Educational and ecological correlates of IQ: A cross-national investigation
Introduction
There is much continuing scientific debate about the meaning and significance of IQ scores (Dickens & Flynn, 2001, Sternberg et al., 2001). One of the key problems is that while IQ scores are strongly genetically heritable, and related to various biological measures including brain size, and reaction times (Lynn & Vanhanen, 2002) they are also quite strongly affected by environmental change, as reflected, for example, in the pronounced secular changes in IQ scores during the 20th century known as the Flynn effect after a researcher who may not have discovered the phenomenon but has written extensively on it (Flynn, 1987). (Such effects are clearly environmental because of the implausibility of an alternate explanation in terms of genetic change). One critical such environmental change in respect to variation in IQ is education. Years at school accounts for approximately two-thirds of the variation in IQ scores. This association likely involves reciprocal causation given that more intelligent people opt for more extensive education, that level of schooling is highly heritable, and that educational interventions designed to raise intelligence generally produce modest effects (Ceci, 1991, Neisser et al., 1995). This paper investigated the cross-national association between education and IQ—as well as ecological differences underlying variation in education, specifically the importance of agriculture as an occupation.
The history of IQ tests is bound up with the problem of predicting educational success. Although modern IQ tests have been validated against “real-world” outcomes from earning capacity to law-abiding, such effects are either quite small, or are obtained in studies lacking appropriate controls for education, socioeconomic status, family structure, personality, and so forth. For example, intelligence tests account for 31% of the variation in overall job performance and are thus the best available predictor in personnel psychology (Schmidt & Hunter, 1998). On the other hand, the correlation between tests of cognitive ability and earning capacity may become trivially small when controls are included for family structure and other background variables (Devlin et al., 2002, Griffin & Ganderton, 1996, Hout, 2002).
Intelligence tests are useful at predicting both academic success and practical outcomes, such as job evaluations (Lynn & Vanhanen, 2002, Schmidt & Hunter, 1998, Sternberg et al., 2001). Success in some cognitively complex tasks, from street children running a business, to racing experts betting on horses, to career success among doctors, may be poorly predicted by intelligence test scores, however (Ceci & Liker, 1986, McManus et al., 2003, Sternberg et al., 2001), although such research has been criticized on methodological grounds (Detterman & Spry, 1988, Sternberg et al., 2001).
If IQ tests measure the cognitive capacity for schooling (as well as a more general capacity to solve problems and achieve economic success), then the fact that these capacities increase with economic development (the Flynn effect) is really no more surprising than the phenomenon of athletic ability increasing with training, at least if one assumes that modern societies are more cognitively demanding (Greenfield, 1998). Nor does it challenge the recognition of heritable individual differences in the capacity to benefit from education-propensities that may have little opportunity to be expressed in a non-literate environment.
The new research paradigm of evolutionary social science suggests that humans are capable of rapid (i.e., in one or two generations) psychological and behavioral adjustment to changing ecological conditions (Barber, submitted for publication). Children raised in poverty experience increased psychological stress that is now known to sculpt the developing brain, for example, and are more vulnerable to anxiety, depression, and impaired immune function throughout their lives as a result (Teicher, Andersen, Polcari, Anderson, & Navalta, 2002). Assuming that anxiety has a protective function in threatening environments, this phenomenon can be seen as a case of adaptive plasticity of the developing brain. Recent evidence suggests that the psychological stress of domestic violence also reduces children's IQ scores and academic performance (Delaney-Black et al., 2002, Koenen et al., 2003).
Increased academic aptitude as a consequence of environmental complexity and increased emphasis on education is another kind of adaptive plasticity during psychological development, in the sense that it can facilitate social success. From this perspective, one would expect intellectual capacity to change in response to the economic significance, and prevalence, of education in a country. This prediction is certainly not unique to evolutionary social science but there are distinct advantages to working within a natural science perspective—including parsimony, more direct relevance to neighboring sciences, breadth of generalization, and novelty of predictions—and these advantages apply even in the case of recent phenomena such as the learning of mathematics (Geary, 1995).
The economic gains of literacy are modest in traditional agricultural societies where farming practices are dictated by time-honored routines propagated via the oral tradition. Literacy is essential for the extensive record-keeping required by a market economy, however, and it is no accident that the rise of universal education accompanied the progressive increase in market economies facilitated by the Industrial Revolution (Justman & Gradstein, 1999). If education increases as economies move away from agriculture, and if educational opportunity increases cognitive abilities, then it is possible to make the following predictions about the cross-national distribution of IQ scores:
- 1.
IQ scores should be lower in countries where large numbers of people make their living from agriculture.
- 2.
IQ scores should be higher in countries having extended educational opportunities (here operationalized as enrollment in secondary school).
- 3.
IQ scores should be lower in countries where more of the population is illiterate.
Such predictions have never been tested in cross-national research due both to lack of suitable data and to skepticism about the cross-national validity of IQ tests (Sternberg et al., 2001; see Method section). The predictions were tested using the IQ data of Lynn and Vanhanen (2002) for 81 countries.
Section snippets
Sample of societies
The countries studied comprised the 81 nations for which IQ data were provided by Lynn and Vanhanen (2002). The average 1996 gross national product (GNP) of these countries was $2514 and this did not differ significantly from that of all 185 countries in the world with populations over 50,000: M=$2020, 95% confidence interval $1660–2847.
Dependent variable
The dependent variable was the IQ score associated with each nation. Lynn and Vanhanen (2002) derived their data from published sources, as described in detail
Results
Correlations among the dependent and independent variables are shown in Table 1 along with their means and SDs. It can be seen that national IQs were significantly negatively correlated with illiteracy, agricultural labor, infant mortality, and the incidence of low birth weights. They were significantly positively correlated with secondary education and GNP.
Regression analyses are shown in Table 2. The first analysis without regional controls found significant negative effects of agricultural
Discussion
All three of the predictions were strongly supported by the data. Countries where large numbers of people made their living from agriculture had lower IQ scores (prediction 1). IQ scores were higher in countries with extended education as indexed by secondary school enrollment (prediction 2). Countries with high levels of illiteracy also had reduced IQ scores (prediction 3). These findings suggest that cross-national differences in IQ scores may be largely determined by ecology, with
Acknowledgments
I am grateful to Earl Hunt and to anonymous reviewers for many detailed and helpful comments that made a substantive contribution to this paper.
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