EVOLUTIONARY PSYCHOLOGY AND DEVELOPMENT Earlier in this chapter we introduced the four questions proposed by

was minimal. These findings suggest that such unexpected stimulation, even if it is not immediately perceived as keenly as by an older individual, can adversely alter the species-typical course of development. Central to the per- spective of evolutionary developmental psychology is the realization that the environment and the organism interact in different ways at different periods of ontogeny. A n implication of this position is that different features may be selected at different points ofdevelopment

(W. D.

Hamilton,

1966).

We now examine aspects of evolutionary psychology that are particularly pertinent to development.

EVOLUTIONARY PSYCHOLOGY AND DEVELOPMENT

Although we believe that many features of childhood serve as prepara- tions for adulthood, it is inappropriate to believe that all adaptive aspects of childhood need to be associated with adult functioning. Rather, we propose, some aspects of infancy and childhood are not preparations for later adulthood but evolved to serve an adaptive value for that specific time in development (Bjorklund, 1997a; Oppenheim,

1981).

Developmental psychobiologists, whose typical subjects are birds or infrahuman mammals, have long held the view that immature aspects of an animal often have adaptive value and were selected in evolution to help keep the young animal alive at that time in development (Gottlieb et al., 1998; Spear, 1984;

Turkewitz & Kenny, 1982). This perspective has been less popular with developmental psychologists who study human ontogeny, whose focus has often been to find behaviors or traits early in development that are predictive of later development (see Kagan,

1996).

Many adaptations are limited to a particular time in development, facili- tating the young organism’s chances of surviving to adulthood and eventually reproducing. This is reflected

by

the concept of ontogeneticdaptations-neuro- behavioral characteristics that serve specific adaptive functions for the devel- oping animal (see Oppenheim,

198 1

). These are not simply incomplete ver- sions of adult characteristics but have specific roles in survival during infancy or youth and disappear when they are no longer necessary. For example, em- bryos of most species have specializations, such as the yolk sac, embryonic excretory mechanisms, and hatching behaviors in birds, that keep them alive but disappear or are discarded once their purpose is served (Oppenheim,

198

1 ).

Such adaptations are not limited to prenatal behaviors. Infant reflexes, such as the sucking reflex in mammals, are obvious postnatal behaviors that serve a specific function and then disappear. Some aspects of human infants’

cognition have also been interpreted as serving a specific function, only to disappear or to become reorganized later in life. For example, newborns’

imitation of facial gestures (Meltzoff & Moore, 1977,1985) has been charac- terized

by

some as an ontogenetic adaptation (Bjorklund,

1987).

Under the appropriate conditions, newborn infants will imitate a range of facial gestures, although imitation of facial expressions decreases to chance levels by about

2

months (Abravanel & Sigafoos,

1984;

Fontaine,

1984;

Jacobsen,

1979).

Several researchers have speculated that neonatal imitation, rather than serving to acquire new behaviors, which seems to be the primary function of imitation in later infancy and childhood, has a very different and specific function for the neonate. For example, Jacobsen (1979) suggested that imitation of facial gestures is functional in nursing, Legerstee ( 1991) proposed that it is a form of prelinguistic communication, and Bjorklund

(1987)

suggested that it facilitates mother-infant social interaction at a time when infants cannot control their gaze and head movements in response to social

stimulation. Heimann (

1989)

provided support for these latter interpreta- tions, reporting significant correlations between degree of neonatal imitation and quality of mother-infant interaction

3

months later. Thus, early imita- tion appears to have a specific adaptive function for the infant (to facilitate communication and social interaction) that is presumably different from the function that imitation will serve in the older infant and child (but see Meltzoff & Moore, 1992, for a different interpretation). Presumably, these different functions for similar behavior at different times in ontogeny were selected over evolutionary time.

Similar examples exist from social development. Take the case of boys’

R&T

play during childhood. One interpretation that we discussed earlier, probably the dominant one (Fagen, 1981; Groos, 1901), is that

R&T

play is an incomplete form of adult fighting. The value of

R&T

play, consequently, has been seen in terms of benefits deferred until after childhood (Smith, 1982). More recently, however, it has been suggested that play generally, and

R&T

play specifically (Martin & Caro,

1985),

has benefits during childhood. In the case of

R&T

play, boys may use it as a way in which to learn and practice social signaling; for example, exaggerated movements and a play face communicate playful intent. Further, it is used as a way in which boys establish leadership in their peer group and assess others’ strength (Pellegrini & Smith,

1998). R&T

play also has immediate nonsocial benefits;

it provides opportunities for the vigorous physical exercise that is important for skeletal and muscle development. (The role of play in development from an evolutionary perspective is the topic of chapter 10.)

From this perspective, cognitive or social immaturity is viewed in a different

light

(Bjorklund, 1997a; Bjorklund & Green,

1992;

Wellman, in press ). For instance, seemingly “immature” behavior, such as play, may have been selected as a way in which young organisms can negotiate the niche of childhood. After all, organisms must survive childhood if they are to mature and then reproduce (Pellegrini & Smith, 1998). As an example from the cognitive domain, young children’s poor metacognition, particu- larly their ability to judge the competency of their own performance, may be more of a blessing than a curse. Children who overestimate their own abilities may attempt a wider range of activities and not perceive their less- than-perfect performance as failure (Bjorklund, Gaultney, & Green, 1993;

see chapter

7).

