We present an account of processing capacity in the ACT-R theory. At the symbolic level, the number of chunks in the current goal provides a measure of relational complexity. At the subsymbolic level, limits on spreading activation, measured by the attentional parameter W, provide a theory of processing capacity, which has been applied to performance, learning, and individual differences data.

Testosterone's connection to sex differences and key evolutionary processes arouses controversy. Effects on humans and other species, though, are not robotically deterministic but are parts of complex interactions. We discuss the societal implications of these findings and consider how the naturalistic fallacy and the person–situation dichotomy contribute to misunderstandings here.

Byrne & Russon's proposal that stimulus enhancement, emulation, and response facilitation should be lumped together as priming effects conceals important questions about nonimitative social learning, fails to forge a useful link between the social learning and cognitive psychological literatures, and leaves unexplained the most interesting feature of phenomena ascribed to “response facilitation.”

I suggest several ways that externalism could influence psychological theorizing. Externalism could just capture our everyday intuitions about concepts and meanings. More profoundly, it could enter into psychology through evolutionary theory, guide our hypotheses about conceptual abilities, and, most significantly, it could influence our accounts of learning and conceptual change.

This commentary focuses on the nature of combinatorial properties for speech and the locus equation. The presence of some overlap in locus equation space suggests that this higher order property may not be strictly invariant and may require other cues or properties for the perception of place of articulation. Moreover, combinatorial analysis in two-dimensional space and the resultant linearity appear to have a “special” status in the development of this theoretical framework. However, place of articulation is only one of many phonetic dimensions in language. It is suggested that a multidimensional space including patterns derived in the frequency, amplitude, and time domains will be needed to characterize the phonetic categories of speech, and that although the derived properties ultimately may not meet the conditions of linearity, they will reflect a higher order acoustic invariance.

Developmental aspects of the frame/content perspective are explored in relation to (1) transitions in early language acquisition, (2) possible differential neurological control for babbling and early and later speech, and (3) development of word production templates in precocious early speakers. Proportionally high frequency of bilabial stops in early stable words versus babble offers advantages for afferent monitoring and supporting “frame dominance.”

A narrow focus on the jaw (or on motor generators) does not account for individual and language-specific differences in babbling and early speech. Furthermore, data from Yoshinaga-Itano's laboratory support earlier findings that show glottal rather than oral stops in deaf infants' babbling: audition is crucial for developing normal syllables.

Millikan's account of substance concepts fails to do away with features. Her approach simply moves the suite of relevant features into an encapsulated module. The crux of the problem for scientists studying human infants and nonhuman animals is to determine how individuals reidentify objects and events in the world.

This commentary focuses on the claim that perceptual demands have caused the linearity exhibited by locus equations. I discuss results of an experiment demonstrating that, contrary to Sussman et al.'s claims, locus equations do not have relevance for the perception of stop consonants. I therefore argue against the plausibility of the orderly output constraint.

Mazur & Booth provide life scientists with an example of the multilevel biopsychosocial approach. Research paradigms have to become more flexible and multidisciplinary if we are to free ourselves from the nature–nurture dichotomy that we have long agreed was simplistic and shortsighted. I point out a variety of kinds of interactions that may be the next frontier for behavioral scientists.

We focus on Byrne & Russon's argument that program-level imitation is driven by hierarchically organized goals, and the related claim that to establish whether observed behavior is evidence of program-level imitation, empirical studies of imitation must use multi-stage actions as imitative tasks. We agree that goals play an indispensable role in the generation of action and imitative behavior but argue that multi-goal tasks, not only multi-stage tasks, reveal program-level imitation.

We discuss how modified dual-task approaches may be used to verify the degree to which cognitive tasks are capacity demanding. We also delineate some of the complexities associated with the use of the “double easy-to-hard” paradigm for testing claim of Halford, Wilson & Phillips that hierarchical reasoning imposes processing demands equivalent to those of transitive reasoning.

The debate on the origin of the locus equation is circular. In this commentary the locus equation is obtained by way of a theoretical model based on acoustics without recourse to articulatory knowledge or perceptual constraints. The proposed model is driven by criteria of minimum energy and maximum simplicity.

Thinking of concepts as explicit lists of features used to pick out referents neatly is indeed mistaken; but there are other alternatives than making concepts mere pointers. These alternatives are suggested by the difference between meaning X and having the concept X, problems of conceptual change, implicit conceptual schemata, the conceptual requirements of the division of cognitive labor, and how concepts figure in perception versus language.

Aggressiveness is a vital component of dominating behavior. We must distinguish adaptive from nonadaptive aggression and must control for skills, intelligence, appropriate context variables, and – most important – whether the aggression displayed was actually suitable for improving a subject's social status. If we do, we may find a consistent positive correlation between adaptive aggressiveness and testosterone.

Byrne & Russon have argued that imitation is not an all-or-none phenomenon but may instead occur at different levels. Although I applaud their theoretical framework, their data provide little empirical support for the theory. Data from studies of human infants, however, are consistent with the view that imitation may occur at different levels. These data may provide better support for Byrne & Russon's hierarchical view of imitation than the nonhuman primate data that their theory was developed to explain.

Synchronization of neural activity in oscillatory neural networks is a general principle of information processing in the brain at both preattentional and attentional levels. This is confirmed by a model of attention based on an oscillatory neural network with a central element and models of feature binding and working memory based on multi-frequency oscillations.

Because the evolution of speech production is beyond our expertise (and perhaps beyond everyone's expertise) we restrict our comments to areas in which data actually exist. We provide articulatory evidence consistent with the claims made about syllable structure in adult speech and infant babbling, but we also voice some disagreement about speech errors and the typing data.

Mazur & Booth present an intriguing model of the relationship between circulating testosterone levels and dominance behaviour in man, but their review of studies on testosterone–behaviour relationships in man is selective. Much of the evidence they cite is correlational in nature. Placebo-controlled manipulations of testosterone levels are required to test their hypothesis that dominance levels are testosterone-dependent in man. The changes in testosterone level that follow behavioural experience may be a consequence of stress. Testosterone levels in man are determined by a wide variety of factors, and a multivariate approach is required.

We argue that Halford et al.'s characterisation of relational complexity offers an unadaptive principle in terms of cognitive economy, that its relation with the empirical evidence is highly selective, and that the task behaviours used in support of a multivector processing space are better described by linear serial processes which do not require n-dimensional mappings for their emergence.