Parasites and Intelligence (Eppig et al. 2010)

This is as good an explanation for global differences in IQ as I have ever seen. It proposes that IQ differences between human groups are created during ontogeny due to an energetic trade-off between brain development and immune response to infectious disease. In regions of the world with a high parasite burden, there is an elevated amount of energy used up to fight disease, at the expense of brain development.

Notice that this is not an explanation requiring genetic adaptation. Parasite burden inhibits cognitive function development. Indeed, if one were to make an adaptive argument (and I will not!), it would be in the direction of greater brain development genetic potential in parasite-heavy locales to counteract the effects of disease, i.e., the selection of individuals that may withstand the rigors of fighting off infectious disease without compromising cognitive function.

The authors write:

Multiple regression shows that, of infectious disease, temperature, evolutionary novelty and AVED, infectious disease is the best predictor of intelligence by a large margin.

...

If the general pathway we propose is correct, there are two plausible mechanisms by which a trade-off in allocation of energy to immune function versus brain development and maintenance may occur. First, parasitic infection may intermittently cause the redirection of energy away from brain development. In this case, during periods of infection, the brain receives fewer energetic resources, but this allocation to brain function will return to pre-infection levels during healthy periods. During periods of infection, whatever aspects of the brain that are growing and developing will suffer reduced phenotypic quality. Second, exposure to infectious agents may cause a developmental pathway that permanently invests more energy into immune function at the expense of brain growth. In this scenario, large amounts of energy would be allocated into immune function during periods of health, as opposed to only redirecting energy during periods of infection. This could operate through a variety of mechanisms. A plausible mechanism is that higher investment in immune system is triggered by individual exposure to infectious disease at some point during ontogeny. This may include triggering from exposure to maternal antibodies while in utero.

...

Our findings suggest that the heritable variation in intelligence may come from two sources: brain structure and immune system quality. Thus, two individuals may possess identical genes for brain structure, but have different IQ owing to differences in immune system quality reflecting their personal allocation of energy into brain development versus immunity.

...

Our findings are consistent with a number of other findings in the literature. In particular, the Flynn effect (Flynn 1987) demands that any hypothesis regarding the worldwide variation and distribution of intelligence must be able to account for some factor that allows for large IQ gains over time spans seemingly too short to be attributed to evolution by natural selection. The parasite-stress hypothesis allows for such a factor in the form of reduced parasitic infection. As societies become modernized, decreased parasite stress may occur through multiple pathways. As national wealth increases, medicine, vaccinations and potable water can be purchased by both the government and by individuals. Moreover, there is cross-national evidence that, as democratization increases, there are corresponding increases in public health legislation and infrastructure. Democratization also increases levels of education, better allowing individuals to seek out and understand information that reduces parasitic infection (Thornhill et al. 2009). This source of endogeneity is not a flaw, but a prediction of our hypothesis.

Related:

• Pathogens and Collectivism
• Religious Diversity and pathogens

Proceedings of the Royal Society B doi:10.1098/rspb.2010.0973

Parasite prevalence and the worldwide distribution of cognitive ability

Christopher Eppig et al.

Abstract

In this study, we hypothesize that the worldwide distribution of cognitive ability is determined in part by variation in the intensity of infectious diseases. From an energetics standpoint, a developing human will have difficulty building a brain and fighting off infectious diseases at the same time, as both are very metabolically costly tasks. Using three measures of average national intelligence quotient (IQ), we found that the zero-order correlation between average IQ and parasite stress ranges from r = −0.76 to r = −0.82 (p less than 0.0001). These correlations are robust worldwide, as well as within five of six world regions. Infectious disease remains the most powerful predictor of average national IQ when temperature, distance from Africa, gross domestic product per capita and several measures of education are controlled for. These findings suggest that the Flynn effect may be caused in part by the decrease in the intensity of infectious diseases as nations develop.

Link

Literature review of Sub-Saharan African IQ

Intelligence doi:10.1016/j.intell.2009.05.002

A systematic literature review of the average IQ of sub-Saharan Africans

Jelte M. Wicherts et al.

Abstract

On the basis of several reviews of the literature, Lynn [Lynn, R., (2006). Race differences in intelligence: An evolutionary analysis. Augusta, GA: Washington Summit Publishers.] and Lynn and Vanhanen [Lynn, R., & Vanhanen, T., (2006). IQ and global inequality. Augusta, GA: Washington Summit Publishers.] concluded that the average IQ of the Black population of sub-Saharan Africa lies below 70. In this paper, the authors systematically review published empirical data on the performance of Africans on the following IQ tests: Draw-A-Man (DAM) test, Kaufman-Assessment Battery for Children (K-ABC), the Wechsler scales (WAIS & WISC), and several other IQ tests (but not the Raven's tests). Inclusion and exclusion criteria are explicitly discussed. Results show that average IQ of Africans on these tests is approximately 82 when compared to UK norms. We provide estimates of the average IQ per country and estimates on the basis of alternative inclusion criteria. Our estimate of average IQ converges with the finding that national IQs of sub-Saharan African countries as predicted from several international studies of student achievement are around 82. It is suggested that this estimate should be considered in light of the Flynn Effect. It is concluded that more psychometric studies are needed to address the issue of measurement bias of western IQ tests for Africans.

