The term dynamic systems theory has recently been introduced to describe a new and well-articulated theory of development (Thelen and Smith, 1994). It is strongly based upon the principles of general systems theory, which means, of course, that, despite its name, it is not unique in its dynamic emphasis. As with Bronfenbrenner's most recent (bioeco- logical) theory, central notions are holism, mutual influence within and between traditionally separate levels of analysis, and a focus on process rather than structure. However, the two theories differ strongly in the emphasis placed upon biology. Although Bronfenbrenner now acknow- ledges the biological (specifically, the genetic) contribution to development, he does not detail biological processes at all. On the contrary, Thelen and
Smith set out deliberately to create a theory that was biologically valid, to the extent that they explicitly reject models of development that use machine or computer analogies, and use biologically appropriate termin- ology instead. They question the common usage of the term 'biological' to refer exclusively to genes, neurology, hormones, etc. - the things that psychologists call 'biological bases' of behaviour. They argue that aspects of the environment, such as social environments that enable language to develop, are no less biological.
In addition to biological plausibility, Thelen and Smith spell out five other requirements for a developmental theory: to understand the origins of novelty; to reconcile global regularities with local variability, complexity and context-specificity; to integrate developmental data at many levels of explanation; to understand how local processes lead to global outcomes;
and to establish a theoretical basis for generating and interpreting empirical research.
They argue that the appearance of development as being orderly, progressive, incremental and directional, which gives the impression of a pre- determined plan, is illusory. For example, the development of infants' ability to walk has long been regarded as a series of invariant stages determined by maturation, as described by Gesell (see Chapter 3); Thelen and Smith pres- ent evidence that, on closer inspection, development is much 'messier' than this, with component processes moving along in fits and starts, and some- times regressing rather than progressing. On closer investigation, then, the grand plan seems to disappear, as it does also if one examines cognitive development. Effectively, group averages are masking individual differences that are important for understanding the true nature of development, a point also made by Hinde (1992a).They observe that descriptions of typical devel- opmental stages have become mistaken for explanations.
Thelen and Smith also argue that the notion that there is a 'plan for the adult' encapsulated in the genes is illogical, as is any 'plan' encoded in the environment. Both these extreme views, they maintain, lead to an impos- sible infinite regress of coded instructions for development, which is not solved by interactionism: a theory of development must explain how novelty and complexity in structure and function arise, and simply claim- ing that the answer is 'through gene-environment interactions' does not explain how new forms and behaviours come about. Their answer is that one has to explain development from the bottom up, not from a top-down plan. Various factors interact to create novelty in the way described above for general systems theory - so that new properties emerge, rather than being predetermined. An important feature of this is that some abilities may be present but hidden, only coming into play when other aspects of development catch up and enable them to operate. For example, it has been discovered that infants can step with alternate legs on a treadmill from early infancy, if their weight is supported; normally, this ability is not
apparent until they can also support their own weight, and balance, in order to walk (Thelen and Smith, 1994). This conception helps to address one of developmental psychology's perennial questions - 'Does develop- ment happen in discrete stages or is it a continuous process?' The answer may be that it is both, with continuity in subabilities or attributes leading to relatively sudden change as the subabilities operate together to form an emergent property.
Piaget's theory fits well with this perspective, being organismic and systemic, but Thelen and Smith note that much research inspired by his theory has been more concerned with structure than process. Apart from general systems theory, other important theories that have influenced their work include Edelman's (1992) theory of neuronal group selection and Eleanor Gibson's work in ecological psychology and the notion of affor- dance (see Chapter 3).
From the perspective of their theory, Thelen and Smith say that one of the questions that has so vexed psychologists over the years - 'What is innate and what is learned?' - is uninteresting. This echoes earlier com- ments by Jeffrey Gray (1985) that the fondness psychologists have for (false) dichotomies often stands in the way of understanding the phenom- ena at hand. Psychologists are as fond of linear causality and traditional research and analytic methods as they are of dichotomies; such approaches do not lend themselves well to investigations in the mould of dynamic sys- tems, and this presents some real challenges for developmental psychology.