Certain aspects of immaturity may thus be adaptive, and attempts to accelerate intellectual development, frequently advocated in the United States, may be counterproductive (see Bjorklund & Schwartz,

1996;

Goodman, 1992).

These and other findings (see Bjorklund, 1997a; chapter

6)

indicate that infants and young children respond to experiences differently than older children and adults. This interpretation, we argue, is consistent with

A NEW SCIENCE OF THE DEVELOPING MIND

39

an evolutionary developmental psychological perspective and is apt to be missed or interpreted otherwise without such a perspective.

Influence of Natural Selection at Different Times in Ontogeny

Related to the concepts of ontogenetic adaptations and the adaptive value of immaturity is the idea that natural selection has had greater impact on some phases of ontogeny than on others. Natural selection operates so that individuals with characteristics that “fit” well with the current environment reach adulthood and leave more offspring than less-well-fit individuals. The criterion for success (for reproductive fitness) is leaving more, as opposed to fewer, copies of your genes (usually in the form of offspring) in subsequent generations. For humans, this means reaching pu- berty, having children, and seeing that those children survive to become parents themselves. (It can also mean that one’s close relatives, such as brothers and sisters, have offspring, who will share a large proportion of your genes.) Thus, characteristics that increase the chances of an individual reaching adulthood, procreating, and rearing children to independence should all have positive selective value. In contrast, characteristics that promote good health beyond one’s ability to reproduce likely have little selective value. The genes that promote long life, for example, as well as those that promote midlife (post-reproduction) death, will not be selected for or against because they are expressed only after a person has had children.

Although aspects of longevity may be inherited, from this perspective it is unlikely that they have been selected in evolution.

Perhaps the best documented example of this is Huntington’s disease, a fatal, genetic disease caused

by

brain degeneration that, unlike most other fatal genetic diseases, is passed on by a single dominant gene. That is, only one gene rather than two recessive genes is necessary to display the symptoms of Huntington’s disease. This means that the likelihood of an individual with the Huntington’s gene passing it to his or her offspring is

50%.

Huntington’s disease occurs in about

1

in every

15,000

people of European descent.

Contrast Huntington’s disease with another disease also caused by a single dominant gene, progeria, which results in premature aging and early death.

Progeria occurs in about

1

in every

8

million births, or about

500

times less frequently than Huntington’s disease (Austad,

1997).

Why

the difference in frequency between the two diseases? The primary reason is that most people with progeria die before reaching their teenage years and thus rarely reproduce. The defective, dominant gene dies with them. New cases of the disease are the result of a random mutation and are thus quite rare. The gene for Huntington’s disease, in contrast, is ex- pressed only later in middle adulthood. By the time the disease strikes a

person, he or she has often had children and thus has a

50%

chance of passing the dominant gene to offspring. Natural selection works against genes that prevent individuals from reaching reproductive age (negative selection). The possessors of genes such as the one responsible for progeria die before reproducing. In contrast, natural selection has no impact on genes that are expressed late in life, such as the one responsible for Huntington’s disease, after individuals have already reproduced.

One way of expressing this effect is to say that the power of natural selection to influence genes wanes with age, or as life span psychologist Paul Baltes

(1997)

put it, “the benefits resulting from evolutionary selection evince a negative age correlation” (p.

367).

As a result, as humans age, there should be more deleterious genes that are expressed. This is seen in contemporary times with the rapid increase in occurrences of Alzheimer’s dementia late in life. During earlier times in human history, people possessing these genes died of other causes long before these late-life genes could be expressed. As a result, there were no selection pressures against such genes.

Their presence today, and that of other late-life genes that contribute to illness and death, is evidence of the failure of natural selection to influence characteristics in the latter part of the life span over human history.

Another possible reason for the phenomenon of senescence and the mitigation against longevity is that genes that promote good health and contribute to one’s selective advantage early in life may have deleterious effects later in life

(G. C.

Williams,

1957).

Genes that have multiple func- tions are referred to as pleiotropic, and it seems likely that not only are multiple genes implicated in affecting any single characteristic of an animal, but also many genes likely influence multiple characteristics.

If

a gene expression affords some benefit early in life, that gene

will

be selected for, even if the expression of that same gene later in life (or the long-term effects of a gene’s earlier functioning) has negative consequences for survival.

For example, there is some evidence that testosterone, so critical for the development of male secondary sexual characteristics and associated with aspects of social status and dominance among males, impairs the immune system (see Geary,

1998)

and increases the risk of certain cancers. Yet, the genes that promote

high

levels of testosterone may be selected because of the early benefit they afford a young male animal at the expense of increased vulnerability to disease later in life.

It

is possible that characteristics of old age may have some positive selective value, if, by living longer, more copies of one’s genes, usually in the form of grandchildren, survive. That is, perhaps long-lived people, although no longer reproducing themselves, may foster their grandchildren’s survival. This possibility has been proposed (e.g., Euler & Weitzel, 1996) and is discussed in chapter 8.

A NEW SCIENCE OF THE DEVELOPING MIND 41