The Impact of Poor Health on IQ

By Maria Perrin
Jan. 8, 2020

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On a chilly, cloudy and windy June 1st in Edinburgh, 1932, almost every 11-year-old child in Scotland was given an IQ test. In all, 87,498 tests were administered, and the over the intervening decades the data has been used to study connections between intelligence and aging, disease, and mortality.

 Lothian Birth Cohort Study

The study of the connection between health and IQ is known as cognitive epidemiology. One of the most striking experiments performed in the field is known as the Lothian Birth Cohort Study, co-founded by Scottish psychologist Ian Deary. In September 1999, Dr. Deary began evaluating the subjects of the decades-old intelligence tests (which were originally used, along with results from similar tests conducted in 1947, to determine if large families cause lowered intelligence—they do not).

What he found was remarkable: those with an IQ 15 points higher than average (IQ=115) were 21% more likely to be alive at age 76 than a person with an average IQ of 100.

Correlation Does not Imply Causation

The “chicken and egg problem.” The question is: does higher intelligence cause people to live longer? Or are people who are predisposed to live longer also predisposed to have higher IQs?

In the first case, it’s possible that those with lower IQs are less educated in healthy habits and therefore are more likely to engage in unhealthy lifestyle choices, such as drug abuse, smoking, drunk driving. Conversely those with higher IQs are more likely to exercise, eat healthily etc. There is evidence for this in the Scottish data—in the 1940s, before the health risks were known, those with higher IQs were as likely to be smokers as the general population. However, once the dangers were widely accepted, those with higher IQs were more likely to stop smoking.

In the second case, it’s possible that the same genetic factors are involved in both IQ variation and healthy/unhealthy lifestyle choices.

Similar “chicken/egg” difficulties have been seen in other studies. For example, those who were breastfed as children are more likely to have higher IQs, possibly as a result of omega-3 fatty acids improving brain structure. However, it could be that mothers who breast-feed are more likely to have higher IQs which they pass on to their children.

The Good News

While the participants in the Lothian Study stayed generally in the same IQ percentile over the course of their lives, Dr. Deary estimates that about 25% of intelligence is genetically predetermined. Those who engaged in healthier behaviors over their lives were more likely to improve their IQ scores as they aged.

This link between health and IQ is yet another compelling reason to engage and educate our communities on the importance of achieving and maintaining a healthy lifestyle.


Post note

In September 2019 almost 300 members of the longest-running study on aging celebrated the 20th anniversary of Dr. Deary’s investigation into their intelligence. By that time the data had produced a staggering 512 peer-reviewed scientific articles.

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