Friday, April 13, 2007

Intelligence and latitude in US

Following the visualization of estimated average IQ by state that I was able to create with the neat site Many Eyes, Fat Knowledge astutely observed that the farther north a state is located, the higher its IQ tends to be.

That is clearly the global trend. At the national level, I found distance from the equator and IQ correlate at a statistically significant .67 using data from Vanhanen and Lynn's IQ and the Wealth of Nations. The relationship is more rigorous even than the contemporary relationship between purchasing power parity and IQ (as an aside, life expectancy shows the single strongest correlation with IQ at .85, followed by infant mortality at an inverse .84, and then--without Mike Judge's endorsement--an inverse relationship with fecundity at .81).

The trend also holds at the state level within the US. Using the latitude of each state's most populous city and comparing it with estimated IQ yields a correlation of .70, made considerably stronger (.81) if only the lower forty-eight are considered. As the relationship is at the global level, much of this is attributable to race, with blacks concentrated in the Deep South and Hispanics in the Southwest.

More interestingly, the relationship between latitude and IQ firmly exists in the US when only whites or only blacks are considered. And the correlations are almost identical: .52 for whites (click here for visual representation of estimated white IQ by state) and .51 for blacks (here for visual).

There's plenty of room for speculation as to why the farther north you go, the smarter the population tends to become.

Regarding whites, the English Puritan families of the northeast that had become established and affluent stayed put while those with less to leave had more to gain by heading out onto the frontier. Whites working in the South as indentured servants made up a large portion of the labor supply early in the 1600s until well into the 18th Century. This was the ticket to the New World for many Europeans with few prospects at home, and sometimes even presented convicts a way to avoid prison time. Give consideration also to how the gullible types, hoping to strike it rich, flocked to California in the 1840s and 1850s with dreams that, for the most part, didn't quite 'pan' out. A rebirth of the California Dream pulled destitute menials from the Heartland to the Southwest during the Great Depression (a la The Grapes of Wrath).

As the more southerly states are more ethnically diverse, it's reasonable to assume that a greater number of whites in these states have some non-European ancestry than in the whitebread upper Midwest and Northeast, especially from Amerindians in the Southwest. Notice also the Scottish/Irish Appalachian streak of banjo-playin' toothless inbreds like those put on display in movie version of Deliverance (I can say that. It's self-deprecation, as my lineage traces back to western Virginia).

Regarding blacks, those in southern states tend to, with exceptions, have less European ancestry than those in the north. Pulling off successful escapes from the antebellum South could have selected for intelligence among those who were able to do so as well.

Contemporarily, for both whites and blacks, warmer climates produce a greater demand for outdoorsy labor (landscapping, lawn mowing, construction, tourism, etc), which tend to have low skill-level requirements. That rougher climates selected for attributes to attenuate the physical threats posed by severe cold, while also selecting for greater intelligence to deal with more scarcity, may mean that, on average, smarter people are less bothered by chilly weather. Personal experience suggests it to me.

The relationship doesn't hold with any rigor for Hispanics in the US (although it trends in the same direction, it's not statistically significant). This illustrates the difficulty in grouping all people of Latin American ancestry under the "Hispanic" umbrella. The highly professional Cuban population in southern Florida provides the best example. Puerto Ricans, concentrated in the Northeast, have a very different profile than the mostly white Cubans of Miami.

9 comments:

JSBolton said...

Another possible explanation could be that malaria-resistant types were selected-for, and that this decreases average IQ. Pushing deformed blood cells through the brain makes thought painful, and a capacity which for generations, is used less in one group, will not build up in the same degree as in an unaffected group, perhaps. Holding race constant, shows that we're not dealing with something that affects different races in the same region, with very high inequality.

Fat Knowledge said...

Very interesting, thanks for doing this analysis.

I also wonder if beyond a genetic reason, there may be an environmental one as well.

We think of people that are emotional as being hot, and those that are rational as being cold. Might a hot climate make you a hot blooded person? I would think that being more emotional would lead to a lower IQ (though I could be wrong on that). I would also guess that more emotional people are likely to have children at younger ages and I wonder if that too would lead to lower IQs (idea being that older parents are more mature and have more resources and are better able to raise their children). I would be interested to know if the average age of mothers at birth increases the farther you go from the equator.

