Back in March, I wrote about a study that was widely touted as showing that catching Covid lowers your IQ. The study found that individuals who’d had symptoms for more than 12 weeks scored about 6 points lower than those who hadn’t caught the virus. Even individuals who’d had an asymptomatic infection, the study found, scored about 2.5 points lower than those who’d never had one.
However, I was sceptical that these findings were explained by Covid having a large negative impact on cognitive ability.
The authors didn’t actually look at change in IQ scores, but rather at the cross-sectional association between Covid infection and IQ scores. Hence their findings could easily be explained by self-selection: people with higher cognitive ability may have been less likely to catch Covid, and may have been less likely to get seriously ill when they did catch it. As evidence, I noted that such people tend to work in professions that had lower risk of exposure to Covid, and tend to be healthier in general.
A new study has confirmed my suspicions: it appears that catching Covid doesn’t lower your IQ after all.
Bas Weerman and colleagues analysed data from the Understanding America Study – an ongoing longitudinal survey with a large sample size that began in 2014. As part of the survey, respondents complete a series of cognitive tests every two years. In March of 2020, they completed a questionnaire about the emerging Covid pandemic, in which they were asked whether they’d ever tested positive for the virus or been diagnosed with the disease by a healthcare professional. Around the same time, they were invited to participate in separate bi-weekly surveys, which also asked about their infection status.
The researchers therefore had data on respondents’ test scores before the pandemic, their infection status during the pandemic, and their test scores after they’d been infected (for those who had). This allowed them to test whether Covid infection was associated with a decline in test scores between the pre- and post-Covid waves of the survey.
Weerman and colleagues began by replicating the finding of the earlier study – that individuals with lower test scores measured before the pandemic were more likely to become infected. They then looked at whether individuals who became infected subsequently scored lower on the cognitive tests, and found they did not. Eight separate measures of cognitive ability were included in the study, and none of them was affected by Covid infection.
Together, these two findings strongly support my interpretation of the earlier study: self-selection is what explains the association between Covid infection and IQ scores. Rather than Covid infection causing lower cognitive ability, it appears that higher cognitive ability caused people not to catch Covid (or to catch the later, milder variants).
All those headlines about Covid making you less intelligent – about even mild infections knocking several points off your IQ – turned out not to be true. Given the media’s general tendency to exaggerate, overhype and sensationalise the threat from Covid, I wouldn’t expect this new study to get nearly as much coverage as the earlier one.
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