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Genetic Influence, Temperament, and Twin Study Insights
Psychology of Personality · Chapter 6: Biological and Evolutionary FoundationsSection 6.2 · lesson 36 of 8310 min · free with a free account
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In this lesson
What a heritability estimate says, and the four things it does not.
- State what heritability is a statistic about
- Explain why temperament supports biological influence
- Describe the logic that lets twin studies separate genes from environment
- Distinguish shared from non-shared environment
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Everything said in the video, section by section (1,396 words). Read along, or search it.
Biological and Evolutionary Foundations
Genes, Temperament and Twins
You will see a sentence like this constantly. Personality is about fifty per cent heritable. Almost everyone who reads it draws the wrong conclusion. They hear that half of who they are was fixed at conception. That is not what the number means, and it is not close to what it means. So this section is as much about interpretation as about findings. We will state what heritability is a statistic about. Explain why temperament supports biological influence on personality. Work through the logic that lets twin studies separate genes from environment. And distinguish shared from non-shared environment, which turns out to be the surprising part.
The Estimate
Forty to Sixty Per Cent, of What Exactly
Behavioural genetics examines how genetic and environmental differences contribute to variation in personality. Two words in that sentence are doing all the work. Differences. And variation. The field studies why people differ from each other. Not what makes up any one person. The central finding is that genes account for a substantial share of those differences. In broad traits, that share is typically estimated at around forty to sixty per cent. Now hold that number carefully. It describes a spread of scores across a population. It is not a claim that forty to sixty per cent of your personality is genetic. That second quantity is not something this method measures at all. It is not a smaller version of the same claim. It is a different kind of claim.
The Route
There Is No Gene for Being Sociable
It also helps to know that genes do not code for traits directly. There is no gene for being sociable, waiting to be found. The influence runs through a chain, and every link loosens it. Genes influence the brain and body systems. Those systems shape temperament, meaning early differences in reactivity and self-regulation. And temperament predisposes characteristic patterns of thinking, feeling and behaving. Predisposes is the operative word in that chain. Genetic influence operates probabilistically. It raises or lowers the likelihood of a disposition. It does not fix one in place. Every step of that route is a place where something else can intervene.
Read It Right
Four Things Heritability Is Not
This is the slide to remember, because heritability is misread more often than any other statistic in psychology. Heritability is a population statistic. It describes the proportion of variation attributable to genetic differences. Within a particular group, and within a particular environment. So here are four things it is not. First, it is not a fixed property of a trait. Change the group, or change the environment, and the estimate changes with it. Second, it says nothing about any single individual. Third, high heritability does not mean a trait is unchangeable. And fourth, it does not imply that environment is unimportant. In fact the arithmetic says the opposite. If genes account for forty to sixty per cent of the variation, the remainder is not a rounding error. That remaining variance points to powerful environmental contributions.
Many, Small
Polygenic Means No Simple Prediction
There is one more reason simple genetic claims about personality keep failing. Modern research increasingly recognizes that personality is polygenic. Meaning many genes of small effect, rather than a few major genes. That architecture explains two things at once. It explains why traits are continuously distributed. Many small contributions sum into a smooth spread, rather than into distinct types. And it explains why simple gene-to-trait predictions remain difficult. No single variant carries enough weight to predict anything much about a person. So when you see a headline announcing the gene for some personality trait. It is running directly against how the trait is actually built.
Before Learning
Temperament Arrives Early
Now to temperament, which is where the biological evidence is strongest. Temperament refers to early-appearing, biologically based individual differences. Specifically, differences in reactivity and self-regulation. These are observable in infancy, and show continuity into later life. The dimensions include activity level and emotional intensity. Soothability. And approach or withdrawal to novelty. Together they form an early scaffold, on which later personality is built. And notice why the timing matters so much for the argument. Temperament appears before extensive social learning has had a chance to operate. Which makes it strong evidence for biological contributions to personality. You cannot explain a two-month-old's reactivity by their upbringing.
Real, Not Fixed
A Scaffold, Not a Blueprint
But a scaffold is not a blueprint, and the source is careful here. Temperament does not rigidly determine adult personality. It sets initial tendencies, which then interact with experience. Take the standard example. A child high in behavioural inhibition may be predisposed toward shyness. Yet supportive parenting, and gradual exposure, can moderate that tendency. So the continuity between infant temperament and later traits is real. And it is probabilistic. Both halves of that matter. It is mediated by the environments a child encounters. And, importantly, by the environments a child helps to create.
The Design
Why Comparing Twin Types Works
Twin studies are a powerful tool for separating genetic from environmental influence. The logic rests on a simple biological difference between two kinds of twins. Identical twins, technically monozygotic, share essentially all of their genes. Fraternal twins, dizygotic, share about half. Like ordinary siblings. Now the inference. And notice that it is a comparison, not a direct measurement. If identical twins resemble each other more than fraternal twins do on a trait. That greater similarity points to a genetic contribution. Studies of twins reared apart provide an especially striking version of the test. Because in that case, shared similarity cannot be attributed to a shared home. That design removes the most obvious rival explanation.
Both Findings
The Result Cuts in Two Directions
And what do those studies find? Two things, pulling in different directions. Across many traits, identical twins show markedly higher correlations than fraternal twins. That holds even for identical twins raised in different families. Which supports substantial heritability. The vivid version of this is the reunion studies. Identical twins separated at birth, meeting again as adults. They often share strikingly similar interests, mannerisms, and emotional styles. Despite having grown up in different homes. But here is the finding that gets left out of the retelling. Identical twins are not perfectly alike. That imperfection is not measurement noise. It is a result. It demonstrates that non-shared environment matters. So twin research delivers a balanced message, and both halves belong in it. Personality is meaningfully shaped by genes, and by the unique experiences that distinguish even people with identical DNA.
The Pattern
Similarity Tracks Genetic Overlap
The pattern across relationship types is worth holding in mind as a whole. Identical twins raised together share about one hundred per cent of their genes, and show high trait similarity. Identical twins raised apart also share about one hundred per cent, and show moderately high similarity. Fraternal twins share about fifty per cent, and show moderate similarity. Unrelated siblings share close to none, and show very low similarity. Compare the first two rows before you look at anything else. Same genes. Different homes. And similarity falls from high to moderately high. That drop is the shared environment becoming visible. Then compare the first row with the third. Half the shared genes, and similarity falls further. That difference is the genetic contribution becoming visible. The design works because those two comparisons can be made separately.
The Surprise
What Makes Siblings Different Matters More
Behavioural genetics draws one more distinction, and it produces the chapter's biggest surprise. Shared environment means influences that make family members more alike. Non-shared environment means unique influences that make family members differ. The finding, which the source calls both surprising and robust, is this. For many personality traits, the non-shared environment matters far more than the shared one. Siblings raised in the same household often differ considerably. In part because each one experiences the family, and the wider world, in a unique way. This reshapes how we think about upbringing. Distinct friendships. Chance events. Even different treatment within the very same family. These individualized experiences appear to exert greater influence on personality. Than the broad features that families share.
Consolidate
How to Read Fifty Per Cent Heritable
So return to where we started. What should you do with fifty per cent heritable? Interpret it as a statement about variation in a population. Measured in a particular group, and in a particular environment. It is not a prediction about any individual. And it is not a claim that the trait cannot change. Held that way, heritability estimates are genuinely useful. Because they guard against two opposite errors at the same time. Genetic determinism on one side. Blank-slate assumptions on the other. And the non-shared environment finding is what reconciles the two. It explains how genetic findings can be strong. While siblings raised in the same house remain remarkably different people.
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About Genetic Influence, Temperament, and Twin Study Insights
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