parasitic prevalence and IQ?

RoyalSociety | Here, we offer a new hypothesis—the parasite-stress hypothesis—to explain the worldwide distribution of intelligence. The brain is the most complex and costly organ in the human body. In human newborns, the brain demands 87 per cent of the body's metabolic budget, 44 per cent at age five, 34 per cent at age ten, and 23 per cent and 27 per cent for adult males and females, respectively (Holliday 1986). Presumably, if an individual cannot meet these energetic demands while the brain is growing and developing, the brain's growth and developmental stability will suffer. Lynn (1990, 1993) has argued that nutrition is vital to high degrees of mental development. Lynn (1990) suggested that nutrition may account for the Flynn effect (large increases in IQ over short periods of time as nations develop; Flynn 1987), and later (Lynn 1993) reviewed evidence showing that undernourished children have smaller heads, smaller brains and lower psychometric intelligence than sufficiently nourished children.

Parasitic infection affects the body, and hence the brain, energetically in four ways. (i) Some parasitic organisms feed on the host's tissues: the loss must be replaced at energetic cost to the host. Such organisms notably include flukes and many kinds of bacteria. (ii) Some parasites inhabit the intestinal tract or cause diarrhoea, limiting the host's intake of otherwise available nutrients. These notably include tapeworms, bacteria, giardia and amoebae. (iii) Viruses use the host's cellular machinery and macromolecules to reproduce themselves, at the energetic expense of the host. (iv) The host must activate its immune system to fight off the infection, at energetic expense. Of these, diarrhoeal diseases may impose the most serious cost on their hosts' energy budget. First, diarrhoeal diseases are the most common category of disease on every continent, and are one of the two top killers of children under five, accounting for 16 to 17 per cent of all of these deaths worldwide (WHO 2004a). Second, diarrhoea can prevent the body from accessing any nutrients at all. If exposed to diarrhoeal diseases during their first five years, individuals may experience lifelong detrimental effects to their brain development, and thus intelligence. Parasites may negatively affect cognitive function in other ways, such as by infecting the brain directly, but we focus only on energetic costs.

The worldwide distribution of parasites is well known. Disease-causing organisms of humans are more prevalent in equatorial regions of the world and become less prevalent as latitude increases. Ecological factors contributing to this distribution include mean annual temperature, monthly temperature range and precipitation (e.g. Guernier et al. 2004). Similar trends of parasite distribution have been shown in other host species (e.g. Møller 1998).

Many studies have shown a negative relationship between intestinal helminth infection and cognitive ability (reviewed in Watkins & Pollitt 1997; see also Dickson et al. 2000). Although several hypotheses have been proposed to explain this phenomenon, none have considered intestinal worms in the larger context of all parasitic infection, nor have they considered fully the energetic cost of infection and its consequences on the brain. Other studies have shown relationships between helminth infection and economic and educational factors that are related to intelligence. For example, Bleakley (2007) studied the effects of eradication of hookworm in the southern US during the early twentieth century, and found that areas where hookworm infections had been greatly reduced had higher average incomes after treatment than areas that had not received treatment. Jardin-Botelho et al. (2008) found that Brazilian children infected with hookworm performed more poorly on cognitive tests than uninfected children, and that children infected with more than one type of intestinal helminth performed more poorly than children infected with only one.

Thus, from the parasite-stress hypothesis, we predict that average national intelligence will correlate significantly and negatively with rates of infectious disease, and that infectious disease will remain an important predictor of average national intelligence when other variables are controlled for. It is the purpose of this study to introduce this hypothesis to describe the worldwide variation in intelligence, and to provide some supportive evidence using correlations and linear modelling techniques.