Migrant studies
- Study design
If people who move take on the rates of their new country, the cause lies mostly in the environment, not in the genes.
What it is
Migrant studies compare the frequency of a disease in people who moved from one country to another with that in people who stayed in the country of origin and in the population of the host country. Migrants keep their genes and their early environment but take on the new environment over time. If their rates move toward those of the host country, and more so in their children, environmental factors weigh more; if they stay as in the country of origin, genes or childhood exposures weigh more.
How it is built
- Age-adjusted rates and the same case definition are needed in all three groups.
- Numerators come from registries (cancer, deaths) and denominators from censuses that record country of birth.
- Results are stratified by age at migration, years of residence and generation.
Worked example
- Age-adjusted rate of a gastric cancer (per 100,000): country of origin 40; host country 8.
- First-generation migrants in the host country: 28. Their children born there: 12.
- Gap between countries: 40 − 8 = 32 points.
- First generation: fell 40 − 28 = 12 points, 12/32 = 37.5% of the gap.
- Second generation: fell 40 − 12 = 28 points, 28/32 = 87.5% of the gap.
- Reading: risk follows the environment, and what happens in childhood (diet, infections) looks decisive.
How to read migrants' rates
| Pattern of the rate in migrants | Likely interpretation |
|---|---|
| Same as the country of origin | Genes or exposures before migrating |
| Intermediate in the first generation, close to the host in the second | Environment, with a critical exposure in childhood |
| Same as the host country in the first generation | Environment acting in adult life |
| Lower than in both countries | Likely healthy migrant effect or under-registration |
Markers
- First generation
- Second generation
- Healthy migrant
When to use it
- Used for diseases with large differences between countries (gastric, colorectal and breast cancer, coronary heart disease).
- They generate hypotheses about environmental factors that are then studied with cohorts or case-control studies.
Common pitfalls
- Comparing crude rates: migrants are usually younger than the population of origin.
- Ignoring that migrating selects healthier people (the healthy migrant effect).
- Forgetting that sick migrants may return home and drop out of the registry.
- Assuming the migrant group is genetically the same as the whole country of origin.
Test yourself
Think of the answer, then tap the question to see it.
Rate 40 in the origin and 8 in the host country; migrants have 28. What share of the gap did they close?
(40 − 28)/(40 − 8) = 12/32 = 37.5%.
What does it suggest when migrants' children reach the host country's rate?
That environmental factors dominate and that exposure in childhood matters.
What is the healthy migrant effect?
People who migrate tend to be healthier than those who stay, which lowers their rates regardless of the new environment.
Why must rates be age-adjusted?
Because migrants usually have a different age structure from both countries.
Elsewhere in DurgiPedia
Connections
Educational content on epidemiological and statistical methods; the examples use made-up numbers to teach the calculation. Not a clinical or legal guide: current Colombian rules are in the official sources.
Sources: Genetics of Gastric Cancer (PDQ®)–Health Professional Version (National Cancer Institute, 2025); Principles of Epidemiology | Lesson 1 - Section 6 (Centers for Disease Control and Prevention, 2012); Age adjustment (National Center for Health Statistics, Centers for Disease Control and Prevention, 2026).
Spotted a mistake?We are always improving for the community. Tell us what is wrong and our team will review it.Report a mistake
Representative model, for educational purposes only.