EVOLUTIONARY DEVELOPMENTAL PSYCHOLOGY, BEHAVIORAL GENETICS, AND INDIVIDUAL DIFFERENCES

One of the lasting contributions of Darwin to psychology was a focus on individual differences (Charlesworth, 1992). Variation is the stuff upon which natural selection works, and differences among individuals and the origins, maintenance, and modification of such differences has been a major focus of developmental psychology (Bjorklund, 2000; Cairns, 1979). The

formal, scientific study of individual differences can be traced to the polymath Francis Galton (1822-191 l), a first cousin of Darwin’s. After reading On the Origin of Species, he wrote to Darwin “your book drove away the constraint of my old superstition as if it had been a nightmare” (cited in Shipman,

1994,

p. 11

1).

Galton’s words were not empty ones. He turned his immense curiosity and energy to a lifetime study of the inheritance of human mental abilities. Thus, the systematic study of human individual differences had an explicitly Darwinian beginning, a perspective that modern behavioral geneticists maintain (Plomin, DeFries, McClearn, & Rutter,

1997;

Scan,

1995a).

Social Darwinism and Other Misuses of Evolutionary Theory

Another early advocate of Darwin’s theory was social scientist Herbert Spencer (

1820-1903),

whose influence in promoting Darwin’s theory, particularly in Great Britain and the United States, was considerable. However, what Spencer is remembered for today in most circles is the social Darwinism movement. Whereas Darwin applied natural selection to biological life, Spencer envisioned a broader application of the concept, believing that it typified every form of existence, from the origin of the solar system to human society (Shipman, 1994). Unlike Darwin, Spencer, who coined the phrase

“survival of the fittest,” was convinced that evolution was progressive, always moving toward perfection. When applied to society, it resulted in the very self-serving position that both social success and failure are inherited characteristics, and it is nature’s way to weed out those who are unsuccessful.

Thus, Spencer argued against social programs such as poverty laws, universal education, and housing regulation that would artificially help poor people (ix., the biologically unfit) and thus impede the progressive improvement of society. Differences in social success between races and social classes were not attributed to differences in educational or economic opportunities

but

to some unspecified mechanism of inheritance. Views such as these were not original to Spencer but had been advocated by people in political and economic power before; what was new was the scientific justification for the notions of inherited superiority and inferiority. Although in these pre- Mendelian days Spencer lacked a mechanism for inheritance, Darwin’s theory provided the scientific “proof’ that the rigid British class system was

“natural” and that attempts to fiddle with it were misguided.

At the same time, Galton was advocating a similar program. In his 1869 book Hereditary Genius, he traced the family histories of successful men over a 200-year span. He observed that “genius” runs in families and, like Spencer, attributed the phenomenon to inheritance, not opportunity.

Galton coined the term eugenics, a proposed science that would improve

72

THE ORIGINS OF HUMAN NATURE

the condition of the human species by selective breeding, much as cattle are improved

by

ranchers.

In the United States during the early decades of the 20th century, the new field of intelligence testing was taking off, prompted in large part

by

Galton’s earlier work (as well as the research of the French psychologists Alfred Binet

[1837-19111

and Theophile Simon [1873-19611). Lewis Ter- man (1871-1956) and Robert Yerkes

(1876-1956),

both giants of early American psychology, undertook a mass testing of

1.75

million recruits to the

U.S.

Army in World War

I.

Two forms of the test were constructed:

the Army Alpha, which was a paper-and-pencil test used for recruits who could read English; and the Army Beta, which was used for recruits who were not fluent in English. Despite many difficulties in administration, the findings of the tests were considered to be reliable, perhaps because they confirmed the preconceived notions of the researchers. The results showed lower levels of intelligence of Black people and more recently arrived immigrants from southern and eastern Europe than for native-born White people or those who had lived in the country for longer periods of time (mostly of northern European descent). Figure 3.1 presents the average mental age

11.01 Î Î Î Î

0-5 ^Years 6-1 ^{0 Years} 11 -1 ^{5 Years} 20+ ^Years Years of Residence

Figure 3.1. Average Mental Age as a Function of Years of Residence in the United States, from U.S. Army Alpha and Beta Tests.

Note. Data from The Mismeasure of Man (p. 221), ^byS. J. Gould, 1981, ^{New York:}

Norton. Copyright 1981 by S. J. Gould. Adapted with permission.

of recruits as a function of years of residence in the United States (adapted from

S. J.

Gould, 1981). As can be seen, mental age varied monotonically with years of U.S. residence. However, rather than viewing these data as a consequence of the testing procedures for non-English speakers or in terms of differing degrees of cultural experience, the researchers proclaimed that the more recent immigrants to the country were innately less intelligent than the earlier immigrants (Brigham, 1923).

