Tuesday, October 22, 2013

Philosophers should be interested in ‘common currency’ claims in the cognitive and behavioural sciences

I'm presenting on this topic at the 2014 conference of the PSSA (Philosophical Society of Southern Africa) to be held in Bloemfontein in January. (If all goes according to plan, my collaborator Blaize Kaye will be presenting something about subsumption architectures and common currencies - watch this space for updates.)

The text below is my submitted abstract. I'll post a working text of the paper in due course. The aim of this paper is simply to document some of the variety in common currency claims, and argue that they are of specific philosophical interest. (That is, I'm not defending any more specific claims about whether any particular currency thesis is true or not.)


A recurring claim made in a number of behavioural, cognitive and neuro-scientific literatures is that there is, or must be, a unidimensional ‘common currency’ in which the values of different available options are represented.

There is striking variety in the quantities or properties that have been proposed as determinants of the ordering in motivational strength. Among those seriously suggested are pain and pleasure, biological fitness, reward and reinforcement, and utility among economists, who have regimented the notion of utility in a variety of ways, some of them incompatible.

This topic deserves philosophical attention for at least the following reasons: (1) Repeated invocation of the ‘common currency’ idiom isn’t merely terminological coincidence because most of the claims are competing explanations for one or the other of two putative kinds of fact. In one case the currency represents a principle of manifest pattern in choices. In the other, it is a functional part of the processes which produce choice. (2) We can’t suppose that the different currency claims within each area are compatible, because there are significant obstacles to identifying pairs of members of either the ‘pattern’ or ‘process’ group. (3) There are, finally, seriously opposed positions about the relationships (generally, and in specific cases including that of humans) between the pattern facts and the process facts.

Philosophical positions both favouring and opposing a common currency exist. Philosophers who incline to view their positions as at least partly empirical, should be more interested in the issues outlined here than they are.

This text was updated slightly on 11 January 2014.

Thursday, August 8, 2013


While following a trail of references relating to the notion of 'lexical' ordering for preferences or values, I came upon a reference to a passage in Aristotle's Nichomachean Ethics which makes an argument about the function of money that is relevant (in a somewhat distant and historical way) to my topic. The passage is the following:
"So money acts as a measure which, by making things commensurable, renders it possible to make them equal. Without exchange there could be no association, without equality there could be no exchange, without commensurability there could be no equality. Strictly speaking no doubt things so widely different can never become commensurable. Still in demand we have a common measure which will be found to work pretty well. Some one standard there must be, and it must be accepted by a general agreement or understanding. Such a standard has the effect of making all things commensurable, since they all can be measured by money." (Book five, Chapter Five, translated by J.A.K. Thomson.)
This passage follows reflections on the requirements for exchanges "according to the right proportion", where Aristotle points out, among other things, that there's no point in trading the same types of things for each other ("Two doctors cannot associate for the purpose of exchanging what they have to give, but a doctor and a farmer can..."). We trade precisely because means and wants differ:
"Where there is not an original equality between them it has to be created. This implies that all products exchanged must somehow be commensurable."
He reasons that money mediates this process, but that the "one standard by which all commodities are to be measured" is demand.

Aristotle's point is, of course, not one about cognitive processing. That said, it's not hard to see how one could use his reflections as a template for an argument about the processes of choosing between different goods.

If there's a historically older statement of an argument for a common currency I'd be very interested to hear about it.

Not dead, just working

I know this blog has been mighty quiet lately. But the project hasn't. In fact, I've been rather busy:

  • I traveled to Australia in July to present an updated version of Intragenomic Conflict and Common Currencies at the AAP in Brisbane. I learned a lot at the conference, and received some useful comments and questions about the paper at the presentation itself. I'm busy reworking the presentation and the paper. I've also got hold of a collection of papers about intragenomic conflict by David Haig to work through. I'll be making additional presentations in August and September here in South Africa, and hope to have a draft that I can circulate by some time in October.
  • I've been developing a talk on 'Sacred' values and common currencies, that is occasioned by the recent wave of empirical research on the topic, including the Berns et al. paper 'The Price of Your Soul' which I posted a notice of back in May. I'm presenting talks on that topic a few times through August, and then once in September.
  • I've done some preliminary work on a paper engaging with Kim Sterelny's treatment of preference in his Thought in a Hostile World. If I make enough headway, I'll present that at a conference in South Africa near the end of September.
So before too long I'll be able to post some updated presentation slides, some new slides, and some brief comments on key research papers.