Link

According to Flynn, nutrition not behind "Flynn effect"

See a recent paper by Lynn where the importance of nutrition for the Flynn effect (secular increase in IQ) was argued. Another paper on Denmark (and another) which also sees a recent decline in IQ scores, a reversal of the Flynn effect.

From this paper's conclusions:

The totality of the evidence supports a summary conclusion. Enhanced
nutrition has made us taller people and poor nutrition has made us more obese. But
our diet today probably does not make us very different people from our grandparents as far as cognitive competence is concerned. Our brains have altered since 1900, and they are better brains for solving the problems of our time. But they have altered rather like a muscle, that is, they have altered because we use them differently than our parents and grandparents did. The causes of this are many and the effects of nutrition, at least since privation has been banished, are too weak to stand out from the crowd.

Economics & Human Biology doi:http://dx.doi.org/10.1016/j.ehb.2009.01.009

Requiem for nutrition as the cause of IQ gains Raven's gains in Britain 1938 to 2008

James R. Flynn

Abstract

The hypothesis that enhanced nutrition is mainly responsible for massive IQ gains over time borrows plausibility from the height gains of the 20th century However, evidence shows that the two trends are largely independent. A detailed analysis of IQ trends on the Raven's Progressive Matrices tests in Britain dramatizes the poverty of the nutrition hypothesis. A multiple factor hypothesis that operates on three levels is offered as an alternative instrument of causal explanation.

The Raven's data show that over the 65 years from circa 1942 to the present, taking ages 5 to 15 together, British school children have gained 14 IQ points for a rate of 0.216 points per year. However, since 1979, gains have declined with age and between the ages of 12 to 13 and 14 to 15, small gains turn into small losses. This is confirmed by Piagetian data and poses the possibility that the cognitive demands of teenage subculture have been stagnant over perhaps the last 30 years.

Link

Nutrition behind Flynn effect?

The Flynn effect is the improvement of IQ scores over time, with people born more recently tending to score higher than the average of previous generations. This paper suggests that better nutrition of pregnant women and infants is behind this phenomenon.

Intelligence doi: doi:10.1016/j.intell.2008.07.008

What has caused the Flynn effect? Secular increases in the Development Quotients of infants

Richard Lynn

Abstract

Results of five studies show that during the second half of the twentieth century there were increases in the Development Quotients (DQs) of infants in the first two years of life. These gains were obtained for the Bayley Scales in the United States and Australia, and for the Griffiths Test in Britain. The average of 19 data points is a DQ gain of approximately 3.7 DQ points per decade. Similar gains of approximately 3.9 IQ points per decade have been present among preschool children aged 4–6 years. These gains are about the same as the IQ gains of school age students and adults on the Wechsler and Binet tests. This suggests that the same factor has been responsible for all these secular gains. This rules out improvements in education, greater test sophistication, etc. and most of the other factors that have been proposed to explain the Flynn effect. It is proposed that the most probable factor has been improvements in pre-natal and early post-natal nutrition.

Link

Reversal of Flynn effect in Denmark

Danish IQ appears to have become lower over a 5-6 year period, providing some evidence that the Flynn effect (secular increase in IQ) not only has ceased to operate but may have actually reversed.

Intelligence (Article in Press)

Secular declines in cognitive test scores: A reversal of the Flynn Effect

Thomas W. Teasdale and David R. Owen

Abstract

Scores on cognitive tests have been very widely reported to have increased through the decades of the last century, a generational phenomenon termed the ‘Flynn Effect’ since it was most comprehensively documented by James Flynn in the 1980's. There has, however, been very little evidence concerning any continuity of the effect specifically into the present century. We here report data from a population, namely young adult males in Denmark, showing that whereas there were modest increases between 1988 and 1998 in scores on a battery of four cognitive tests–these constituting a diminishing continuation of a trend documented back to the late 1950's–scores on all four tests declined between 1998 and 2003/2004. For two of the tests, levels fell to below those of 1988. Across all tests, the decrease in the 5/6 year period corresponds to approximately 1.5 IQ points, very close to the net gain between 1988 and 1998. The declines between 1998 and 2003/4 appeared amongst both men pursuing higher academic education and those not doing so.

Link

Richard Lynn's speculations

Richard Lynn is back with more speculation about the origin of racial IQ differences. I will not dwell on his data, which should be taken with a heavy grain of salt. Rather, I will deal with his suggestions that racial IQ differences have something to do with cold winters during human evolution.

