In the largest twin study to date, researchers in the U.S. looked at the Swedish Twin Registry to see which factors lead to Alzheimer’s disease (AD).

When scientists are trying to determine whether genetic or environmental factors influence disease risk they will often study twins.

All 11 884 twin pairs were aged 65 years and older and included 392 pairs in which 1 or both members had AD.

All individuals were screened for cognitive dysfunction and suspected cases of dementia and their co-twins received complete clinical diagnostic evaluations for AD.

Estimates of heritability, shared environmental influences, and nonshared environmental influences, adjusting for age, were derived from the twin data.

The researchers also looked at how common it was for identical twins to both have Alzheimer’s, or for only one to be affected.

Two-thirds of adults aged 65 years and older with dementia have Alzheimer’s and that number is expected to rise with the growing older adult population.

Identical (monozygotic) twins share all their genes so if a disease does have a strong genetic basis, it is likely to be seen in both or neither.

If only one has a condition, it is likely that environmental factors have a greater influence.

The researchers found that genetic factors appear to determine when a person developed the condition.

It seems it was more common for identical twins to both have Alzheimer’s compared to non-identical, and a statistical analysis of risk rates in the groups gave the researchers their estimate of how significant genetic factors are.

The researchers estimated heritability for Alzheimer’s is high and that the same genetic factors are influential for both men and women.

However, nongenetic risk factors also play an important role and might be the focus for interventions to reduce disease risk or delay disease onset.

The study is published in the Archives of General Psychiatry.