Behavioral momentum is a useful metaphor reminding us that with constant conditions, ongoing behavior – in the form of response rate – would be expected to remain constant. But despite an impressive array of behavioral experiments, the concept has not yet been applied in a way that would make it useful as a general behavioral law.

The metaphor of “behavioral momentum” exemplifies modernism at its best and follows in the wake of countless other applications of Newtonian mechanics and “the machine metaphor” to virtually every aspect of the human condition. Modernism, however, has fallen on hard times. Some of the chief reasons why are implicit in the target article by Nevin & Grace.

Nevin & Grace's primary argument against theory and research on behavioral momentum is that preference and resistance to change may not covary. The method for evaluating preference and resistance to change seems problematic. Moreover, the theory fails to account convincingly for effects of average overall time to primary reinforcement on choice and preference for unsegmented schedules.

The research Nevin & Grace report is impressive in its integrative power, but it also shows the current limits of operant theory: There is tremendous concentration on understanding how existing behavioral relations are modulated in rate or time allocation, but little on dealing with the origin of the behavioral relations themselves. Specifying what should count as a behavioral unit will require source principles sensitive to the composition of the units being related.

Metaphors “highlight and hide” different aspects of phenomena. A behavioral economic metaphor for excessive consumption highlights the contextual features of behavioral-environment relations. Can the behavioral momentum metaphor generate a representation of context that is at least as useful as that generated by behavioral economics? Maybe, maybe not; or maybe a mixed metaphor will do a better job than either alone.

Many studies have demonstrated differential resistance to change in the context of negative behavioral contrast. That is, as a result of introducing a disruptor, response rates decrease to a greater extent when the maintaining reinforcement schedule is leaner. Resistance to change also applies to positive contrast, in that increases in response rate are greater in leaner schedules. The negative contrast effects seen in studies of intrinsically motivated behavior reflect an increase in resistance to change as a result of adding extrinsic reinforcers.

Two important assumptions of behavioral momentum theory are contradicted by existing data. Resistance to change is not due simply to the Pavlovian contingency between a discriminative stimulus and the rate of reinforcement in its presence, because variations in the response-reinforcer contingency, independent of the stimulus-reinforcer contingency, produce differential resistance to change. Resistance to change is also not clearly related to measures of preference, in that several experiments show the two measures to dissociate.

In reply to the comments on our target article, we address a variety of issues concerning the generality of our major findings, their relation to other theoretical formulations, and the metaphor of behavioral momentum that inspired much of our work. Most of these issues can be resolved by empirical studies, and we hope that the ideas advanced here will promote the analysis of resistance to change and preference in new areas of research and application.

We propose a conceptual model that maps the causal pathways relating biological evolution to cultural change. It builds on conventional evolutionary theory by placing emphasis on the capacity of organisms to modify sources of natural selection in their environment (niche construction) and by broadening the evolutionary dynamic to incorporate ontogenetic and cultural processes. In this model, phenotypes have a much more active role in evolution than generally conceived. This sheds light on hominid evolution, on the evolution of culture, and on altruism and cooperation. Culture amplifies the capacity of human beings to modify sources of natural selection in their environments to the point where that capacity raises some new questions about the processes of human adaptation.

Though the target article is not without fertile suggestions, at least two problems limit its overall validity: (1) the extended gene-culture coevolutionary framework is not an alternative to standard evolutionary theory; (2) the proposed model does not explain how much time is necessary for selective pressure to determine the stabilization of a new aspect of the genotype.

The phenogenotype, a routinely co-occuring combination of a cultural and genetic trait, is unlikely to survive over time because of the potentially varying evolutionary pressures upon cultural as opposed to genetic traits. This is because the production and evaluation of cultural inputs will themselves be based on information previously acquired culturally. As a result, treating both cultural and genetic inheritance in a single recursion may be problematic.

Because human cultures are far more similar than they are different, culturally constituted niches may work to limit or prevent the development of genetically based psychological differences across populations. The niche approach further implies that we may remain relatively well-adapted to contemporary environments because of the latter's cultural niche continuity with ancient environments.

Economic institutions governing such activities as food sharing among non-kin, the accumulation and inheritance of wealth, and the division of labor and its rewards are human-constructed environments capable of imparting distinctive direction and pace to the process of biological evolution and cultural change. Where differing structures of these institutions take the form of distinct conventions sustained by (near) mutual adherence, small initial differences may support divergent evolutionary trajectories even in the absence of conformist behaviors.

Consistent with the “niche construction” hypothesis, human beings tailor their behavior to local circumstances in ways beneficial to their inclusive fitness. However, the fact that any human being seems equally capable of adopting any of these context-dependent fitness-enhancing behaviors makes niche construction theory implausible in practice. The human capacity for exhibiting context-specific behavior remains in need of an explanation.

The teleological language in the target article is ill-advised, as it obscures the question of whether ecological and cultural inheritances are directed or random. Laland et al. present a very broad palette of explanatory possibilities; evolutionary simulation models could help narrow down the processes important in a particular case. Examples of such models are offered in the areas of language change and the Baldwin effect.

Cultural and dual-inheritance models of evolution present ambiguities not typically present in biological evolution. Criteria and the ability to specify the adaptive value of a trait or cultural practice become less clear. When niche construction is added, additional challenges and ambiguities arise. Its dynamic nature increases the difficulty of identifying adaptations, tracing the causal path between a trait and its function, and identifying the links between environmental demands and the development of adaptations.

The capacity for flexible niche construction increases suddenly and dramatically when discrete memories and sensorimotor associations become woven into an interconnected worldview. Ontogenetic learning is as vital to culture as social learning because it is the wellspring of cultural novelty. Human altruism may result from pressures exerted at the level of cultural rather than biological replicators.

Laland et al.'s model of human evolution and cultural change needs to incorporate theory and research in human psychological development. The psychological approach that focuses on sociocultural contributions to growth offers a way of thinking about individual development that would complement and extend this model.

I distinguish different versions of the “niche construction” idea. Some are primarily scientific, while others are more philosophical. Laland, Odling-Smee & Feldman's is mostly scientific, but given that fact, there are some changes they could make to their account. I also compare the target article to Lewontin's classic 1983 paper.

Laland et al.'s bidirectional model is a welcome starting point that can be enhanced by a full incorporation of systems thinking into its framework. Systems thinkers note that culture is not transmitted linearly in chunks but is co-constructed within subgroups. Niche construction, particularly among primates, should be studied primarily through the effects that social relationships have on selection pressures.