Lynn's speculations implicitly assume that IQ differences are of deep evolutionary origin. In other words, he assumes that Caucasoid-Mongoloid-Negroid aptitudes, as well as subracial cognitive distinctions (e.g., between Northcentral Europeans and Southeastern Europeans and Arctic vs. Far Eastern Mongoloids) date to the prehistoric period and accompanied racial differentiation.

Of course, this assumption is entirely unfounded. Indeed, increasingly, we are realizing that recent human evolutionary change, in post-Neolithic times was rapid, and the characteristics of populations were not already fixed (as the evolutionary psychologists hold) at the onset of the Neolithic, when racial differentiation was already in place.

But, the real problem is that there are only about 100 years of IQ data for some human populations and data of good quality (e.g., measured with culture-fair instruments) are at most 1-2 generations old. At the same time, it has been amply demonstrated that human IQ has been rising rapidly in several human populations even over the short time span for which IQ data exist, the so-called Flynn effect.

We thus know that (a) IQ is volatile, and can change rapidly, and (b) we have data that spans only perhaps 2% of human history.

The combination of these two factors renders any extrapolations from recent IQ data into the Stone Age or even a few centuries ago entirely unusable and untrustworthy. A good analogy would be someone's attempt to estimate the price of the IBM stock in 1935 by taking into account its value in the last year.

Richard Lynn must be commended for collecting and compiling such a volume of IQ data, but it's time to move beyond an early-20th century statistical understanding of human cognitive differences based on the reification of g and tackle the really hard problems of (a) figuring out whether individual racial ancestry predicts IQ, and (b) identifying some of the genes contributing to intelligence.

As I explain here, I see little hope of (b) occurring to any significant degree any time soon, but determining the racial (ancestral) contribution to IQ is wholly feasible. Let's hope that the race/IQ enthusiasts take up the challenge.

IQ falling?

The Flynn effect refers to the increase in IQ in recent history in several populations around the world. Several explanations have been proposed for this effect, but a new study from Denmark, replicating a previous one from Norway indicates that the Flynn effect has ceased in the 1990s, and indeed IQ has slightly dropped in recent years to early '90s levels.

Personality and Individual Differences (Article in Press)

A long-term rise and recent decline in intelligence test performance: The Flynn Effect in reverse

Thomas W. Teasdale and David R. Owen

Abstract

In the 1980s reviewed evidence indicated that, through the preceding decades of the last century, population performance on intelligence tests had been rising substantially, typically about 3–5 IQ points per decade, in developed countries. The phenomenon, now termed the ‘Flynn Effect’, has been variously attributed to biological and/or to social and educational factors. Although there is some evidence to suggest a slowing of the effect through the 1990s, only little evidence, to our knowledge, has yet been presented to show an arrest or reversal of the trend. Substantially replicating a recent report from Norway, we here report intelligence test results from over 500,000 young Danish men, tested between 1959 and 2004, showing that performance peaked in the late 1990s, and has since declined moderately to pre-1991 levels. A contributing factor in this recent fall could be a simultaneous decline in proportions of students entering 3-year advanced-level school programs for 16–18 year olds.

Link

Better nutrition of less privileged causes generational IQ gains

A new article uses full-distribution data (as opposed to a simple comparison of means) to determine that IQ gains across thirty years (also known as the "Flynn effect") are due to better nutrition of the less privileged classes. There has been no substantial increase at the higher-IQ end of the distribution, unlike the less privileged classes which are "catching up" in terms of IQ.

Interestingly, better nutrition among the less privileged has served to reduce the spread of IQ which has a lower standard deviation in the more recent sample than in the one taken 30 years earlier. We know that the standard deviation of the IQ distribution shows both great variation in different populations and geographical structuring. Therefore, it appears that populations with the same mean IQ may have a different potential for enhancing their IQ via better nutrition in the future.

Intelligence (Article in press)

The generational intelligence gains are caused by decreasing variance in the lower half of the distribution: Supporting evidence for the nutrition hypothesis

Roberto Colom et al.

Generational intelligence gains are one intriguing finding in science. Nutrition and cognitive stimulation are among the most remarkable causes of the upward trend in intelligence. The nutrition hypothesis predicts a primary impact on the most deprived, producing disproportionate gains at low intelligence levels. The cognitive stimulation hypothesis predicts gains along the intelligence distribution. However, data from the entire distribution are rarely available. The present study compares a sample of children tested in 1970 with an equivalent sample tested 30 years later. Data for the entire distributions were available. The results are consistent with the nutrition hypothesis, because the gains were mainly concentrated in the lower and medium halves of the distribution and were negligible in the very top half of the distribution. Moreover, an impressive gradual decrease in the gains was observed from the lower half to the top half of the distribution.

Link