A Scottish study of intelligence that began 80 years ago has now found that genetic variants accounted for nearly one-quarter of the differences in cognitive stability — how well intelligence is maintained over time — of the study’s surviving participants.

Research psychologist Ian Deary of the University of Edinburgh and colleagues tracked down the original participants, collected DNA samples, and performed fresh intelligence tests in nearly 2,000 of the original participants, then aged 65 or older.

Deary and his team tested the DNA samples for the presence of more than half a million common genetic variants, each affecting only a single letter in the DNA sequence of the genome. The team then calculated whether these variants were associated with cognitive stability.

The sample size of 2,000 people was too small to grant the statistical power needed to track down individual genetic signatures associated with cognitive stability. But it was enough to estimate how much genetics contributes to cognitive aging.

The team found that these variants accounted for nearly one-quarter of the differences in cognitive stability. However, the genetic factors could also be interacting with the environment, making a person more or less susceptible to environmental effects on intelligence, says Yaakov Stern, a neuropsychologist at Columbia University Medical Center in New York, who was not involved in the study.

Ref.: Ian J. Deary et al., Genetic contributions to stability and change in intelligence from childhood to old age, Nature, 2012 [DOI: 10.1038/nature10781]