The evolutionary continuity between the prespeech functions of premotor cortex and its new linguistic functions, the main thesis of MacNeilage's target article, is confirmed by the recent discovery of “mirror” neurons in monkeys and a corresponding action-observation/action-execution matching system in humans. Physiological data (and other considerations) appear to indicate, however, that brachiomanual gestures played a greater role in language evolution than MacNeilage would like to admit.

Dolphins exhibit both action-level imitation (ALI) and program-level imitation (PLI). Dolphins may use ALI primarily for social cohesion, whereas PLI seems more likely to occur in goal-directed, problem-solving contexts. Both PLI and insightful problem solving require a recognition of the functional relations between actions and outcomes. Insightful problem solving, however, involves the creation of a program in the absence of a model, and therefore requires a higher order appreciation and application of the relations between actions and outcomes.

Halford, Wilson & Phillips argue that cognitive development is driven by increases in processing capacity. We suggest that changes in relational knowledge are as important or more so. We present evidence that 3-year-olds' analogical abilities are sharply improved by teaching them relational labels; over a 30-minute experimental session children gained approximately 2 years in effective performance. These results mandate caution interpreting age-related change as indicating maturational change, and call for a deeper consideration of the role of epistemological change in cognitive development.

Parrots prepare for vocalization by a ventro-caudal retraction of the larynx. This laryngeal movement, which “frames” vocal sequences, is similar to a movement used by pigeons as a preparation for suction drinking. The air-pressure events involved in such movements can trigger either suction drinking or vocalization. This suggests a possible evolutionary link between these ingestive and vocal mechanisms.

Millikan's account of substance concepts is based on a notion of “substance” expanded from realist notions of individuals and natural kinds. Her metaphysical notion, based on “inductive potential,” is shown to be too puristic and needs to incorporate cognizing subjects. This could preserve the realist/nondescriptionist insight that the extension of substances is determined by the world.

Byrne & Russon's account of program imitation in primates involves propositional attitudes (expectations and goals), which limits its falsifiability. Yet their account of priming shows exactly how imitation without attitudes would look. The challenge is to upgrade the notion of priming to give an account of low-level program imitation without invoking propositional attitudes.

The orderly output constraint (OOC) is extraneous. Talkers “speak in lines” in its absence. Further, there is no perceptual motivation for an OOC; perceivers ignore the linearity between F2 at consonant-vowel onset and F2 in the vowel. In any case, the analogy with bat and barn owl localization systems underlying the theory is extreme, Sussman et al.'s comments to the contrary notwithstanding.

Men's interest in sex partners' status traits and commitment (investment thoughts) declines with number of sex partners and permissiveness of attitudes; women's investment thoughts do not seem to decline. Testosterone, dominance, sexual attractiveness, and number of sex partners are correlated in men but not in women. It is plausible that these sex differences are part of sexually dimorphic feedback systems. This type of feedback is consistent with both reciprocal and basal models of testosterone.

We commend Byrne & Russon for their effort to expand and clarify the concept of imitation by addressing the various levels of behavior organization at which it could occur. We are concerned, however, first about the ambiguity with which these levels are defined and second about whether there is any particular need for comparative cognition to keep focusing on imitation as an important intellectual faculty. We recommend stricter definitions of hierarchical behavioral levels that will lend themselves to operational definitions and continued study of how animal subjects organize their goal-directed behavior as opposed to whether it is or is not imitation.

This commentary deals with the relation between human language and nonverbal signals used by nonhuman primates. It suggests that human language could have developed through the interaction of procedural learning with a preexisting system for socio-affective communication. The introduction of “content” into existing “frames” requires a neurobiologically plausible learning mechanism.

This commentary focusses on the evidence used by Halford et al. to support their postulated links between relational complexity and age differences in children's understanding of concepts. None of their developmental claims is consistent with recent cognitive-developmental research. Relational complexity must be an important variable in cognition, but it does not provide a satisfactory metric for explaining cognitive development.

The problems that Millikan addresses in theories of concepts arise from an extensional view of concepts and word meaning. If instead one assumes that concepts are psychological entities intended to explain human behavior and thought, many of these problems dissolve.

This Response focuses on the strength of the testosterone (T) dominance relationship, the circumstances under which aggression accompanies dominance, the viability of the basal model, mediators and moderators of the T-dominance relationship, and the research that is needed.

Although an individual speaker's productions obey locus equations, whether listeners' perceptions are based on them needs further exploration. Comparing the results from the perceptual experiments to predicted identifications, one sees qualitative similarities and some discrepancies. However, the variability of locus equations and individual consonant-vowel (CV) tokens across speakers seems problematic if listeners are using locus equations for perception.

Talents that selectively facilitate the acquisition of high levels of skill are said to be present in some children but not others. The evidence for this includes biological correlates of specific abilities, certain rare abilities in autistic savants, and the seemingly spontaneous emergence of exceptional abilities in young children, but there is also contrary evidence indicating an absence of early precursors of high skill levels. An analysis of positive and negative evidence and arguments suggests that differences in early experiences, preferences, opportunities, habits, training, and practice are the real determinants of excellence.

The discovery of a quaternary complexity limitation to mature cognitive operations raises important theoretical issues. We propose that cognitive limitations arise naturally in a complex dynamic information processing system, and consider problems such as the distinction between parallel and serial processes and the representativeness of the empirical data base used by Halford et al. to support their relational complexity scheme.

This commentary focusses on MacNeilage's arguments and evidence that the development of cerebral cortical controls over cyclic ingestive movements has provided substrates for the evolution of speech production. It outlines evidence from experimental approaches using cortical stimulation, inactivation, and single neuron recording in primates that lateral frontal cortical regions are indeed crucial for the generation and guidance of cyclic orofacial movements.

Byrne & Russon's notion of program-level imitation is based on the ability of apes to plan novel sequences of behavior and on how information gleaned by observation can aid the planning process. Byrne & Russon would have made a stronger case by focusing on social learning and planning and expending less effort interpreting their results as a new category of imitation.

Millikan's theory of substance concepts has advantages for psychological theories, including those in cognitive development. However, the disadvantage is that it cannot be generalized even to some of the most common concepts that children acquire in the early years of life. For a general theory we must get beyond substances.

Understanding spoken language involves far more than decoding a linear sequence of phonetic elements. In view of the inherent variability of the acoustic signal in spontaneous speech, it is not entirely clear that the sort of representation derived from locus equations is sufficient to account for the robustness of spoken language understanding under real-world conditions. An alternative representation, based on the low-frequency modulation spectrum, provides a more plausible neural foundation for spoken language processing.