Concordance Rate of Schizophrenia in Identical Twins
The concordance rate for schizophrenia in monozygotic (identical) twins is approximately 33%, with heritability estimated at 79%. 1, 2
Key Epidemiological Data
The most robust and recent evidence from a 2018 nationwide Danish twin study provides the definitive answer:
- Probandwise concordance rate in monozygotic twins: 33%
- Probandwise concordance rate in dizygotic twins: 7%
- Heritability estimate: 79% 2
This data comes from 31,524 twin pairs born between 1951-2000, followed until 2011, using advanced statistical methodology that accounts for censoring—making it the highest quality evidence available.
Understanding What This Means
The 33% concordance rate demonstrates that genetics alone do not determine schizophrenia risk. Despite identical twins sharing 100% of their DNA, two-thirds of monozygotic co-twins do not develop schizophrenia even when their twin is affected. This highlights the critical role of environmental factors 1.
Factors That Modify Concordance Rates
The concordance rate varies based on specific risk factors 3:
- When a parent has psychotic disorder: Concordance increases to 62.5% (versus 38.6% when no parent affected)
- Earlier age of onset (<23 years): Concordance of 54.3%
- Later age of onset (>30 years): Concordance drops to 29.5%
These variations indicate that familial liability markers can identify twins at particularly high risk.
Historical Context and Range
Earlier studies reported concordance rates ranging from 30.9% to 48% in monozygotic twins 4, 5, with the variation largely due to methodological differences in ascertainment, diagnostic criteria (DSM-III-R versus DSM-IV), and statistical approaches. The 2018 Danish study's 33% figure represents the most methodologically sound estimate using modern diagnostic criteria and accounting for follow-up censoring.
Clinical Implications
For genetic counseling: When one identical twin has schizophrenia, the unaffected co-twin has approximately a 1 in 3 lifetime risk of developing the disorder—substantially higher than the general population risk of 0.6-0.7% 1, 6, but far from deterministic.
The high heritability (79%) combined with modest concordance (33%) indicates that while genetic factors create substantial vulnerability, environmental triggers, prenatal factors, and epigenetic mechanisms play essential roles in disease manifestation 1.