Advances in the Psychology of Human Intelligence

Volume 5

Robert J. Sternberg, Robert J. Sternberg

• Publisher: Psychology Press
• Year: 2014
• ISBN:
  9781317784517
• Edition: 1
• Topic: Psychology
• Subtopic: History & Theory in Psychology
• Index: Psychology

Volume five continues to mark the significant advances made in the psychology of human intelligence, problem solving, and thinking abilities. Papers contributed by leaders in the field reflect a diversity of perspectives and approaches to the human intelligence. Subjects discussed include: * genetic and environmental contributions to information-processing abilities
* development of children's conceptions of intelligence
* skill acquisition as a bridge between intelligence and motivation
* information-processing abilities underlying intelligence
* costs of expertise and their relation to intelligence
* the nature of abstract thought

1	Genetic and Environmental Determinants of Information Processing and Special Mental Abilities: A Twin Analysis

Matt McGue
Thomas J. Bouchard, Jr.

University of Minnesota, Minneapolis

INTRODUCTION

The evidence supporting the existence of genetic influences upon human cognitive abilities is overwhelming (Bouchard & McGue, 1981; Teasdale & Owen, 1984; Vandenberg & Vogler, 1985; Wilson, 1983). Analyses of combined twin and family data indicate both that the data cannot be explained without allowing for genetic effects, and that the magnitude of these effects is such that approximately 50%-60% of the variance in IQ is associated with genetic differences among individuals (Loehlin, Willerman, & Horn, 1988). Results from twin and adoption studies also underscore the importance of environmental influences upon measured intelligence. Analyses of a combined data set of familial correlations indicate that approximately 30% of the variance in IQ is associated with familial environmental factors (i.e., environmental factors that are shared by family members; for example, socioeconomic status [SES], diet) and approximately 20% of the variance is due to nonfamilial environmental factors (i.e., environmental factors that are not shared by family members; for example, accidents, birth order effects).

Two not necessarily incompatible hypotheses have been proposed to characterize the nature of genetic involvement in cognitive functioning. The first, as exemplified in the writings of, among others, Wilson (1983), argued that genetic influences upon IQ largely reflect inherited differences in the structure and function of the brain (cf. also Reed, 1984). In short, individuals differ in the extent to which they inherit a nervous system that supports the higher level cognitive processing that is assessed by intelligence tests. We call this position the INS theory (innate neurological structure). The second hypothesis, exemplified in the writings of, among others, Scarr (Scarr & McCartney, 1983), argued that the genetic effect is in determining the range and quality of an individual’s environmental experiences. We call this the EPD theory, after Hayes (1962), who discussed the same issues under the term, experience-producing drives. In this view, individuals who “inherit a high IQ” actually inherit a propensity to seek out intellectually stimulating environments.

The INS theory implies that the genetic influences upon cognitive abilities will ultimately be accounted for by individual differences in neurological structures. Evaluation of this theory requires the existence of individual-difference measures of neurological processes that are (a) reliable, (b) related to cognitive test performance, and (c) inherited and that account, at least in part, for the inheritance of cognitive abilities.

Cognitive scientists have recently developed experimentally derived individual-difference measures of the speed with which simple cognitive processes (e.g., retrieval of an item from memory) are performed. The moderate but consistent and significant association between these cognitive-processing measures and psychometrically assessed cognitive ability (Vernon, in press) has been interpreted to indicate that the development of cognitive ability depends upon the efficient execution of these simple processes (Hunt, 1983). It has, moreover, been hypothesized that the cognitive-processing measures assess the efficiency of basic neurological mechanisms (Hunt, 1983). Substantial genetic effects upon these measures are, consequently, expected, and if the INS theory applies, the association between these measures and psychometric test scores should primarily be mediated by genetic factors. The present report explores the influence of genetic factors upon variation in cognitive processing measures and covariation between cognitive-processing measures and psychometrically assessed cognitive abilities.

The EPD theory suggests that there are inherited individual differences in the reinforcement salience of various cognitive activities (e.g., reading, listening to stories, disassembling and reassembling toys). These individual differences determine, in part, the frequency and range of cognitive experiences that will, in turn, affect subsequent intellectual achievement. Evaluation of this theory requires individual-difference measures of the frequency and range of intellectual experiences and the demonstration that these measures are both partially inherited and predictive of subsequent achievement.

Most attempts to characterize environmental influences upon cognitive functioning have focused upon static characteristics of the child’s rearing environment rather than upon the dynamic processes implicated as important by the EPD theory. Thus psychologists have focused primarily upon SES and its correlates in identifying relevant environmental influences upon individual differences in cognitive abilities. SES is repeatedly used in various research programs as a measure of the intellectual “environment” of the child, and intervention programs are typically targeted to the children of lower social class families in the hope that the program will overcome the “impoverished intellectual environment” in which the child is reared (Bouchard & Segal, 1985).

An association between any characteristic of a parent and any characteristic of a biological child he or she rears may reflect the influence of genetic mechanisms and/or common environmental factors. The mere observation of parent-offspring resemblance is not, consequently, sufficient evidence for inferring the existence of genetic influences. Perhaps not as widely appreciated is the fact that parent-offspring correlations do not provide unambiguous evidence for the presence of environmental influences. The correlation between parental SES and offspring cognitive ability is approximately .33 (White, 1982). The basis for this correlation may be a common genetic effect and/or a common environmental effect. The study of intact nuclear families cannot provide data that will allow us unequivocally to decide between these two explanations (cf. Plomin, Loehlin, & DeFries, 1985).

Adoption studies, by relating characteristics of the rearing parents to the characteristics of their nonbiologically related adopted offspring, are able to resolve the genetic and environmental contributions to parent-offspring resemblance (DeFries & Plomin, 1978). This chapter reports results from an application of a powerful variant of the adoption design: the study of twins reared apart. Barring sel...