Imagine one of medicine’s most powerful natural control groups: two people of the same age and sex who began from the same fertilized egg and therefore share almost all inherited DNA. Decades later, one has Parkinson disease and the other does not. Or Alzheimer disease. Or MS. Or ALS.
The recurring question is not proof that genes are unimportant. Often the opposite is true: identical twins are more similar than fraternal twins. The scientific opportunity comes from the remaining discordance — the part of the story that shared DNA does not fully explain.
Quick answer
Why can identical twins develop different diseases?
Because inherited DNA is only one layer of biology. Over time, twins can diverge through somatic mutations, epigenetic regulation, immune history, infections, vascular and metabolic health, hormones, behaviours, treatments, exposures, ageing and stochastic cellular events. The importance of each layer differs by disease.
Key takeaways
- Identical-twin concordance is well below 100% for many complex diseases even when heritability is substantial.
- Heritability and concordance answer population questions; neither is an individual destiny score.
- Identical twins can diverge epigenetically with age, and their cells can also accumulate different postzygotic genetic changes.
- A difference observed between discordant twins may be a cause, a consequence of disease or an unrelated correlate — timing and replication matter.
- The broader value of twin research is not confined to twins: it helps identify mechanisms that may matter for everyone.
Start with the numbers — then resist oversimplifying them
Classic Swedish Alzheimer research reported 67% concordance in monozygotic compared with 22% in dizygotic pairs. A Swedish Parkinson study reported 11% versus 4%. Canadian MS research reported 30.8% versus 4.7%. An ALS twin analysis estimated substantial heritability even though most monozygotic pairs with an affected twin were discordant.
These figures differ because the diseases differ — and because study design, age, ascertainment and statistical models differ. The shared lesson is not one percentage. It is that genetic influence and individual discordance can coexist.
Identical does not mean biologically frozen
Monozygotic twins begin with nearly the same inherited genome, but bodies are dynamic. Cells divide, DNA changes, tissues age and molecular regulation adapts. A landmark molecular study showed that epigenetic patterns can diverge between identical twins over the life course.
Somatic mutations create additional genetic mosaicism after conception. In diseases such as Huntington’s, even the length of a disease-causing repeat can expand differently across tissues during life. “Same DNA” is therefore useful shorthand, not literal lifelong molecular identity.
Immune systems live different lives
Every infection, vaccination, inflammatory event and immune exposure adds history. This is especially relevant for immune-mediated disease. In MS, genetic susceptibility is substantial, yet most identical pairs are discordant and Epstein–Barr virus has emerged as a central causal trigger that is still insufficient on its own.
Two twins can therefore share susceptibility while accumulating different immune trajectories. Co-twin studies allow researchers to compare those trajectories with unusually good control of inherited background.
Vascular, metabolic and behavioural differences can accumulate for decades
Blood pressure, diabetes, smoking, physical activity, hearing, depression, social isolation, body composition and many other factors can diverge between twins. A Swedish Twin Registry study found that a greater number of nine potentially modifiable factors was associated with higher future dementia incidence, with within-pair analyses arguing against an explanation based only on shared familial factors.
That does not prove a simple prevention recipe. It shows that biology after conception matters enough to deserve rigorous study.
Sometimes the missing factor is chance
Biology contains stochastic processes. DNA damage, protein folding, immune-cell expansion, gene expression and cellular survival are not perfectly synchronized between two bodies over eighty years.
Scientists should not use “chance” as an excuse to stop investigating. It is a reminder that even complete knowledge of genes and measured exposures may not produce a perfectly deterministic prediction for an individual.
Nature versus nurture is the wrong match
The old debate asks whether disease comes from genes or environment. Modern biology more often looks like genes × environment × time × stochastic events. Genes can influence responses to exposures; exposures can influence gene regulation; ageing changes both.
Twin studies are valuable because they make that interaction visible. They can reduce genetic noise, identify candidate modifiers and show which apparently simple associations become weaker once familial background is controlled.
