredity. This is has led to many discoveries regarding heritability vs environmental impact.

Identical twins share 100 percent of their genetic makeup, freepik.com

Adoption Studies

Adoption studies offer another ideal situation for researchers to study the impact of genes and environment on characteristic differences. Suppose identical twins were adopted by different families and raised in different environments. In that case, researchers can use these circumstances to identify similarities that remained regardless of the environment, and what differences were created due to the environment. Researchers also might observe correlations between adopted children and their biological parents to understand the heritability of certain traits that might have been passed down.

Heritability Coefficient in Psychology

Heritability is expressed as a proportion, which we often refer to as the heritability coefficient.

As a proportion, the numerical value of the heritability coefficient ranges between 0.0 and 1.0. A heritability coefficient of 0.0 means that genes do not affect the individual phenotypic differences. On the other hand, a heritability coefficient of 1.0 means that the individual variations are solely due to genetic makeup.

Examples of Heritability in Psychology

Heritability in psychology explores a broad range of character traits and behaviors. Let's look specifically at the heritability of Intelligence and personality.

Heritability of Personality

Behavior genetics studies also seek to understand the heritability of personality. Personality is widely broken down into five main traits:

• Extraversion-Introversion (outgoing/reserved; processing the world around us internally or externally)
• Neuroticism (emotional stability)
• Conscientiousness (dependability/responsibility/motivation)
• Agreeableness (cooperation/helpfulness)
• Openness to experience (imagination/artistic sensitivity/curiosity)

Twin studies come into play once again. Many studies have used identical twins to determine the heritability of personality. For example, Johnson (2016) found that personality traits are 40-60 percent heritable.

Heritability of Intelligence

If Intelligence were a 100 percent heritable trait, people with the same genotype would have identical test scores. Twin studies are extremely valuable in this case because they recreate an otherwise hypothetical situation. Many studies have explored the correlation between adopted children and their biological parents to identify the heritability of intelligence. For example, a study done by Petrill (2003) found evidence to support that 50-70 percent of the IQ variation within a group is due to genes.

Other studies have observed the correlation of IQ scores between identical twins who grew up together and those who did not, versus full-siblings who grew up together and those who did not. If environment played a more significant role in intelligence than genes, there would be very little difference in the correlation of IQ scores of twins and siblings who grew up together. However, Bouchard & McGue (1981) found a noticeable drop, more evidence to support the claim that intelligence can be a highly heritable trait.

What Is Low Heritability?

If genes do not heavily determine the variation of a trait, it has low heritability. In this case, environmental factors greatly impact the trait's variability. The proportion of environmental factors that affect the variability of a trait can be determined by subtracting the heritability coefficient from 1.0. If a personality trait's heritability is 40 percent, or .40, then the environmental factors have a 60 percent or .60 effect on the variation of that trait.