Although Gould and Lewontin's (1979) paper stimulated an extraordinary response, the current study of adaptation is – and should be – more than a defense against their criticisms. Adaptations are studied by biologists in new and exciting ways, including experimental manipulations of populations in the field and laboratory, comparative analyses of taxa with known evolutionary relationships, and quantitative genetics. These techniques go beyond ascertaining whether or not a trait is an adaptation.

The mother sacrificing herself while rescuing someone else's child is a red herring. Neither behaviorism nor cognitivism can explain it. Unlike behaviorism, however, the cognitive process of projection can explain cooperation in one-shot social dilemmas.

Wynn shows that intentionally standardized artifacts (handaxes) provide evidence of the ability to conceptualize form (symmetry). However, such conceptual ability is not sufficient for the actual production of these forms. Stone knapping is a concrete skill that is acquired in the real world. Appreciation of its perceptual-motor foundations and the broader issues surrounding skill acquisition may lead to further important insights into human cognitive evolution.

Rachlin shows that experiments about social cooperation may fruitfully be grouped with experiments on self-control, and that this suggests interesting possibilities for practical behavioral controls. The concepts of selfishness and altruism, however, that inform his theorizing about these experiments, do not serve to provide understanding of the behavior that commonly is referred to, derogatorily, as selfish.

Opposed to Norman's proposal, processing of affordance is likely to occur not solely in the dorsal stream but also in the ventral stream. Moreover, the dorsal stream might do more than just serve an important role in motor actions. It supports egocentric location coding as well. As such, it would possess a form of representational memory, contrary to Norman's proposal.

Implicitly, Wynn's target article starts from the transformational definition of symmetry. Unlike his suggestion, this traditional definition and the recent holographic definition are relevant to the discussion on the cognitive evolution of visual symmetries. These definitions reveal underlying properties and, thereby, they support the natural selection hypothesis. The holographic definition even agrees with an indirect test of this hypothesis.

In our target article, we discussed the standards of evidence that could be used to identify adaptations, and argued that building an empirical case that certain features of a trait are best explained by exaptation, spandrel, or constraint requires the consideration, testing, and rejection of adaptationist hypotheses. We are grateful to the 31 commentators for their thoughtful insights. They raised important issues, including the meaning of “exaptation”; whether Gould and Lewontin's critique of adaptationism was primarily epistemological or ontological; the necessity, sufficiency, or utility of design evidence, phylogenetic analyses, homology, and molecular genetics in distinguishing exaptations from adaptations; whether adaptationists accept adaptationist hypotheses too quickly; and the real utility of adaptationism to human behavioral science. We organize our response along the major points of the target article, in some situations defending our original claims and in others modifying them. While debate on these issues will undoubtedly continue, we are cautiously optimistic that the main points of the target article (as modified by our response) will help move the debate in a positive direction.

Using connectionist modelling, Thomas & Karmiloff-Smith (T&K-S) claim that developmental disorders in children are characterised by atypical trajectories and an ultimate functional architecture that is fundamentally different from normal. We argue that there is no empirical evidence for these claims in any developmental disorder and that the available evidence provides support for Residual Normality in both developmental and acquired disorders. We also refute the claim that modular accounts cannot encompass developmental trajectories in children with developmental disorders.

Berkeley and Helmholtz proposed different indirect mechanisms for size perception: Berkeley, that size was conditioned to various cues, independently of perceived distance; Helmholtz, that it was unconsciously calculated from angular size and perceived distance. The geometrical approach cannot explain size-distance paradoxes (e.g., moon illusion). The dorsal/ventral solution is dubious for close displays and untestable for far displays.

Functional magnetic resonance imaging studies of developmental disorders and normal cognition that include children are becoming increasingly common and represent part of a newly expanding field of developmental cognitive neuroscience. These studies have illustrated the importance of the process of development in understanding brain mechanisms underlying cognition and including children in the study of the etiology of developmental disorders.

Wynn suggests that the imposition of symmetry on stone tools is indicative of the evolutionary development of cognitive abilities of the tool makers, particularly that of creating mental images. I suggest that it is more likely indicative of the evolutionary development of the perceptual ability to detect resources for behavior of hand-held objects.

Altruism by definition involves the self's evaluation of costs and benefits of an act of the self, which must include cost to the self and benefits to the other. Reinforcement value to the self of such acts is greater than the costs to the self. Without consideration of a self-system of evaluation, there is little meaning to altruistic acts.

Wynn's model for the evolution of spatial cognition is well supported by fossil evidence from brain endocasts, and from neurological studies of the cerebellum and the posterior parietal region of the cerebral cortex. Wynn's intriguing hypothesis that the spatial skill reflected in artifacts is an index of navigational ability, could be further explored by an analysis of lithic transport patterns.

Connectionist models aiming to reveal the mechanisms of atypical development must in their undamaged form constitute plausible models of normal development and follow a developmental trajectory that matches empirical data. Constructivist models that adapt their structure to the learning task satisfy this demand. They are therefore more informative in the study of atypical development than the static models employed by Thomas & Karmiloff-Smith (T&K-S).

Few disagree that altruism exists. The frequency and source of altruistic behavior remain mysterious, however.

Norman relates two theoretical approaches, the constructivist and ecological, to two cortical visual streams, the ventral and dorsal systems, respectively. This commentary reviews a similar approach in order to increase our understanding of complex skill development and to advance Norman's goal of stimulating and guiding research on the two theoretical approaches and the two visual systems.

In response to our target article, many of the commentators concentrated on our notion of Residual Normality. In our response, we focus on the questions raised by this idea. However, we also examine broader issues concerning the importance of incorporating a realistic theory of the process of development into explanations of developmental deficits.

Rachlin adds to the already long list of proposals for reducing what might be seen as social motivation to some roundabout form of self-interest. But his argument exhibits the usual limitations, and prompts questions about what drives this apparently unending quest.

The underdetermination of intentional explanation by motor behavior complicates inferences drawn from preserved artifacts in the archaeological record to intentions in their production. Without knowledge of a producer's intentions, inferences drawn from those intentions to required cognitive abilities for having those intentions is also complicated.

Perception of (and during) events is a necessary feature of any perceptual theory. Norman's dual-process approach cannot account for the perception of events without substantial interactions between the dorsal and ventral systems. These interactions, as outlined by Norman, are highly problematical. The necessity for interactions between the two systems makes the distinction useless.