TwinPare perspective: twins are the research model; everyone is the beneficiary
If researchers learn why one genetically susceptible twin remains healthy longer than the other, the eventual value is not limited to twins. The mechanisms can inform prevention, biomarkers, treatment targets and better risk models for the wider population.
That is the deeper TwinPare Research idea: the most powerful lesson from identical twins is not that people are copies. It is that starting from almost the same inherited blueprint can reveal how many layers of biology still shape what happens next.
Source notes
The sources have been verified and editorially reviewed for this article. The limitations below show which level of conclusion the sources support.
- [polderman-2015] Meta-analysis of the heritability of human traits based on fifty years of twin studies Tinca J C Polderman et al.. Nature Genetics, 2015. Evidence type: Meta-analysis of twin studies across 17,804 traits Limitation: The often cited average heritability near 49% is an average across many traits and study settings and must never be interpreted as an individual destiny percentage. PubMed DOI
- [fraga-2005] Epigenetic differences arise during the lifetime of monozygotic twins Mario F Fraga et al.. Proceedings of the National Academy of Sciences, 2005. Evidence type: Molecular study of monozygotic twins Limitation: Demonstrated age-related epigenetic divergence between genetically similar twins. It does not establish that one particular lifestyle exposure caused a specific disease. PubMed DOI
- [gatz-1997] Heritability for Alzheimer’s disease: the study of dementia in Swedish twins Margaret Gatz et al.. Journal of Gerontology: Medical Sciences, 1997. Evidence type: Swedish twin study Limitation: Reported Alzheimer concordance of 67% in monozygotic versus 22% in dizygotic pairs, illustrating strong genetic influence without full concordance. PubMed DOI
- [wirdefeldt-2011] Heritability of Parkinson disease in Swedish twins: a longitudinal study Karin Wirdefeldt et al.. Neurobiology of Aging, 2011. Evidence type: Longitudinal Swedish Twin Registry study Limitation: Reported Parkinson concordance of 11% in monozygotic versus 4% in same-sex dizygotic pairs and estimated heritability at 34%. PubMed PMC DOI
- [alchalabi-2010] An estimate of amyotrophic lateral sclerosis heritability using twin data Ammar Al-Chalabi et al.. Journal of Neurology, Neurosurgery & Psychiatry, 2010. Evidence type: Twin meta-analysis Limitation: Estimated sporadic ALS heritability at 61%, while 44 of 49 monozygotic pairs with at least one affected twin were discordant. PubMed PMC DOI
- [sadovnick-1993] A population-based study of multiple sclerosis in twins: update A Dessa Sadovnick et al.. Annals of Neurology, 1993. Evidence type: Population-based Canadian twin study Limitation: Reported MS concordance of 30.8% in monozygotic and 4.7% in same-sex dizygotic pairs, demonstrating strong genetic susceptibility alongside substantial discordance. PubMed DOI
- [friedman-2005] Monozygotic Twins Discordant for Huntington Disease After 7 Years Joseph H Friedman et al.. Archives of Neurology, 2005. Evidence type: Case report of genetically confirmed monozygotic twin sisters with the same HTT CAG repeat length Limitation: One twin with 39 CAG repeats had developed Huntington disease while her genetically confirmed identical co-twin with the same repeat count remained neurologically healthy at least seven years later. Differences in smoking and industrial exposures were hypotheses, not proven explanations. PubMed DOI
- [tomata-2020] Joint impact of common risk factors on incident dementia: A cohort study of the Swedish Twin Registry Yasutake Tomata et al.. Journal of Internal Medicine, 2020. Evidence type: Swedish Twin Registry cohort with discordant-pair analysis Limitation: Nine potentially modifiable factors were jointly associated with incident dementia, and within-pair analyses suggested shared familial factors did not fully explain the association. PubMed DOI
Editorial source review
This section shows how the article's key factual claims are linked to the source.
Phrasings that require caution
- Do not treat discordance as proof that lifestyle caused or prevented a specific disease.
- Heritability, concordance and population risk-factor associations must not be used as individual diagnosis or prognosis.
- The relative roles of somatic change, epigenetics, immune history, behaviour and chance differ greatly between diseases.
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