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In a post titled Why Vegetarians Are More Intelligent than Meat Eaters, evolutionary psychologist Satoshi Kanazawa claims:

Among the British respondents in the National Child Development Study, those who are vegetarian at age 42 have significantly higher childhood general intelligence than those who are not vegetarian at age 42. (Childhood general intelligence was measured with 11 different cognitive tests at three ages before 16.) Vegetarians have the mean childhood IQ of 109.1 whereas meat eaters have the mean childhood IQ of 100.9. The difference is large and highly statistically significant.

The Evening Standard discusses a similar study.

I am skeptical of this claim, so I searched for scientific data investigating this issue, but I couldn't find any solid scientific study conducted over long period or with bigger sample. The most articles I encountered were just speculations made people to support their own claim.

I found the following related research while searching:

  1. IQ in childhood and vegetarianism in adulthood: 1970 British cohort study

  2. Schooling, educational achievement, and cognitive functioning among young Guatemalan adults.

  3. Malnutrition can affect development of brain in early stages.

  4. The Hindu-CNN-IBN State of the Nation Survey 2006 : India

Is vegetarianism correlated to intelligence (as measured by IQ or similar methods)?

CrownedEagle
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    I wonder. If India has 40% vegetarians, is it the smartest country in the world? – GEdgar May 08 '16 at 20:36
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    @GEdgar- In Britain, where the study was conducted, vegetarianism is (I think) mostly for moral reasons. In India it's for religious reasons. – PointlessSpike May 09 '16 at 07:30
  • @PointlessSpike - Exactly my point.. for Religious Region!! That's why I posted the question. Moral reasons can be justified to some extent but with religious reasons it's a unquestioning following which after few generation become Gospel Truth. – CrownedEagle May 09 '16 at 09:06
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    @CrownedEagle- It's also worth mentioning that even moral reasons can become a sort of tradition. Vegetarianism might be taken up for social reasons among those considered to be more intelligent. It might not say anything at all even if you're correct. – PointlessSpike May 09 '16 at 09:20
  • *for Religious Reasons instead of Religious Region - Typo in my above comment. – CrownedEagle May 09 '16 at 09:26
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    After reading the article I'm sure you got it backwards. The question is "Are intelligent people more likely to go vegetarian?", not the other way around. – Agent_L May 09 '16 at 09:55
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    The question whether Intelligent people more likely to go vegetarian is answered/studied by BMJ study (see 1st link in my question) as it concluded: "Higher scores for IQ in childhood are associated with an increased likelihood of being a vegetarian as an adult." I am interested in the validity of inverse! – CrownedEagle May 09 '16 at 10:14
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    I wonder if this is true for any difficult, active lifestyle choice motivated by moral considerations, that people with higher IQ from childhood are more likely to make active decisions to change their lifestyle and are more likely to suceed in sticking to them? Trying to think of a suitable equivalent; maybe something like volunteering or supporting charities, or regularly giving blood – user56reinstatemonica8 May 09 '16 at 10:55
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    I think you need the study to control for income level. Perhaps vegetarian diets are more expensive than "eat whatever you can get" diets---maybe only people above a certain income level have the choice. And maybe income level is also correlated with IQ. Also, is the question supposed to be about "in Britain" or not? – GEdgar May 09 '16 at 17:18
  • @GEdgar - No, It is a general question. Not particularly focused on Britain. It just happened that I was able to find only study by BMJ in Britain. Anyway you have made a Very Good Point about the additional factor of Income Level. – CrownedEagle May 10 '16 at 08:40
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    @GEdgar - High IQ possibly leading to High Income, hence those people prefer being Vegetarian i.e. High IQ >> High Income >> Vegetarianism. [But this choice (as PointlessSpike mentioned above) can be based on moral reasons]: This is what BMJ study said/concluded. But I am searching for the links between Vegetarianism and High IQ. Whether vegetarian diet has any effect on intelligence? – CrownedEagle May 10 '16 at 09:18
  • As I mentioned before. To investigate whether vegetarian diet has any effect on intelligence, examine the case of India. – GEdgar May 10 '16 at 13:58
  • If people do things for unquestioned reasons, that does not point to intelligence. In one of Larry Niven's books (SciFi), one of his characters (carnivore) says to another (vegetarian) "*How much intelligence does it take to sneak up on a leaf?*" –  May 12 '16 at 00:43
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    `Vegetarians have [...] IQ of 109.1 whereas meat eaters have [...] 100.9` Uhmmm... How exactly can both vegetarians *and* carnivores have *above average* IQ? Are there some flexitarians/vegans with really low IQ to balance this out? :P – fgysin May 12 '16 at 13:26
  • @fgysin - may be BMJ's sample consists ppl with above avg. IQ – CrownedEagle May 12 '16 at 15:13
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    @CrownedEagle: That would not speak for their sample size... – fgysin May 17 '16 at 05:38
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    From an evolutionary perspective, hunting should take more brainpower than farming plants that cant move or defend themselves. So, perhaps it is the meat eating that gave rise to the intelligence but then the intelligent are able to think more deeply and have more empathy. – Souradeep Nanda Aug 25 '17 at 02:43
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    @SouradeepNanda: That's "stands to reason" / pub talk level of arguing. I could just as well say "hunting just takes a pointy stick and testosterone, while farming takes more brainpower as it requires an understanding of crops and soil." Or, with other words, [citation needed]. – DevSolar Oct 29 '19 at 09:04
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    @fygsin: Actually, getting a measure of 100.9 from sampled IQ tests would be considered acceptably close to expected mean for, say, a sample size of 1000. Whilst an 8 point difference between two groups at the same sample size would still be highly significant. What might be interesting is to look at the recruitment/filtering methodology to see if it could introduce some bias independently of diet. – Neil Slater Oct 30 '19 at 10:46
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    @SouradeepNanda Hmmm... I would think the majority of meat-eaters are neither hunters nor farmers... – komodosp Nov 01 '19 at 09:04

1 Answers1

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Unknown.

A 2019 meta-review The effects of plant-based diets on the body and the brain: a systematic review looks at the available evidence. The final sentence of the abstract is

Still, putative effects of plant-based diets on brain health and cognitive functions as well as the underlying mechanisms remain largely unexplored and new studies need to address these questions.

It goes on to discuss ways that a non-meat diet could be both positive and negative for cognition. However the larger studies seem to have focused on mortality and disease rather than brain function.

Regarding the first two articles in the question, both refer to the IQ in childhood and vegetarianism in adulthood: 1970 British cohort study, also mentioned in the question. This looks at a link between IQ in childhood and a tendency to vegetarianism in adulthood: it does not look at IQ in adulthood, so the vegetarians-are-more-intelligent titles of the Psychology Today and Evening Standard articles are misleading.

Barrington
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    Given the combination of how vague intelligence is and how you could measure plant-based diets between a thousand other variables just involving food I think it is close to impossible to really study this. – Borgh Jan 09 '20 at 08:17