The theories of the IQ-testers and of Spencer and Galton extended beyond the ivory tower and influenced social policy

by

those who were all too willing to believe and influenced the formulation of social policy

by

ethnically biased legislators. In the United States, the

1924

Immigration Act limited immigration to

2%

of those of the same nation that were living in the United States in 1890, when northern Europeans predominated.

Sterilization laws were passed in 16 states between 1907 and 1917, making it legal to sterilize many classes of criminals as well as people with epilepsy, people with a drug addiction, people with mental retardation, and those considered insane (Shipman, 1994).

Improvements in

IQ

testing continued throughout the 20th century and became the primary arena for debates concerning the heritability of intelligence. The field of behavioral genetics emerged, which examines the similarity of characteristics among people of different degrees of genetic relatedness and provides statistical estimates of the degree to which variation in these characteristics are attributed to inheritance. In Great Britain, the major proponent of a gene-based theory of intelligence was the influential educational psychologist Sir

Cyril

Burt (1883-1971). In a series of studies conducted both before and after World War

11,

Burt demonstrated that the IQs of groups of identical twins who had been separated early in life were remarkably similar, even though they had been reared in different environments. The results, clearly supporting a strong genetic view of intelligence, were seriously questioned after his death in 1971. Burt had published three studies comparing IQ-test scores of three groups of separated identical twins.

The numerical results in the three studies were exactly the same-a coinci- dence so unusual to be considered impossible, due either to fraud (Kamin,

1974)

or, according to more recent evaluations, carelessness (Mackin- tosh, 1995).

Excesses of social Darwinism were also found in some segments of European society. In Germany, Haeckel became an ardent proponent of Darwin’s theory and promoted it vigorously in his homeland. However, as

did

many researchers of his time, he saw evolution as being progressive, and for Haeckel, this meant that the different races reflected different levels of perfection, with the pure German, or Aryan, race (the Volk) being “imbued with power and intrinsic goodness and mystically tied to their holy German landscape’’ (cited in Shipman, 1994, p. 95). Evolutionary theory justified

74 THE ORIGINS OF HUMAN NATURE

Parent-Adoptive Offspring Adoptive Siblings Parent-Off spring-Apart Siblings- Apart Parent-Off spring-Together Si blings-Together Dizygotic Twins-Together Monozygotic Twins-Apart Monozygotic Twins-Together

0.0 0.2 0.4 0.6 0.8 1

.o

Correlations figure 3.2. Average Correlations of Familial Studies of Intelligence.

Note. Adapted from data from “Familial Studies of Intelligence: A Review,” by T. J.

overt racism for Haeckel, and he advocated that German authorities be given the power to enhance and protect the race through eugenics (Ship- man,

1994).

Given these early misappropriations of evolutionary theory to human variation, it is little wonder that scientists were reluctant to apply Darwinian principles to the study of human individual differences. Gradually, however, Darwinian thinking returned to the issues of individual differences and has produced an impressive corpus of results illustrating the heritability of a wide range of characteristics from intelligence through religiosity (see McGuffin et al., 2001; Plomin et al., 1997).2 For example, research conducted since the uncovering of the Burt controversy has found that

IQ

scores between people vary predictably as a function of their degree of genetic relatedness (Bouchard

& McGue,

1981;

Bouchard, Lykken, McGue, Segal, & Tellegen, 1990;

Plomin & Petrill,

1997),

reflecting a substantial heritability of intelligence (see Figure

3.2).

Theory in modem behavioral genetics is explicitly Dar-

’Heritability is the extent to which differences in any trait within a population can he attributed to inheritance. Heritability is expressed as a statistic that ranges from 0 (none of the differences in a trait are attributed to inheritance) to 1.0 (100% of the differences in a trait are attributed to inheritance). It reflects the proportion of variance in an observed trait that is due to genetic variability. Heritability is a population statistic, in that it describes average differences among people within a population. It does not refer to how much of any one person’s intelligence (or height or personality characteristics) can be attributed to genetic factors, only what percentage of the difference in a trait within a specific population can be attributed to inheritance, on average.

Heritability of a trait can vary over time as environments vary. The more homogenous environments are among people within a population, the higher heritability will be.

winian, at least among some prominent practitioners (Harris,

1998;

Scarr, 1995a).

In contrast, evolutionary psychologists have generally steered away from individual differences. Rather, evolutionary psychology has primarily been concerned with species universals-characteristics that typifY all members of a species (or all of one sex of a species) at certain times in development.

