Identical Twins are more Similar in IQ than Personality

Identical, but Not Quite: Why IQ Marches in Step while Personality Wanders Off

It is a familiar observation: identical twins often present with astonishing similarities, not only in appearance but also in mannerism and speech. Friends and even parents may struggle to tell them apart in the early years.

Yet as they mature, the illusion of interchangeability fades. One twin excels in mathematics, the other in literature; one becomes a public speaker, the other prefers a solitary vocation. What begins in near-perfect alignment gently diverges. But how far does the divergence go, and why?

The Data

Behavioural genetics offers an empirical framework to think about these questions. Here are the observed correlations for identical (monozygotic, MZ) twins reared together:

Trait	MZ Twin Correlation (r)
Height	0.90 – 0.95
IQ (Full-Scale)	0.85 – 0.88
Big Five Personality Traits	0.40 – 0.60

The conclusion is immediate: intelligence is more tightly correlated between identical twins than personality traits. Height, as a proxy for straightforward genetic architecture, sits at the top. Personality, despite its deep integration with brain function, reveals a lower correspondence. This is not liberal ideology, merely observation.

1. Measurement Reliability

IQ tests are among the most robust tools in psychometrics. Instruments such as the WAIS-IV report test–retest reliabilities upwards of 0.95. In contrast, personality inventories, while well-validated, typically hover around 0.80–0.85 for domain scores. This psychometric limitation alone reduces the maximum possible observable correlation.

2. Neurobiological Substrates

General intelligence is strongly associated with the fronto-parietal control network. A broadly connected and globally efficient set of hubs in the brain supports reasoning, problem-solving, and learning. Minor improvements in network integrity can shift the overall performance upwards across all subdomains of IQ.

Personality traits, in contrast, arise from a mosaic of partially independent systems. Dopaminergic tone influences Extraversion and Openness; serotonergic regulation is linked to Neuroticism and Agreeableness. The anatomical correlates are more diffuse and variable. A tweak in one circuit may have little bearing on others. The architecture is simply less centralised.

3. Genetic Additivity versus Interaction

IQ is highly additive in its genetic structure. Genome-wide association studies identify thousands of single nucleotide polymorphisms (SNPs) that contribute small, independent effects. The result is a stable, predictable transmission across generations and high resemblance in MZ twins.

Personality is less cooperative. Dominance effects, epistasis, and gene–environment interactions dominate the landscape. These introduce a degree of stochasticity—twins may inherit the same genes, but the way those genes interact during development can vary.

4. Developmental Contingency and Environmental Divergence

Identical twins share a genome but not a placenta. Even intrauterine environments can differ subtly in nutrient distribution or hormonal exposure. Postnatal environments are even more idiosyncratic: peer groups, teachers, illnesses, and accidents all impose small but cumulative effects on the formation of personality. The “non-shared environment” typically explains 40–50% of variance in personality traits, compared to perhaps 20% in intelligence.

5. The Surface Illusion

In early childhood, identical twins are often treated as interchangeable. The clothing, names, routines—sometimes even the expectations—are aligned. This creates an illusion of profound sameness. However, deeper measurement reveals divergence beneath the symmetry.

Conclusion

From a neurobiological and psychometric perspective, it is no surprise that IQ shows greater concordance between identical twins than personality. Intelligence reflects centralised brain infrastructure and a dominantly additive genetic script. Personality, by contrast, emerges from a web of interacting systems, developmental contingencies, and environmental nuance.

The genome loads the dice for both, but it throws more heavily in the case of IQ. Personality is the shadow cast by chance, history and context over a shared biological template.