Evolutionary developmental psychology
As mentioned in Chapter 2, it has been proposed that evolutionary devel- opmental psychology is another approach that has the potential to act as an integrating force. Geary and Bjorklund (2000) describe it as an emerg- ing interdisciplinary field, a sister discipline to evolutionary developmental biology, with the goals of identifying 'the social, psychological, cognitive, and neural phenotypes that are common to human beings, and to other species, and to identify the genetic and ecological mechanisms that shape the development of these phenotypes and ensure their adaptation to local conditions' (2000: 57).
The above definition makes it clear that this field is inclusive of all aspects of development traditionally studied as separate subdisciplines.
Geary has proposed a hierarchically organized system of modules and sub- modules of the mind that enable the individual to manage the social en- vironment (e.g. theory of mind) and the natural environment (e.g. spatial representation). These modules have evolved to subserve survival and reproduction, and one issue for evolutionary developmental psychology is to relate to modern environments mechanisms that evolved under past evolutionary pressures. This level of explanation is, of course, like genetic
determinism, concerned with ultimate functions of behaviour and develop- ment, rather than with mechanisms, understanding of which must draw upon other theoretical frameworks. In making this point, Dunbar has applauded the potential of evolutionary psychology to 'weld together the innumerable cracks that threaten to tear psychology apart' (2001: 421).
We discussed in Chapter 2 the criticism that evolutionary psychology creates post hoc explanations. As noted by Ketelaar and Ellis (2000), critics observe that it accounts for an endless range of phenomena, and when the phenomena change the explanations change: essentially, evolutionary- derived hypotheses are unfalsifiable. While Ketelaar and Ellis accept that the field was indeed open to such charges in the past, they argue that it has now matured, and is able to produce surprising and testable new hypoth- eses. The essence of their argument rests upon adopting the philosophy of science articulated by Lakatos, which we outlined in Chapter 1. They suggest that evolutionary theory constitutes a 'metatheoretical research program', based on certain core assumptions. This core is surrounded by a 'protective belt' of middle-level theories such as attachment theory, parental investment/sexual selection theory and reciprocal altruism theory.
Rather than experimental results leading to all-or-none acceptance or rejection of the basic metatheory, they should contribute to making a decision about whether the metatheory in general is progressive or degen- erative. Thus, even if the weight of evidence were, say, to destroy attach- ment theory, provided the weight of evidence still favoured other middle-level theories, the basic evolutionary metatheory would still stand.
Ketelaar and Ellis argue that evolutionary theory has the status of a pro- gressive metatheory, in being able both to accommodate major anomalous findings and to generate novel predictions and explanations. An example of the former is altruism, which was initially considered a major threat to evo- lutionary theory, but was later accommodated within it. An example of the latter was a challenge to the accepted 'fact' that males are superior to females in spatial ability: it was predicted, and supported by various stud- ies (e.g. McBurney et a/., 1997), that males would tend to be superior on mental rotation tasks, based on the assumption that ancestral males need- ed such skills to hunt and kill animals, while females would be superior in remembering static locations of objects, based on an ancestral role in gathering food from static sources.
In adopting an evolutionary approach, it will be important for psych- ologists to ensure that their theories about the operation of ancient societies do not become outdated. Also, postmodern approaches to scientific inquiry should lead us to maintain a healthy scepticism about taken-for- granted archaeological assumptions; for example, a feminist approach might lead us to inquire closely into the truth of tales of brave, strong (male) hunters. As explained by Dincauze (2001), research has shown that archaeological sites with large animal bones are more highly valued and
researched than those without them; there has thus been a privileging, in research, of carnivory and hunting over other nutritional strategies such as gathering plant materials or trapping small animals. One might therefore question just how much evolutionary pressure really came from hunting. It has also been proposed that where hunting of large animals did occur, it served the purpose of achieving and validating male status. If so, a hunting-based explanation of male spatial rotation superiority would have to posit a cultural rather than a nutritional selection pressure.
If the overall position that evolutionary theory acts as a metatheory is accepted, this adds philosophical weight to the argument that it has the potential to act as a unifying force across many diverse areas of psychology.
We began this book with the suggestion (not entirely unchallenged) that Darwinian theory can be taken as the starting point for the various schools of developmental theory that have arisen. It is fascinating, therefore, that the wheel has turned full circle and the evolutionary approach is now being proposed as a novel means of reunifying the field.