Lifespan Development Research Paper

1. Evolutionary Assumptions

Evolutionary theorizing considers striving for optimal reproductive success as the ultimate goal also of human development. This assumption not only specifies that the proximate regulation of physical and biological systems, but also social behavior and the complex psychology of human beings, follow a reproductive logic. Reproductive success is expressed in terms of inclusive fitness which comprises the individual’s own procreation (Darwinian fitness) and the procreation of relatives with whom the individual shares genes (indirect fitness), thus establishing the principle of kin selection (Hamilton 1964). Therefore, the evolutionary perspective focuses on the gene as the unit of analysis. However ‘… it is not the naked gene, that is exposed to selective forces directly’ (Mayr 1994, p. 206), but rather the ‘realized animal, that lives or dies, breeds or helps the relatives’ (Daly and Wilson 1983, p. 32). Although individuals are considered as ‘vehicles’ of their genes (Dawkins 1976), the importance of the phenotypical individual for determining reproductive success has recently been (re)emphasized (Laland et al. 2000). This view has implications for the metatheoretical perspective on the nature of humans. It can be assumed that genetic ‘egoism’ is correlated with respective psychological characteristics, thus leading to the consequence that humans are not regarded as altruistic and cooperative per se, but rather as a result of—possibly implicit and non- intentional—selfish cost–benefit calculations.

However, it is not the adult individual who represents the product of evolution; rather, the whole lifespan with the different developmental phases is shaped by selective forces with the eventual exception of senescence, which itself seems not to be a product of selection. Aging is a concomitant of sexually reproducing individuals because the generational change increases the speed of evolution. The inevitability of death due to environmental stressors during human history has resulted in a decrease of individuals with increasing age, so that the power of natural selection decreased also (Medawar 1952).

Life-history theory (Alexander 1990) regards the developmental course of the individual lifespan as the result of a trade-off between investment into one’s own growth and development (somatic effort) and investment into reproduction, comprising the functional systems of mating and parenting (reproductive effort; Chisholm 1996). Somatic efforts accumulate reproductive potential whereas reproductive efforts decrease the residual reproductive value. Evolution has thus shaped humans into being reproductive strategists who constantly have to make ‘decisions’ about the best allocation of investment (Voland 1998). The heuristics of reproduction are contingent upon environmental demands, which comprise material and ecological forces as well as social complexity (see Dunbar 1996). Especially the evolution of intelligence as a necessary prerequisite for exploring environ- mental resources and regulating the social behavior of expanding groups favored learning as a second asset of adaptation. Yet learning is not random, but follows (genetically imposed) biases and constraints (‘central tendencies,’ MacDonald 1997; ‘epigenetic rules,’ Lumsden and Wilson 1981) to acquiring specific environmental information during specific phases of development allowing easy learning at these points in time (Boyd and Richerson 1985).

Psychological development across the lifespan can simultaneously be interpreted as a structured sequence of developmental tasks in order to improve social and cognitive skills as well as the formation of a reproductive style. Evolutionary socialization models have described the ontogeny of individuals’ reproductive styles as a meaningful passage through the developmental tasks of childhood and adolescence. In particular, the resource situation and the family climate during early childhood are considered to be predictive of attachment formation. The quality of attachment in turn influences psychological adjustment during late childhood. Psychological development also influences neurophysiological, maturational processes, leading to the somatic consequence of defining the beginning of puberty. The beginning of puberty channels the reproductive style, especially with regard to the age at which first giving birth, the spacing, and number of children with the respective implications for parental investment (Belsky et al. 1991, Keller 1996). Empirical studies from different parts of the world have supported the view of different developmental trajectories as specified by these models. The new perspective of evolutionary theorizing is thus the assumption that neurophysiological and psychological regulations of behavioral development (proximate causes) are influenced by adaptive, i.e., phylogenetically shaped, mechanisms which serve the ultimate function of optimizing reproductive success. The postreproductive phase is therefore not subject to the same degree of selection pressure as the prereproductive and reproductive life phases (Baltes 1997, Geary and Bjorklund 2000). Accordingly, old age is not a distinct phase in stage conceptualizations of the life-history approach (Bogin 1997).