Friday, May 31, 2013

Brief Notices 3

There's a fair-sized philosophical literature on the possibility of incommensurable values. According to some accounts of what it would mean for a value to be incommensurable, such values would represent a failure of a common currency.

Joseph Raz, for example, says two values (or bearers of value) are incommensurable if it is false that “either one is better than the other or they are of equal value” (Raz, 1986, The Morality of Freedom, p. 342).

I'm not - for now - taking a position on whether there are indeed incommensurable values. It's just useful, either way, to have some philosophical work clarifying what it might mean for values to be incommensurable at all, and considering some arguments against and in favour of the possibility. I'll say more about some of the arguments in the future.

If you want to look at a clear overview of the issues, then Nien-hê Hsieh has written an entry on Incommensurable Values for the Stanford Encyclopedia of Philosophy.

Wednesday, May 29, 2013

The price of your soul: neural evidence for the non-utilitarian representation of sacred values

One common way that people reject the claim that there is a common currency in which all available options are represented is by raising the possibility that some options are ‘incommensurable’. For two things to be incommensurable in general means that there isn’t some standard of comparison which applies to both of them. So the relevance of such claims to the common currency thesis is pretty direct.

Barry Schwartz, for example, holds that “some sets of commodities are simply incomparable or incommensurable” (Schwartz 1986, p154). Schwartz has in mind, among other things, comparisons of outcomes we would regard as morally significant, such as charitable giving, and cases of more ordinary consumption.

Perhaps the most common context in which people will claim that some option (or type of option) is incommensurable in this way is with reference to options that are moral, or purportedly sacred. (The two are often supposed to coincide, so a moral view about the value of life might well be expressed as a claim about the 'sanctity' of life.)

It is clear enough what vague idea is being expressed here. We might reasonably and calmly suppose that there is some definite number of milkshakes that I would have to be given in order to forgo a piece of cake. But most people actively resent the suggestion that there is any answer to the question how many milkshakes you’d need to give them before they sell one of their children. (And they resent both the suggestion that there is a definite answer, as well as the attempt to get them to reveal their price.)

The vague idea, then, is this: Some values are of different kinds, and they don’t even have ‘prices’ in more mundane things people might want. It is, furthermore, somehow wrong - not merely incorrect - to suppose anything else.

If something like this is true, then there isn’t a single common currency. There might be two currencies – one with conventional consumption goods on it, and another with moral values. There’s reason to doubt that such a picture is actually coherent, or at least that it could be worked out in detail coherently, which is why I say that the idea is vague, even if it is clear what it is. (I’ll pick up this point in a future posting on the generic ‘anti dualist’ argument.)

One point to make here is, of course, that talk is cheap. People routinely avow their commitment to standards of conduct of which in practice they fall well short. (The majority of sworn undertakings of fidelity in the course of marriage ceremonies are probably entirely sincere at the time.) So the mere fact that many people (at least) assert that they have values that are unconditional or have no pragmatic price does not establish that anyone does.

That said, there are certainly many prima facie credible examples of people who seem to place some value or other far beyond other considerations and temptations, and upheld these values in the face of death, or under torture, etc. We could perhaps say that these people were never offered a great enough temptation, and suppose that if they had been then their allegedly unconditional values would have gone the way of many marriage vows. But we'd be speculating, and it would be fairer to admit the behavioural evidence, in at least many cases, just doesn’t settle the question either way.

It would be good to have some evidence of different kinds, including evidence about what is going on ‘under the hood’, which is to say in the brains of people purportedly responsive to values of different kinds, including putatively 'sacred' ones.

A recent paper in Philosophical Transactions of TheRoyal Society B offers just that. It’s by a set of collaborators headed by Gregory S. Berns, and purports to show ‘neural evidence for the non-utilitarian representation of sacred values’.

