Thursday January 19 2012
The results of the study remain uncertain
The Daily Mail has reported that “lifestyle is key to staying alert into old age”, and that genes are not to blame for changes in intelligence as we age. The newspaper adds that our environment is the most important factor controlling changes in intelligence – in other words, we can control it.
This news is based on an unusual long-term study that examined how a group of nearly 2,000 Scottish people varied in intelligence across the course of their lives, and whether genetic and environmental factors could be responsible. To do this, researchers mapped the genetics of elderly participants who had first taken general intelligence tests when they were 11 years old. The participants again had their intelligence tested between the ages of 65 and 79, with researchers looking at how closely their genetics were tied to changes in test performance over their lifetimes. The researchers estimated that 24% of the variation in intelligence from childhood to old age is accounted for by genetics.
This research combined relatively new genetic technology with a long-term study of intelligence. It sought to determine how much of the change in intelligence over a lifetime is due to genetic factors. However, while the results were intriguing they were not statistically significant, therefore we cannot tell whether they occurred due to chance. Further research will be now needed to tell if the association holds true.
Where did the story come from?
The study was carried out by researchers from the Universities of Edinburgh and Aberdeen, along with other institutes throughout the UK and Australia. It was funded by the UK’s Biotechnology and Biological Science Research Council, the Engineering and Physical Sciences Research Council, the Economic and Social Research Council and Medical Research Council. The study was published in the peer-reviewed scientific journal Nature.
The research was generally covered appropriately in the media, with the Daily Mail reporting that the results didn’t reach statistical significance and that the study didn’t examine individual genetic and environmental factors that may account for any changes in intelligence. Coincidentally, several media outlets ran stories reporting on a genetic test that can predict whether a person will live to be 100 years old, but that was a different test and is not reviewed here.
What kind of research was this?
This was a genome-wide association study that looked for links between genetic variation and changes in intelligence among members of a long-running cohort study in Scotland. Such studies attempt to link genetic changes to observable differences across a population. Applying these techniques to lifetime changes in intelligence has not been possible before, as the technology required is relatively new.
This study was exploratory in nature: the researchers did not have pre-specified genes or DNA sections that they were assessing. They also didn’t examine the role of individual genetic or environmental factors on changes in intelligence.
What did the research involve?
Researchers identified 1,940 unrelated, living participants of three long-running cohort studies; the 1936 Aberdeen Birth Cohort, and the 1921 and 1936 Lothian Birth Cohorts. The three studies had administered intelligence tests to participants when they were 11 years old. The current study re-administered these and other intelligence tests between 54 and 68 years later (aged 65-79), and used these two results to calculate how the participants’ intelligence changed over time.
After their second test, participants then provided blood samples for genome mapping, thereby allowing them to see how particular variations in their DNA related to their test performance. Our DNA is essentially a long code made up of sequences of four molecules called nucleotides. Each of the four nucleotides is represented by a single letter. The researchers identified “single nucleotide polymorphisms” (SNPs), or single-letter variations in the participants’ genetic codes, and determined which of these differences occurred more frequently than would be expected. The researchers then created a statistical model to identify associations between genetic variations and changes in intelligence over the lifetime.
Further analyses were conducted to attempt to quantify the extent that environment and specific genetics influenced differences in intelligence over time. A genetic correlation score of 0 would indicate that childhood intelligence and intelligence later in life are governed by separate genes, and that any variation in intelligence over the lifespan is due partly to genetics. On the other hand, a genetic correlation score of 1 would indicate that childhood and late-in-life intelligence are governed by the same genes and that any variation in intelligence over the course of a lifetime is accounted for by environmental factors.
What were the basic results?
Researchers found that there was a significant correlation between intelligence at the age of 11 and intelligence in old age. When controlling for intelligence at age 11, they found that approximately 24% (a score of 0.24) of the lifetime change in general intelligence is caused by genetic variation. This result was not, however, statistically significant.
Analysis on the role of environment and genes in changes in intelligence were not significant, which meant that researchers could not rule out either an entirely genetic or an entirely environmental cause behind the observed change.
How did the researchers interpret the results?
The researchers said that despite the lack of statistical significance, their results indicate that both “genetics and environment could contribute substantially” to an association between intelligence at age 11 and later in life, and that genetic factors could explain the differences in intelligence over a person’s lifetime. They also said that “genetic factors seem to contribute much to the stability of intelligence differences across the majority of the human lifespan”.
This study tried to calculate how strongly genetic variations are associated with changes in intelligence over a lifetime. However, none of the study’s results proved statistically significant and therefore it’s not possible to say for certain where the balance of genetic and environmental influences lies.
Despite the lack of statistically significant results, the researchers said that the strengths of this study are that it had an extremely long follow-up and measured intelligence at two points in time, providing an estimate of cognitive change and stability over time, and providing estimates of the relative roles of genetics and the environment in this change.
The researchers also said that further studies are needed to validate their findings, as their cohort was too small to have sufficient “statistical power” to detect a significant association, despite the fact that approximately 2,000 people participated in the research.
Furthermore, the study did not examine individual genetic variations or specific environmental factors that account for the observed changes in intelligence over time. The researchers said that further studies will need to be conducted in order to determine what factors account for change in intelligence.
Overall, this was an interesting combination of a long-running cohort study and new genetic technologies. However, the lack of statistical significance means that we cannot be certain that the results were not due to chance, and the lack of examination of individual environmental factors limits the relevance of this study to our daily lives and decisions.