Personality and Individual Differences
Twin-Study Simulator
Set the truth, generate fictional twin pairs, estimate it back — and then break the assumptions one at a time and watch the estimates go wrong in specific directions.
Simulated — fictional pairs from a seeded generator, no real study
Learning objective
By the end you should be able to say what heritability is a property of, and which way each assumption pushes the estimate when it fails.
About 30 minutes. Nothing you do here is saved or sent anywhere.
The one sentence to take away
Heritability is the proportion of variance in a population attributable to genetic variance in that population under those environmental conditions. It is not the proportion of any individual's trait caused by their genes, and it says nothing about whether a trait can be changed.
First, the definition
A study reports that a trait has a heritability of 0.50.
The simulator
You set the true variance components. The simulator generates 250 pairs in each group from them, and Falconer's formulae estimate them back from the correlations. Real studies only ever see the right-hand column.
Key terms
- Heritability (h²)
- The share of variance in a trait, in one population at one time, associated with genetic variation. It is a property of that population, not of any person in it.
- A, C and E
- Additive genetic, shared environment and non-shared environment: the three components this model divides the variance into.
- Equal environments assumption
- That identical and fraternal twins share their environments to the same degree. The slider here lets you break it.
- Assortative mating
- Partners resembling each other on the trait, which makes fraternal twins more alike than the model expects them to be.
Fictional twin and sibling pairs
250 pairs per group, generated from a seed. No real family, study or dataset is involved.
Pair resemblance
| Group | Pairs | Correlation |
|---|
Truth against estimate
| Component | Truth | Estimated | Error |
|---|
What the violations are doing
Challenge — diagnose the inflation
What this demonstrates
A heritability estimate is a ratio, and a ratio has a denominator: it describes how much of the variation in this population, as sampled, on this measure tracks genetic variation. Change the range of environments and the same genes give a different figure. It is not a statement about any individual, and a high value does not mean a trait is fixed.
For teaching elsewhere: take this activity as one self-contained block of HTML, on the clipboard or as a file. Either way it is styled so that it will not disturb the page you put it into.