Elsevier

Intelligence

Volume 32, Issue 1, January–February 2004, Pages 65-83
Intelligence

The secular rise in IQ: Giving heterosis a closer look

https://doi.org/10.1016/S0160-2896(03)00058-8Get rights and content

Abstract

Although most discussions today start from the assumption that the secular rise in IQ must be environmental in origin, three reasons warrant giving the genetic phenomenon heterosis a closer look as a potential cause. First, it easily accounts for both the high heritability and low shared environmental effects seen in IQ, findings that are difficult to reconcile with environmental hypotheses. Second, numerous other highly heritable traits, both physical as well as psychological, have also undergone large secular changes in parallel with IQ, which is consistent with the occurrence of broad-based genetic change like heterosis. And third, a heterosis hypothesis for the trend can be tested in several straightforward ways. The paper also provides a hypothetical example, based on data from a real population, of how heterosis can result from demographic changes like those that have taken place throughout the developed world in recent history and shows that under certain conditions, even a small demographic change could cause large genetically based phenotypic changes.

Introduction

Identifying the cause of the secular rise in IQ test scores represents one of the most puzzling questions facing the field of intelligence research today. This paper explores the potential role of one possible cause, the genetic phenomenon heterosis, often referred to as hybrid vigor. Although heterosis has occasionally come up in discussions of the IQ trend, it has never been examined in any detail. For example, both Jensen (1998, p. 327) and Kane and Oakland (2000) list heterosis as one of several possible causes of the trend; Jensen also provides a brief description of the phenomenon. Flynn (1998) and Dickens and Flynn (2001) state that heterosis may have been a partial cause of pre-1950 IQ gains, although they provide no detailed explanation as to why the effects of heterosis must have stopped in 1950. However, the most common treatment of heterosis in current discussions of the IQ trend is to simply ignore it. Such discussions usually start from the assumption that IQ trend must be entirely environmental in origin thus dismissing genetic factors like heterosis without even explicitly stating that they have been ruled out Fernandez-Ballesteros et al., 2001, Neisser, 1998, Wahlsten, 1997.

Three reasons justify giving heterosis a much closer look than has been the case up to now. These reasons, which together form the outline of the present paper, are as follows:

  • 1.

    Because it is a genetic mechanism, heterosis easily accounts for the high heritability, the lack of any secular change in estimates of the heritability, and the low shared environmental effects seen in IQ, findings that are difficult to reconcile with environmental hypotheses.

  • 2.

    Because it alters a very fundamental aspect of a population's genetic structure, heterosis has the potential to account for a number of other unexplained secular trends that have largely paralleled the IQ trend. Like IQ, several of these other traits have also displayed consistently high heritability and low shared environmental effects.

  • 3.

    Because it posits a very specific causal factor, a heterosis hypothesis for the IQ trend generates several straightforward predictions, which if confirmed would be difficult to explain except as the result of heterosis. That is, a heterosis hypothesis for the trend is testable.

Section snippets

The “IQ paradox” and the need for a “factor X”

In a recent article, Dickens and Flynn (2001) use the term “IQ paradox” to refer to the fact that estimates of the heritability of IQ have remained consistently high over extended periods of steadily rising IQ in many countries. This finding would seem to suggest that any presumed environmental factor(s) responsible for the IQ trend could not have been a measurable source of environmental variance within any given generation. There are two different ways to look at the situation, depending on

Secular trends in other traits

The genetic phenomenon heterosis increases the ratio of heterozygous to homozygous genotypes throughout the entire genome of a population (Dahlberg, 1942, Jensen, 1998; see also the discussion in the next section). Because recessive alleles affect the phenotype primarily in the homozygous state but have little effect in the heterozygous state, heterosis tends to decrease the phenotypic expression of all recessive alleles and increase the expression of all dominant alleles in the population and

Conditions necessary for heterosis

This section provides an example of how the process of heterosis can cause secular trends in traits like IQ. The example starts with genetic data from a real population. It then posits demographic changes similar to those that have occurred in recent history throughout the developed world and estimates the likely genetic effects of such changes. It is intended primarily to identify the basic conditions that must be in place for heterosis to occur; I will then argue that these conditions have

Testing the heterosis hypothesis

Because heterosis is a very specific causal factor, a heterosis hypothesis makes several clear predictions that can be tested in straightforward ways. This section discusses three approaches to testing the heterosis hypothesis for the IQ trend, as well as the trends in other traits. Although there are other possible tests of the heterosis hypothesis, the three approaches described below are likely to yield the most definitive results. That is, positive results on these tests would be difficult

Conclusion

IQ and a number of other highly heritable traits, from height and growth rate to autism and myopia, have shown large parallel secular trends in populations across the industrialized world. At present, no compelling environmental explanation exists for even one of these trends. In the cases of several trends, decades of research have brought science no closer to an explanation than when the trend was first found to be occurring. Because the populations exhibiting these trends have undergone

Acknowledgements

Part of this work was supported by a grant from the Pioneer Fund. I would like to thank Douglas Detterman, James Flynn, Robert Plomin, Lee Thompson, and an anonymous reviewer for helpful comments on earlier drafts. Special thanks go to Linda Gottfredson for her helpful comments and invaluable guidance over the past several years.

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