Conventional wisdom in the neurosciences has long held that specific brain regions have specific functions. However, several recent studies have undermined the claim that cognitive functions can typically be mapped in straightforward ways to highly specialized brain areas, leading Anderson (Reference Anderson2007c) to propose his massive redeployment hypothesis (MRH). In the target article, Anderson has considered his theory, along with others that posit similarly, that existing neural structures are normally reused/recycled/redeployed as new brain functions develop. This new approach has enormous potential for helping neuroscientists rethink the relationship between brain structures and their functions, as well as for helping those interested in the development and/or evolution of behavioral organization to understand changes in that organization across ontogeny and phylogeny.

Anderson uses the MRH to predict that a brain area's phylogenetic age should correlate with how often that area is deployed for various cognitive functions, and that a cognitive function's phylogenetic age should correlate with how localized that function is in the brain. However, although Anderson recognizes that neural reuse theories bear on questions of development, his article focuses on phylogeny to the virtual exclusion of ontogeny. Brief mentions of development are made, and a note points out that neural reuse “is broadly compatible with the developmental theories of Piaget” (target article, Note 10); but, in fact, neural reuse should interest all developmental psychologists because the approach is compatible with most current theories of development and could contribute to theoretical progress in the field in general. Anderson cites Dehaene's “neuronal recycling” theory as having potentially identified a “fundamental developmental … strategy for realizing cognitive functions” (sect. 1, para. 3); but, like other promissory notes in Anderson's text, this one is never fully redeemed. Neither Anderson nor Dehaene and Cohen (Reference Dehaene and Cohen2007) fully consider the implications of neural reuse theories for understanding development.

The idea of neural reuse could have profound and general implications for the understanding of behavioral development. In particular, we believe that neural reuse produces a type of developmental homology, and that just as evolutionary biology has profited from the discovery and analysis of evolutionary homologies (Hall Reference Hall2003), so developmental psychology may profit from the identification of developmental homologies, some of which likely arise as a result of neural reuse. Because two or more psychological characteristics present at a given point in development might both (re)use neural circuits formed much earlier in development, thinking about such characteristics in terms of developmental homology could well illuminate their relationship to each other (as well as to other psychological characteristics present earlier in development that also depend on these circuits). Consequently, we believe that importing the concept of homology into developmental psychology has the potential to help behavioral scientists understand when, how, and why specific traits have common developmental origins.

Within biology, several types of homology have been identified, including among others (1) taxic homology (Griffiths Reference Griffiths2007), in which characteristics in different species (e.g., bat wings and human forearms) have derived from a characteristic present in a common ancestor; (2) serial homology (Rutishauser & Moline Reference Rutishauser and Moline2005), in which parts of an individual organism are of the same type (e.g., the corresponding bones in a person's right hand and right foot, or any two vertebrae in mammalian spinal columns); and (3) ontogenetic homology (Hoßfeld & Olsson Reference Hoßfeld and Olsson2005), in which distinct individuals of the same species have differing features that nonetheless derive from common embryonic tissues (e.g., human ovaries and testes). Developmental homologies arising from neural reuse would be most similar to the kinds of homologies identified by Bertalanffy in 1934 (described in Hoßfeld & Olsson Reference Hoßfeld and Olsson2005), and would include pairs of psychological characteristics, both of which emerged from a common characteristic present earlier in development. In addition, much as human forearms are homologous to the forearms of extinct Australopithecines, psychological characteristics of adults could be recognized as homologues of psychological characteristics present in juveniles in various developmental stages. Such homologues could arise in ways that would not require neural reuse – after all, “a structure that is homologous across species can develop based on non-homologous genes and/or developmental processes, and vice-versa” (Brigandt & Griffiths Reference Brigandt and Griffiths2007, p. 634) – but any characteristics known to emerge following the redeployment of a specific neural circuit would seem prima facie to be homologous, at least structurally if not functionally.

Several examples of possible developmental homologies may be identified. Temporal cognition in the form of episodic thinking develops later than spatial cognition and makes use of related conceptual structures (Clayton & Russell Reference Clayton and Russell2009). The discovery that these mental processes also make use of certain shared neural circuits would indicate that they are homologous, thereby shedding light on the nature of their developmental relationship. Linguistic structures, likewise, may well depend upon earlier-developing social interactive communicative structures. Tomasello (Reference Tomasello2003), for example, argues that syntax can be understood as a form of joint attention, a conceptualization that implies that these are homologous psychological characteristics, their different appearances notwithstanding. Still other psychological characteristics that appear similar across age have been assumed to be homologues, such as the neonatal imitation reported by Meltzoff and Moore (Reference Meltzoff and Moore1977) and later-developing forms of imitation observed in older children and adults. Even so, studies of the neural circuits that contribute to neonatal and later imitation might or might not support this conclusion; a finding that adult imitation normally recruits neural circuits previously used during neonatal imitation would certainly qualify as support for the contention that these behaviors are homologous.

As Anderson suggests, neural reuse might be a fundamental organizational principle of the brain; and just as this idea can be used to formulate testable hypotheses about the evolution of both the brain and its function, we think it could also influence the study of psychological development in significant ways. Similarly, importing the idea of homology from evolutionary biology into developmental psychology could help researchers conceptualize behavioral development in new and potentially informative ways. Taken together, the concepts of neural reuse and developmental homology could be used to further our understanding of brain development, psychological development, and the relationships between these phenomena.