A new study shows that children of mothers who drank as little as 1 to 6 units of alcohol per week had children whose IQs at age 8 were statistically significantly lower than those of children who mothers abstained.
This finding was widely reported by news media yesterday. Google “IQ alcohol” and you will find many articles which uncritically describe the findings of the study. As is the case in many such articles, quotes from the press release accompanying the paper’s publication appear to have been liberally used.
The major problem with this study is one that I’ve written about before. Results that are statistically significant may not necessarily be clinically significant.
The difference in IQ between the two groups of children was 1.8 points. Do you really think that an IQ difference of 1.8 is going to be a life-altering finding for a child? I don’t.
IQ tests are often unreliable and if taken more than once can yield different results. A difference of 1.8 points is well within the margin of error of such tests. One source I found states that the margin of error of the IQ test used in this study is a minimum of 5 points. The children in this study were 8 years old when they took the test.
The study, done by a group in Bristol, England, was published on PLoS One and the full text is available here. Caution is advised if you plan to read it. There are many other problems to consider.
Data collected for this paper were pulled from another study which was done from 1992 to 2000 about the genetics of alcohol metabolism. The primary focus of the original study was not the hypothesis of the IQ study.
A unit of alcohol was defined as 8 grams by the authors. Since I was not familiar with what 8 grams of alcohol really meant, I looked it up. A “standard drink” was said to contain anywhere from 8 (in the UK) to 14 grams (in the US) of alcohol.
Here’s a quote from the paper’s “Methods” section, “At approximately 18 and 32 weeks of pregnancy women were also asked on how days during the past month they had drank [sic] 2 pints of beer (or the equivalent amount of alcohol), any women who reported doing this on at least one occasion was classified as a binge drinker in our analysis of the association between genotype and binge drinking.”
Really? Two pints of beer on one occasion makes a woman a binge drinker? Depending on the type of beer, a pint contains 2 to 3 units or 16 to 24 grams of alcohol.
To help you understand the paper better, here is a table:
I asked my wife what she thought of this study and she said, “I think the women who drank were probably more fun to be with.”