In the following, the functional significance of the different developmental phases for reproductive success will be analyzed. The developmental phases of infancy, childhood, adolescence, adulthood, and old age will be briefly characterized with respect to their possible evolutionary roots. Old age is included in this analysis, since the assumption of structural continuity implies that earlier experiences shape later ones, so that the quality of old age could be regarded as a consequence of the earlier reproductive life (Keller 1991).

2. Developmental Phases Across The Lifespan

2.1 Infancy

The extreme helplessness (‘altriciality’) of the human newborn can be considered as part of a reproductive strategy to becoming a better adult (‘better adult hypothesis,’ Alexander 1990), since it allows the baby to invest all available resources in growth and development. This view is in contrast with the common interpretation of infants’ helplessness as a consequence of a physiological preterm birth, due to hominid brain growth. Early development is shaped by the investment that parents allocate to each child individually, depending on their own and the child’s reproductive values (e.g., age of parent, social support, sex, and birth position of child). Early experiences form the basis of the psychological adaptation to the respective physical and social environment in prompting phenotypic development. Accordingly, infants adopt internal representations of social relationships and a primary conception of the self, focusing on the expectations of reciprocity as the balance of give and take that can be expected in future social relationships.

2.2 Childhood

At about 2 years of age, a developmental transition with a new focus on the extension of the physical and social range of children’s activities can be observed in many cultural contexts. This marks the beginning of children’s apprenticeship which involves the adoption of promising strategies for acquiring resources in their particular environment. Mediated through the culturally primed beliefs and values (‘ethnotheories’) of their social environment, children acquire and construct their environmental and social competence through participatory learning as well as by adopting and creating developmental niches (‘niche picking’) which include the implicit and explicit reflection of sex differences as concomitants of adult reproductive life.

2.3 Adolescence

Adolescence constitutes the transition from childhood to adulthood. The beginning of adolescence is universally defined by entry into puberty. The duration of adolescence, however, differs substantially across cultures, varying from transitional rituals during a few weeks to a stage which last several years, depending mainly on economic constraints and educational provisions (see Schlegel and Barry 1991). The attainment of sexual maturity is a biological marker variable with a hereditary component and it is contingent upon ecological factors, such as nutritional status and physical condition, as well as previous socialization experiences, especially the security and stability of the early home environment. At the same time, this marker variable is believed to be predictive of future reproductive decisions (e.g., age at first sexual intercourse and age at first childbirth, duration of interbirth intervals). Although similar developmental trajectories for males and females have been empirically identified, sex-specific reproductive strategies with their respective costs make it likely that environmental factors influence female fecundity more strongly than male. Therefore, and owing to the greater accessibility of menarche as an empirical datum, female developmental trajectories are overrepresented in the literature. Psychosocial stress in the family of origin, especially marital conflict and father absence, has been documented as decreasing the age of menarche in different contextual systems. Particularly the divorce of parents during a child’s first five years of life could reliably be related to early menarche (Chasiotis et al. 1998). The perception of this environmental scenario may support the development of a more quantitative oriented reproductive style with lesser parental investment whereas marital stability could be considered as part of a qualitative style with higher maternal and paternal investment. Thus, low ecological stress (i.e., good nutritional status and a balanced physical condition) as well as high psychosocial stress (e.g. divorce of parents before the age of five years) can result in earlier maturation, whereas high ecological stress and/or low psychosocial stress can lead to later maturation. The main reproductive function of the variability of sexual maturity is to reduce the costs of premature reproduction. If an individual ‘waits’ for sexual maturity, the resources can be allocated to growth which will increase the reproductive value at maturity. On the other hand, the delay might decrease the probability of surviving until the reproductive age (Charnov 1989).

The onset of reproduction varies with the duration of adolescence across cultural contexts. Especially in traditional rural societies, reproduction begins soon after sexual maturity and seems to be more quantitatively oriented, whereas reproduction in urban educated societies is delayed until the end of an educational moratorium and seems to be more qualitatively oriented.