Here’s what they did

Subjects (32 in scanner, and an additional 11 outside the scanner) participated in an experiment separated into four phases, with neural (fMRI) data acquired in all stages:

In phase one, “participants were presented with value statements phrased in the second person, one at a time.” In this stage participants made no choices.

There were 62 pairs of statements including ones about mundane preferences (‘You are a Coke drinker’) and ones about what Berns et al call ‘sacred’ matters (‘You believe that all Jews should have been killed in WWII’).

In phase two “complementary statements were presented together, and for each pair, the participant had to choose one of the statements.”

In phase three, participants were asked hypothetically if there was a dollar amount that they would accept in order to reverse their choices from phase two.

The fourth phase was an auction about which subjects had not been told in the briefing for the preceding phases. (So that expectations about the auction did not influence earlier choices.) Here subjects “were given the opportunity to sell their answers from the active phase for real money.” (The ‘active’ phase is phase two.) Selling an answer meant signing a statement disavowing the original answer. This phase used a Becker–DeGroot–Marshak (BDM) auction mechanism, with bids ranging from $1 to $100.

A follow-up survey between 6 and 14 months after the experimental session (without scanning) attempted to assess the stability in people’s answers (in phase two), and to ask for a rationale for each answer, where the three options were: “(i) right and wrong; (ii) costs and benefits; and (iii) neither.”

In addition to this, an on-line survey with 334 subjects was used to validate aspects of the design, especially the classification of statements.

Here’s what they found

First, the ‘sacred’ values were put up for sale (hypothetically and actually) far less frequently than the mundane ones. See figure 1 (from the paper). The inset bar graph shows fraction of responses sold as a function of being deontic (identified as based on ‘right/wrong’), utilitarian (‘cost/benefit’) or neither, and shows that there was a much higher opt out (don't sell) rate for statements classified as 'deontic'.

Figure 1 of the paper.

Second, there seems to be a different neural division of labour for statements about right and wrong, and statements that about more mundane preferences. In the paper this is expressed as a distinction between ‘deontic processing’ and ‘utilitarian processing’.

The main evidence for this claim is a fairly rich set of comparisons where the ways subjects explained their choices (right/wrong vs. cost/benefit), their actual choices, as well as what they hypothetically said they might disavow for money, and what they were actually prepared to disavow for how much money, were all used in different ways to partition the neural data, which was then examined for contrasts. A number of possible confounds were identified, and attempts made to control for them. You should read the paper for the details.

The simple bottom line from the neural analysis is, as the authors put it in the first paragraph of their discussion:

“These results provide strong evidence that when individuals naturally process statements about sacred values, they use neural systems associated with evaluating rights and wrongs (TPJ) and semantic rule retrieval (VLPFC) but not systems associated with utility. The involvement of the TPJ is consistent with the conjecture that moral sentiments exist as context independent knowledge in temporal cortex. Both the left and right TPJ have been associated with belief attribution during moral judgements of third parties. Our results show that it is also involved in the evaluation of personal sacred values without decision constraints. Thus, one explanation for the reduction in morally prohibited judgements when the TPJ is disrupted by transcranial magnetic stimulation is because disruption impairs access to personal deontic knowledge.” [TPJ = left temporoparietal junction; VLPFC = ventrolateral prefrontal cortex.]

Here’s what I think

This post is a bit long already, so I’ll be brief for now. (I plan to write about some related papers on the ‘sacred values’ topic in the future.)

(1) Before I say anything critical, let me say that I think this is a good and valuable paper, that helps open up a really worthwhile line of enquiry. One way to deal with a problem of underdetermination (in this case the problem that we can’t tell from behavior whether people really have any unconditional values) is to get different kinds of data, that can help constrain how we interpret the data we do have.

That general methodological principle informs many design decisions in this paper, where a mixture of passive, active, and self-report tasks are related with behavioural measures of varying kind (hypothetical avowals, and real bids at auction) and all correlated with neural data. My brief overview attempts to describe the gist, but the details are well worth working through.

