After conducting fresh intelligence tests on the surviving participants, the researchers tested the DNA samples they had collected 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 — how well intelligence had been maintained over time.
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 ageing.
The team found that these variants accounted for nearly one-quarter of the differences in cognitive stability. The results are published online today in Nature (abstract http://dx.doi.org/10.1038/nature10781) .