2.4 Adulthood

In evolutionary terms, adulthood is the phase of reproduction with the challenge to resolve various allocation conflicts. First, the conflict between current and future reproduction (i.e., investment in growth or reproduction) has to be addressed, culminating in the decision of when to start reproduction, and the related issue of nepotistic effort, i.e., investing in one’s own offspring or temporarily (or even permanently) adopting the role of a ‘helper in the nest’ (Voland 1998). The second allocation conflict concerns the question of investment in quality or quantity of offspring, which determines the number and spacing of children. Since these decisions not only have to be made generally as an expression of reproductive style, but also individually for each offspring separately, parent–child conflict is an inevitable consequence (Trivers 1974). Survival and psychological adaptation are facilitated when not only maternal, but also paternal investment is spent on the infant. It is assumed that partner bonding and family cohesion evolved as psychological mechanisms to facilitate the development of ‘better adults.’ This view seems to confirm the relationship between father’s absence and (increased) infant mortality in traditional societies, e.g., among the Ache Indians in South America (Hill and Hurtado 1996).

However, the determinants of reproductive success differ for males and females. Females per se make a higher investment owing to intrauterine pregnancy and nursing, supporting a qualitative parenting style with higher investment in fewer offspring. They have to decide whether they want to continue to look for a better mate or start reproducing. The prototypical male strategy, on the other hand, should be more quantitative, that is, to invest less in more offspring, due mainly to the fact that it is difficult for a male to know precisely whether he is the father of an infant (‘paternity insecurity’). Moreover, males show greater variability in their reproductive rates, from having no children to having many children. Men have to decide between the ‘cad versus dad’ strategies, i.e., maximizing fertilization or parental investment (Voland 1998). The trade-off that individuals have to accept therefore entails the balance of mating and parenting efforts. Studies on partner selection have documented universal preferences for the two sexes: females prefer partners with characteristics indicative of economic success, whereas males prefer females with characteristics that indicate reproductive capacity (youth, health). In addition, there is preference by both males and females for mates who are similar to themselves in a variety of characteristics (‘assortive mating’). Parental investment constitutes the intergenerational link through which the offspring experiences the socialization style that the parent has developed as a result of their own past as well as current experiences. In the case of human beings, not only time and energy as parameters of investment decisions have to be taken into consideration, but also psychological care systems which prepare the psychology of the phenotypic adult (Keller 2000).

2.5 Old Age

Old age covers the postgenerative phase of the lifespan. Yet it has been proven meaningful to differentiate a third young–old age span from about 60 to 75 years where the vital capacity has improved to compensate for about five years due to cultural evolution, from a fourth age span from around 80 to 100 years (cf., Baltes 1997) which has emerged due to increasing life expectancy, but where senescence has predominantly detrimental effects. These might originate as byproducts from possibly two processes: (a) selective neutrality of the genome by no effect at early life and deleterious effect at later life and (b) antagonistic pleiotropic gene action that supports reproductive success during the reproductive life stages (Martin et al. 1996). One detrimental factor that has been identified across species may be metabolic oxidative damage operating in macromolecules. On the other hand, it has been argued that the extension of the lifespan serves individual fitness by supporting the development of the child’s and grandchild’s generation. Although this ‘grandmother’ hypothesis (Peccei 1995) is controversial, researchers nevertheless report that being a grandparent increases life satisfaction in old age. The physical deterioration especially of the old–old age requires extra societal cultural resources which at the same time become more difficult to be exploited effectively. Accordingly, the regulation and compensation of losses become a major mechanism for ontogenetic processes (Baltes 1991). Relating interindividual differences in regulatory competencies to the earlier reproductive life histories might be a challenge for lifespan psychology.

3. Outlook

Although essential questions such as the interplay of genetic and social influences on phenotypic development remain open, the evolutionary view on the human lifespan offers a stimulating perspective on many controversial issues of psychological development, e.g., the nature of sex differences or the normative character of secure attachment relationships. During their ontogenetic development, individuals, based on universal genetic programs (e.g., for language acquisition, partner preferences, parenting styles), acquire the contextually relevant ecological and social and cultural information (e.g., mortality rate of their population, economic constraints, cultural values) both intentionally and intuitively to promote their psychological development so that it is adaptive for the reproductive style that promises best inclusive fitness in the given situation. Old age challenges the cultural nature of humans in a very special way, since trade-offs are no longer associated with epigenetic rules or central tendencies. Humans therefore have to use the accumulated knowledge of the previous life stages in order to redefine and redirect the allocation of resources and balance universally evolved behavioral heuristics, ontogenetically acquired styles, and situational regulations, making each life history unique. The question of assumed ultimate causes, ‘how do these behavioral regulations or these motivational systems contribute to reproductive success,’ might allow us to reframe empirical findings and adapt research designs accordingly.

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