(2) That said, I find some of the ways claims are expressed in the paper unfortunate. Utilitarianism is a theory – or a family of theories – about right and wrong, and so supposing a dichotomy between utilitarian and right/wrong processing seems confused. In addition, deontological and consequentialist theories are not primarily descriptive theories about how people think, but normative ones about how they should behave. Both types of theory are clearly committed to some views about how it might be possible for people to act in accordance with those theories, but they are not about cognitive processes. Most of the formulations of claims about ‘deontic’ and ‘utilitarian’ processing in the paper can be taken as elliptical for more careful statements that aren’t overtly problematic. But it’s still annoying to see the terminology used so loosely, and also to see worked out and appropriate distinctions (such as the notion of 'lexical preferences') not being deployed.

(3) Competing values need to … well … compete. It’s all very well suggesting that some values are unconditional, but if they motivate action then it seems as though they need to show up in the same place other motivations do, and come with some motivational 'force' that is alike in kind even if different in degree from whatever temptations bring with them. Reason, Hume said, must be the slave of the passions. The sacred, we might analogously say, must do so too, if it wants to be motivating.

George Ainslie quotes the same claim about incommensurability from Barry Schwartz that I quote above, and goes on to say:
But if behaviors are not selected according to a single standard of choosability, the standard summarized by the term “reward” […], how are they selected? The organism’s means for expression are limited. A single channel of attention, if not a single set of muscles, is needed for the variety of behaviors that physically can be substituted for one another. Assuming that the selection of these behaviors is determinate, there must be a means of comparing them along a common dimension (Ainslie 1992, 31).
I'm not saying that there is a decisive and general argument in favour of a common currency that automatically applies to putatively sacred values. (I don't think there is a general argument in favour of a common currency, but - in case you were wondering - that it's a contingent fact that humans approximately have one some of the time.) My point is simply that there is an argument worth taking seriously here, and that it would be welcome to see those discussing sacred values taking it more seriously.

(As a last note on this point, there's growing evidence that choices across a wide range of different modalities have a common neural value representation in humans. For more, see immediately below.)

(4) Here are some experiments that I’d like to see in the future (and may well be in the pipeline)

(a) What happens when ‘sacred’ values are in conflict with each other? People manifestly do end up in this condition. On the one hand it is the basis of some of our most compelling literature (consider Antigone’s position, between sacred duty to her dead brother Polyneices and similarly sacred duty to obey Creon, the king of Thebes). On the other, we have various kinds of trained specialist, such as triage nurses, whose job requires trading off competing moral (maybe 'sacred') values.

(b) What neuroeconomic sense can be made of this data, or the same phenomene in a more specifically neuroeconomic setting? It would be very interesting to know more about how conflict between ‘sacred’ values is neurally represented. And also to know how (if at all) are values associated with ‘sacred’ options are represented in relation to the long and growing list of rewards in various modalities that do seem to have a common basis for representation. See this (soon to be expanded) evidence rack.

That’s all I’ve got time for now.

Full text of the abstract of the paper:

Sacred values, such as those associated with religious or ethnic identity, underlie many important individual and group decisions in life, and individuals typically resist attempts to trade off their sacred values in exchange for material benefits. Deontological theory suggests that sacred values are processed based on rights and wrongs irrespective of outcomes, while utilitarian theory suggests that they are processed based on costs and benefits of potential outcomes, but which mode of processing an individual naturally uses is unknown. The study of decisions over sacred values is difficult because outcomes cannot typically be realized in a laboratory, and hence little is known about the neural representation and processing of sacred values. We used an experimental paradigm that used integrity as a proxy for sacredness and which paid real money to induce individuals to sell their personal values. Using functional magnetic resonance imaging (fMRI), we found that values that people refused to sell (sacred values) were associated with increased activity in the left temporoparietal junction and ventrolateral prefrontal cortex, regions previously associated with semantic rule retrieval. This suggests that sacred values affect behaviour through the retrieval and processing of deontic rules and not through a utilitarian evaluation of costs and benefits.

Related posts (some forthcoming)

The generic ‘anti-dualist’ argument


Ainslie, G. 1992. Picoeconomics, Cambridge: Cambridge University Press.

Schwartz, B. 1986. The Battle for Human Nature: Science, Morality and
Modern Life. New York: Norton.