To

many evolutionary psychologists, individual differences are simply noise (such as whether a belly button turns inward or outward)-uninteresting variation based on underlying evolved universals (Buss, 1995). But variation is at the very core of Darwinian theory, and so any evolutionary account that views variability among individuals as simply noise is likely missing some important points. According to developmental psychologist and behavioral geneticist Sandra Scarr ( 1995a), evolutionary psychologists have applied Darwin’s idea of natural selection to understanding human nature but have virtually ignored variation. A notable exception is Gottlieb ( 1983), who considered the study of individual differences to be a defining principle in the study of development.

Developmental Systems Approach and Behavioral Genetics:

Different Views of Individual Differences

Child

developmental psychology is concerned

both

with universals (developmental function) and with individual differences, and an evolutionary approach to development encounters the same criticisms as does evolutionary psychology in general with respect to how it deals with individual differences. In fact, a perspective that emphasizes the expression of epigenetic rules through interaction with the environment, as does the developmental systems perspective favored here, has been criticized for making individual differences interpretable only after the fact. There are so many interacting factors at so many different levels of development that individual differences can never be predicted with any accuracy (Scarr, 1993). This is in contradic- tion to what is known about the stability of many individual differences over time (Bayley, 1949; Schneider, Perner, Bullock, Stefanek, & Ziegler, 1999) and about the heritability of many psychologically important traits (Plomin et al.,

1997).

Behavioral genetics research provides a means of predicting and explaining individual differences

by

using an underlying evolutionary theory that, at one level, is substantially different from that advocated

by

the developmental systems approach.

Perhaps the best developed behavioral genetics approach with respect to development is Scarr’s genotype + environment theory (Scarr & Mc- Cartney, 1983; Scarr, 1992,

1993;

see also Plomin, DeFries, & Loehlin, 1977). In this theory, there are three types of genotype ⁺environment

76

^THE^ORlGlNSOF HUMAN NATURE

effects: passive, evocative, and active. Passive effects occur when genetically related parents provide the rearing environment of the child. When biological parents rear a child, the effects of genetics and environment cannot be separated because the people who provide the genetic constitution for a child also provide the environment. The influence of passive effects is proposed to decline with age. Evocative effects occur when the child elicits responses from others that are influenced

by

his or her genotype. For example, people respond differently to an irritable child than to a well-tempered child, and the type of attention received

by

an infant who likes to cuddle is different from that received by an infant who does not want to be held.

Evocative effects are proposed to be stable over development.

For our purposes in this book (and for Scarr’s), active genotype ⁺ environment effects are the most important. According to this model, a child will seek environments that are compatible with his or her genotype.

This

is captured by the phrase “genes drive experiences.” The experiences children have

will

shape their personalities and intellects, but it is one’s genotype that determines, to a large extent, which environments are sampled (see also Bouchard et al., 1990; Harris, 1998; Plomin & Daniels,

1987).

Active effects increase with age, as children are increasingly able to choose their own environments. Children thus develop consistent with their genotypes. Children with identical genotypes (monozygotic twins) develop simi- larly, and children with different genotypes develop differently.

At one level, the behavioral genetic position gives a substantial role to the environment in shaping individual differences-experience is most directly responsible for crafting important aspects of cognition and personality. But genes drive children to seek these environments (McCartney, Harris,

& Bernieri, 1990; Plomin & Daniels, 1987). Scarr (1995a) contended that children

will

develop characteristics consistent with their genotype in all but the most impoverished environments. Similar to the developmental systems approach, a species-atypical environment will restrict children’s options and thus limit their development. But differences among ordinary environments

will

have little impact on the expression of characteristics, and individual differences will best be predicted based on a child’s genotype.

This is because, according to Scarr (1992, 1993), evolution does not permit easy environmental modification of characteristics that are essential for survival. Rather, important aspects of social, emotional, and cognitive development are

highly

canalized and are not much influenced

by

the vagaries of parenting behavior.

It

would be a short-lived species that had an extended childhood and a narrow range of parental behaviors necessary to rear a child to adulthood.

Based on these observations, Scarr (1992) proposed the concept of

“good-enough” parenting. Because children will develop according to their

genotype in all (‘ordinary” environments, it is not necessary to be “superpar- ents” to rear healthy and (re)productive members of the community. More is said about Scarr’s ideas about parenting in chapter

8.

Behavioral genetics research has generally confirmed many of Scarr’s observations, particularly for intelligence as measured

by IQ.

For example, identical twins reared apart are highly similar to one another on a wide range of traits (see Bouchard et al., 1990; Plomin et al., 1997), whereas siblings become less alike the older they get (McCartney et al., 1990; Plomin

& Daniels,

1987;

Scarr & Weinberg,

1976;

Turkheimer & Waldron, 2000),

presumably because the differences in their genotypes (they share

SO%

of their genes compared to the

100%

for identical twins) lead them to different experiences, with these active genotype ^--tenvironment effects increasing with age for many characteristics.

