The other day I came across this table of National IQs for all the countries in the world.  (Drawn from Richard Lynn's and Tatu Vanhanen's "Intelligence and the Wealth and Poverty of Nations", via Gweilo, via razib.)  This is fascinating information, particularly when combined with population growth rates.

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For the purposes of this discussion, let's stipulate that someone's IQ is just something you can measure, which may or may not have some correlation to anything else you can measure.  Whether it has anything to do with intelligence or any particular cognitive ability will not be addressed.
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The U.S. Census Bureau has a terrific website called the International Data Base (IDB).  This includes a facility to create a table of National populations for any year between 1950 and 2050.  Let's assume that countries are (first-order) self-breeding and that measured IQ was/will be stable in each country during the 100 year period.  Combining all these data yields the following graph:

The dark blue line is the average IQ of the world.  I've also plotted the population growth of the five most populous countries, India, China, the U.S., Indonesia, and Nigeria; the average IQ of each of these countries is in parenthesis.  (Nigeria is currently ninth, with Brazil (87), Pakistan (81), Russia (96), and Bangladesh (81) intervening, but by 2050 it will be fifth.)  As you can see, in a 100 year period the world's average IQ will have dropped from 92 to 86, a change of 6%.  That is pretty darn significant.  And all because of differential population growth.

I extrapolated the population growth of each country another 50 years to the year 2100 (lightly shaded region of graph).  At that time the world's average IQ will have dropped below 84.  Within this time period of 150 years, extremely short by any evolutionary standard, an incredibly significant change in this key metric will have occurred.  And there is no sign of the trend bottoming out, because the growth rate of countries with lower IQs exceeds the growth rate of countries with higher IQs.  The most populous country today is China, which has a high IQ (100), but its growth is actually projected to be negative because of their "one child" policy.  After about 2030 India will be the most populous country, and it has relatively low IQ (81).  At current growth rates by 2100 Nigeria will be the third most populous country, and it has a low IQ (67).

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If you're interested in playing with these numbers yourself, here's the Excel spreadsheet with all these data.  If you publish further analysis or commentary, I would appreciate it if you'd link back to this page.
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There were two assumptions we made up front, and I'd like to revisit them.  First, we assumed countries are self-breeding.  With modern vehicles and opportunities for travel this is becoming less and less true, but for the bulk of the world's population it is definitely a safe assumption.  The two largest countries, China and India, are both relatively undeveloped and by-and-large people do not travel in or out of them.  The third largest country, the U.S., is the only possible exception to this assumption, because so many people immigrate into the U.S. (in 1990 8% of the U.S. population was foreign-born).

The second assumption is more interesting; we assumed measured IQ was/will be stable in each country.  The Flynn Effect predicts this is false, and that measured IQ will increase over time.  (Historical data provide significant evidence for this.)  Many explanations have been offered for this effect, including steady improvement in testing procedures, and there is some evidence that in recent years the Flynn Effect has diminished.  If the overall world IQ changes due to differential birth rates among populations with different IQs (that is, separate countries), then it seems plausible that a country's IQ could change due to differential birth rates within its sub-populations as well.  In most countries and under most circumstances the birth rate of poorer and less educated people is significantly higher than the birth rate among wealthier and more educated people.  (China is the primary exception; due to their "one child" policies the birth rate within all sub-populations is essentially the same.)  Given the positive correlation between measured IQ and wealth, and between measured IQ and education, these differential birth rates would suggest that individual countries' IQs would decrease as populations expand.  If true, this would obviously accelerate the overall decrease in world IQ over time.

There are other factors at work.  For example, AIDS is presently the most common cause of death in Nigeria, which is one of the most populous and fastest growing countries.  Wealthier and more educated people are less likely to become infected by AIDS, because of awareness of the known transmission mechanisms and available protections, and also more likely to survive infection, because of availability of treatment (at least to the point of having and raising healthy children).  Because of this the effective birth rate among wealthier and more educated people in Nigeria is probably higher than poorer, less educated people.  There is a substantial correlation between wealth, education, and measured IQ.  Thus the AIDS epidemic may have the effect of raising the average IQ of Nigerians.

The human race has been in existence for approximately 150,000 years, during which time natural selection has incrementally increased human intelligence and cognitive ability.  It is not possible to give IQ tests to humans from 100,000 years ago - at least not yet :) - so we can only surmise that there would have been a corresponding increase in measured IQ as well.  Only recently - within the last 10,000 years or so - has this trend been halted, primarily by organized agriculture which enabled a small group of humans to provide food for a larger group.  It now appears that very recently - within the last 100 years or so - this trend has been reversed.  I call this Unnatural Selection, since it appears that societal rather than evolutionary effects are at work.  The consequences of this overall decrease in world IQ have yet to be quantified, but they are bound to be significant.

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