Berns, G., Bell, E., Capra, C., Prietula, M., Moore, S., Anderson, B., Ginges, J., & Atran, S. (2012). The price of your soul: neural evidence for the non-utilitarian representation of sacred values Philosophical Transactions of the Royal Society B: Biological Sciences, 367 (1589), 754-762 DOI: 10.1098/rstb.2011.0262

Tuesday, May 21, 2013

Brief Notices 2

Here are two passing mentions of a 'common currency' in slightly unusual settings:

Carotenoids Are Cornerstone Of Bird's Vitality

This piece reports on research suggesting that carotenoids don't merely contribute to the bright pigments that play a role in sexual selection in some birds, but also play a role in colour perception, and have other nutritional benefits. One of the researchers describes the emerging view as follows:

"We are proposing a positive fitness feedback loop for these 'self-loving molecules,' given how high carotenoid accumulation can improve one's state and one's interest in selecting carotenoid richness in mates and food. This provides a window into how major sexual selection models, such as sensory biases and assortative mating, may be explained by a common, nutritional and narcissistic currency".
Now this doesn't sound like a common currency in the full sense that I'm interested in it, but presumably the claim made is still relevant: the net benefit (nutritional and narcissistic) of carotenoids is suppose to be a component of the fitness value, which in turn is one of the things that many behavioural ecologists take to determine a common currency that it is part of the job of scientists to describe. So some parts of a full 'currency for fitness' might map onto the factors described here.

The human machine: probing the mechanics

This is a cool notice of some recent work on cellular energy metabolism, and in particular conversion of electron to proton currents. I don't know enough molecular biology to give much of a gloss of the content, but it's a lively and interesting read. It also refers to Adenosine triphosphate (ATP) as "the single energy currency of the cell".

This might seem like an irrelevant use of the same term ('currency') but there's reason not to rush to this conclusion. But as I noted elsewhere on this blog, "Since any behavior has some energy cost (or gain), and any allocation of metabolic resources has an opportunity cost (in actions rendered unavailable, or made possible) it’s plausible to think that in a highly aggregated way, ATP represents an important overall budgetary bottom line."

You can get the actual article on the Nature website here (possibly behind a paywall): Crystal structure of the entire respiratory complex I.

Monday, May 20, 2013

What is to be done? Why reward is difficult to do without

The text below is the un-edited preprint of a commentary I wrote on Andy Clark's 'Whatever Next? Predictive Brains, Situated Agents, and the Future of Cognitive Science' in Behavioral and Brain Sciences. The published version of Clark's paper, with commentaries and a response, is here (probably behind a paywall.) Because behavioral and brain scientists were just falling over themselves to comment on Clark's target article, my commentary ended up appearing in Frontiers in Philosophical and Theoretical Psychology. That journal is open access, and so you can read the final version of my commentary at this link. You can read Clark's response to all of the commentaries at this link.

I've included the preprint text on this blog because it's one of the recent pieces in which I've directly argued for a thesis that has something to do with common currencies.

Commentary text

Clark’s synthesis of much recent work on sensory and motor systems in the brain is at once radical and curiously traditional. It is radical, among other things, concerning what representations are, how they are constructed, and what sensory and motor representations have in common. But it is traditionally cognitivist in viewing the main task of brains as being that of representing the world.

What this traditional orientation tends to neglect is the role of the brain as a system for selecting among available actions. This phenomenon has an ultimate aspect regarding the external standards relevant to assessing actions. Various behavioral ecological schemes for ranking actions in terms of their contribution to quantities such as fitness, and economic models of revealed preference, are the leading theoretical players here. The phenomenon also has a proximal aspect, which concerns the specific biological mechanisms, including neural ones, by means of which the values of different available actions might be represented, and selections between them made. On this topic the recent explosion of neuroeconomic research on decision processes in the brain is urgently relevant.

Natural agents have limited means of action, and those means have alternative – sometimes mutually exclusive – uses. That is to say the predicament of natural agents is fundamentally an economic one, even if it is not necessary that selection converge on a system for responding to the predicament in which economic variables are explicitly represented. Furthermore there is considerable evidence from behavioural ecology and other fields that many vertebrate behaviours in natural settings are economically efficient.