These findings should not be viewed as indicating that postnatal environment has little influence on children’s developing intellects and personalities. Most behavioral geneticists would place the heritability of intelligence at between

.S

^and

^.6

and the heritability of personality somewhat lower

(J. R.

Harris, 1998; Plomin & Daniels, 1987; Plomin et al., 1997; Plomin

& Petrill,

1997). This

means that

SO%

or more of the variance for important

psychological characteristics are attributable, presumably, to postnatal environmental factors. However, the

bulk

of environmental effects on

IQ

and personality remain unspecified (so-called nonshared environmental effects; see discussion below).

A

smaller proportion of the differences in

IQ

and personality is attributed to shared environment effects, a result of siblings growing up in the same family. Shared environment, the family environment that siblings share, independent of shared genes, is also discussed below.

The modest effects of shared environment on

IQ

are illustrated in research by Segal

(2000),

who developed the virtual-twin method to assess the joint contribution of genes and shared environment on

IQ.

Virtual twins were defined as unrelated children of about the same age who grew up in the same family from early infancy. For instance, two adopted siblings, or a biological child and an adopted child, fewer than

9

months apart in age would be virtual twins. In such cases, they share the same family environment, similar to that shared by monozygotic or dizygotic twins, but are genetically unrelated. Segal reported a correlation between the IQs of the virtual twins of .26-significant, but substantially less than that found between monozygotic twins

(.86),

dizygotic twins

(.60),

or full siblings

( S O ) .

This finding is consistent with the argument that shared environments have only small effects on intellectual development.

How does a behavioral genetics approach differ from the developmental systems approach? Although both take an explicitly evolutionary perspective, they have a different view of what underlies development. As advocates of a developmental systems approach have emphasized (Gottlieb, 1991a,

78

THE ORIGINS OF HUMAN NATURE

1998; Lickliter &Berry, 1990; Oyama, 2000a,

2000b),

there is no straightfor- ward relation between a gene and a behavior. Development proceeds via the bidirectional relation between all levels of organization, including the genetic. Moreover, the effects of genes and environment on any particular phenotypic characteristic cannot literally be partitioned, as is done via statistical techniques

by

behavioral geneticists.

As

we have stressed throughout this book, genes are part of a developmental system, with continuous and bidirectional interaction between adjacent levels within the system, ranging from the

DNA

through the culture.

Behavioral geneticists, in contrast, look for genetic links to behavior and have actually found genes associated with

IQ,

and thus, to certain aspects, associated with intelligence (see Chorney et al., 1998; McGuffin et al.,

2001;

Plomin & Rutter,

1998).

Gottlieb (1995a) has interpreted the claims of behavioral genetics as being unidirectional, genes ⁺behavior, rather than bidirectional, as postulated by the developmental systems approach.

Of

course, behavioral geneticists understand that the connection between genes and behavior is multilevel and complex; they believe that Gottlieb has misinterpreted contemporary behavioral genetics theory and exaggerated the uncertain course of development, and they balk at the label of “genetic determinists” (Burgess & Molenaar, 1995; Scarr, 1995a;

Turkheimer, Goldsmith, & Gottesman, 1995). Nonetheless, we believe that Gottlieb is correct in stating that behavioral genetics research typically simplifies the notion of “the environment,” in part

by

not explicitly acknowl- edging the role that self-produced function may play in early development and

by

failing to objectively define specific aspects of children’s environments that contribute to the characteristics under study. For example, in most behavioral genetics models, nonshared environmental effects account for the greatest amount of nongenetic variance (Harris,

1998;

McCartney et al., 1990; Plomin & Daniels,

1987;

Scarr & Weinberg, 1976; Turkheimer

& Waldron,

2000).

Nonshared environments refers to siblings’ unique experi-

ences, those not experienced

by

the whole family. Such nonshared environmental effects are usually not objectively measured, but when they are, they account for only a small portion of the nongenetic variance (Turkheimer

& Waldron,

2000).

One interpretation of this finding is that there are other

(nonshared) environmental factors that are not being measured in current behavioral genetics research.j

’In addition to proposals about the source of nonshared environmental effects, some theorists have proposed that shared family environment effects are larger than have been assumed. For example, Stoolmiller (1999) proposed that statistical anomalies, associated with restricted range of family environmental qualities in adoption studies, greatly underestimates the degree of shared family environment, and when this is taken into consideration, the amount of variance accounted for by shared family environment increases substantially.

Dalam dokumen The Origins of Human Nature Evolutionary Developmental Psychology (David F. Bjorklund, Anthony D. Pellegrini) (Halaman 75-89)