Inside the laboratory psychologists use a control group to isolate the effects of specific variables. But many important real world problems can't be captured in the lab. Ageing is a good example: if we want to know what predicts a healthy old age, running experiments is difficult, even if only for the reason that they take a lifetime to get the results. Questions about potentially harmful substances are another good example: if we suspect something may be harmful we can hardly give it to half of a group of volunteer participants. The question of the long-term effects of alcohol consumption on cognitive ability combines both of these difficulties.
If a problem can't be studied in the lab then everything you measure could be affected by other things you can't. So, for example, if you find that older people who drink have a worse memory than older people who don't drink, you don't know if the drinkers had a worse memory to begin with, or if people who from certain social groups are likely to have both a worse memory and drink more. These potential confounds mean that you can't be sure if drinking is having any effect on cognition, even if you find a difference between groups in both how much they drink and their cognitive abilities.
A new study from the University of Edinburgh uses an unique longitudinal dataset – the Lothian Birth Cohort – and an ingenious analysis technique called Mendelian Randomisation to disentangle the causal influence of drinking alcohol on cognitive change in older age.
The Lothian Birth Cohort consists of Scots born in 1936 studied during the Scottish Mental Survey of 1947 and then between 2004 and 2007 when Professor Ian Deary of the University of Edinburgh followed up participants in the original survey. The data these participants provided included an IQ test at around the age of 11 and another at around the age of 70. Importantly, possible confounds on IQ test score such as socioeconomic status and education were also recorded, as well as a measure of alcohol consumption (at the age of 70, 'over the past few months').
With this data you could assess how alcohol affected cognitive change – testing whether those who tended to drink more had suffered a greater drop in their cognitive ability compared to those who drunk less (as some studies have found), or whether they stayed the same or even improved their cognitive function (as some other studies have found).
The neatest part of the study, however, was to look at the influence of genetics on the effects of alcohol. Certain rare genetic variants are known to be related to the body's ability to process alcohol. Individuals with more of these variants are worse at processing alcohol and so, for a given amount of alcohol consumption, will have higher exposure to the potentially damaging effects of alcohol. The analysis, led by Dr Stuart Richie, showed that individuals with a poor ability to process alcohol did indeed suffer a cognitive decline if they drunk more alcohol. But individuals with a good ability to process alcohol actually showed the reverse effect – for these individuals, higher alcohol consumption predicted actual improvements in cognitive ability.
The authors warn that the study is blind to the other, known, detrimental effects of alcohol consumption (such as heart and liver disease) and speculate that the results may be due to an anti-inflamatory effect of alcohol, which – only in those with the right genes – means the cognitive protective effects can outweigh the cognitively harmful ones.
Ritchie SJ, Bates TC, Corley J, McNeill G, Davies G, Liewald DC, Starr JM, & Deary IJ (2014). Alcohol consumption and lifetime change in cognitive ability: a gene × environment interaction study. Age (Dordrecht, Netherlands) PMID: 24652602
Post written for the BPS Research Digest by guest host Tom Stafford, a psychologist from the University of Sheffield who is a regular contributor to the Mind Hacks blog.