I am also curious if diet would impact how emotional people are or their IQ. Might people who live at lower latitudes eat spicier food, to increase sweeting to lower body temperature, that also would impact how their brain functions?

I wonder what happens to the IQ of someone who lives around the equator if you move them to Canada or Russia? Maybe there is no impact on them, but you would see an impact in their children or grandchildren. Fortunately, with global warming such data should be available soon. :)

All conjecture on my part, but an interesting topic for thought.

Audacious Epigone said...

FK,

Another plausible environmental role is that rougher weather is more conducive to staying inside reading, doing schoolwork, etc, whereas warmer weather encourages stomping around outside, playing sports, going out, etc.

Regarding smart parents being able to provide their children with better environments--it's a virtuous circle, but adopted children only gain a modest IQ advantage from a much better environment (5-8 points on average).

Regarding diet--at a certain level it definitely does. Insuring adequate nutrition, especially iodine and Vitamin A, appears to be a viable strategy for improving the situation in Africa.

Jason Malloy said...

Another interesting analysis, AE.

Something that I still don't know but want to investigate:

Do animal populations (within and/or between species) increase in intelligence as you move north?*

What if you just the pet dogs of people up north and down south? Would the northern dogs be smarter too?



* It doesn't appear that way, i.e. primates/homo sapiens, but then again, I haven't done any legwork.

Audacious Epigone said...

Jason,

Thanks.

Beats me. Dog breeds don't appear to follow the intelligence/life expectancy trend that is so pronounced across human populations (with IQ and life expectancy correlating at .85)--the sharper dogs seem to, through eyeing the rankings, die sooner than the dumber ones. And dogs that've historically lived in cooler climates (malamutes, huskies, chows, etc aren't on the sharp-dog shortlist).

BCG said...

I think the positive relationship between IQ and degrees of latitude you suggest is indeed probably causal, all else being equal - but other factors can overwhelm it. For instance in this hard-to-find paper by Richard Lynn -

Lynn, R. (1979). The social ecology of intelligence in the British Isles. British Journal of Social and Clinical Psychology,18,1-12.

He finds that in the British Isles IQ is highest in London and falls off with distance from London and teh South East - his suggestion is that this is due to many centuries of high IQ individuals migrating to London.

So within the UK the IQ map probably goes in the opposite direction to the USA - but I would like to see this checked against (easily avaiable) national age-specific school testing results - I may need to try and do this myself, but I'm not sure where to look.

The 'puritans' who settled New England mostly came from South East England, I believe.

Audacious Epigone said...

Undoubtedly there are exceptions, especially when population density is taken into consideration (suburban metropolitan areas consistently show higher aptitude/IQ test scores than rural areas do, at least in the US). West Virginia is a big 'anomaly' in the US, for example.

But to the extent that the relationship is causal, I presume it is influenced primarily by climate (in a broader sense that weather alone). How much difference in climes between SE England and the rest of the English mainland exists? Also, would that pattern have anything to do with Dutch/Saxon ancestry versus Celtic as we move northwest? That, in addition to migration, is similar to my working assumption for Ireland's
relatively low IQ scores.

Thank you for the heads-up on the Lynn paper.

BGC said...

The SE English climate is not very different from the rest of the British Isles, by intenational standards. But the UK is a very extreme latitude place compared with most developed countries, and I suspect that we may be on a cusp of latitude where day-length differences begin to have a bigger effect on human psychology via seasonal affective disorder, and probably whatever biology underlies the tendency towards binge-drinking.

Your idea about different ancestry in different parts of the UK and Ireland may be significant - I would have thought it unlikely until recently, but the incoming data on human genetic differences has changed my 'prior probabilities'. Now I think if the genes are measurably different (which they are) then the psychological traits will be different - it is just a question of finding which traits and how much they differ...

shahrbaraz said...

The relationship with northern latitudes is quite apparent;the two associations are cold and earth's magnetic pole.The third is that more industrious and adventurous people would follow the recession of ice northward in search of new land and pastures after the last LMG 12000 yrs ago.This seems to at least explain the rise of USA;the transatlantic journey in 16th century and the harsh terrain was not for the faint hearted.