Neither the ultimate nor the proximal aspects of the problem of selecting between behaviours play a significant role in Clark’s account. Natural selection, fitness and biological descendents are not mentioned at all, and cognate concepts like adaptiveness feature in diluted form. There’s similarly little mention of decision and choice as theoretically understood in economics including neuroeconomics, none of incentives, and reward and utility appear only in the course of musing over whether it’s possible that cognitive neuroscience could do without reference to either (section 5.1). Clark does make some important points about action-centric representations, but even here does not consider the problem of action selection.

Of course, no survey can cover anything that anyone thinks is relevant, and it’s very easy to complain about things that are left out. Clark’s lack of engagement with neuroeconomics means missing a specific opportunity to make his general case even more compelling, because what is emerging in that field complements his case about sensory and motor systems in deep ways.

In his section (3.2) Clark apparently takes seriously the concern that an agent with the sort of brain that he’s been describing would be expected to ‘seek a nice dark room and stay in it’. Clark disposes of the worry by pointing out that creatures with real biological needs should ‘expect’ to follow exploratory strategies, and that these expectations themselves should recruit both perception and action. This is part of a reasonable and interesting response, but action selection under those conditions (as with most others) would still require some way of dealing with specific questions, such as where and how to forage, and how to trade off foraging with other expected behaviours such as predator avoidance and reproduction.

A related move appears later, in section (5.1) when he considers an austere vision of cognition that does without reference to goals and rewards, in favour of comprehensive analysis in terms of expectations. Clark correctly holds back from endorsing this possibility, but for relatively generic reasons to the effect that even if some description is in principle replaceable, it may be convenient to continue using it. This misses the main chance. Recent work on the neural implementation of decision in various vertebrates including humans has produced a body of results highly congenial to the unifying vision Clark supports.

Consider saccadic movements in rhesus monkeys. A key component in the neural implementation of these movements is the lateral intraparietal area (LIP), which comprises a topographic map integrating locations in the visual field and aspects of the muscular plans that would effect the centering of gaze on those locations. It, along with a network of other maps with varying topographies in the frontal eye fields, superior colliculus and related areas, provides a striking illustration of what Clark calls an ‘action-centric’ representation. In addition, as studies including Platt & Glimcher (1999) and Dorris & Glimcher (2004) have shown, some activity in LIP neurons of rhesus monkeys on visually identical trials varies in precise ways with the relative expected rewards (or relative subjective value) from saccades to the represented location. These representations are not merely ‘action-centric’ insofar as they combine answers to the questions ‘where is it?’ with ‘how do I gaze at it?’ They also include identifiable activity corresponding to the answer to ‘what’s it probably worth for me to look at it?’

There’s more. The expected relative reward values attached to saccadic and other movements are not sui generis. They’re predictions, and ones that get updated in the light of ongoing experience. Among the key findings on this topic is that dopamine neurons do not – as previously supposed – directly encode hedonic value (because if they did they would respond in the same to expected and unexpected rewards of equivalent hedonic worth). Rather it turns out that they encode some aspects of the difference between experienced and expected reward (Montague et al 1997, see also Bayer & Glimcher 2005). While many details about the operation of this system, and its interaction with other neural systems, have yet to be determined, it is nonetheless clear that crucial features of the neural systems for attaching values to sensory events and actions operate by means of prediction error. In this respect they suggest a way of expanding the scope of Clark’s claim about the importance of minimizing prediction error as a general goal of neural systems.


Bayer, H.M. and Glimcher, P.W. (2005). Midbrain dopamine neurons encode a quantitative reward prediction error signal. Neuron 47, 1 – 13.

Dorris, M.C. and Glimcher, P.W. (2004). Activity in posterior parietal cortex is correlated with the subjective desirability of an action. Neuron 44, 365 – 378.

Montague , P.R., Dayan , P., and Sejnowski, T.J. (1997). A framework for mesencephalic dopamine systems based on predictive Hebbian learning. J. Neurosci. 16, 1936 – 1947.

Platt, M.L. and Glimcher, P.W. (1999). Neural correlates of decision variables in parietal cortex. Nature 400, 233 – 238.

Version notes:
First posted May 20, 2013. Link to Clark's reply to commentaries added May 21, 2013.

Things that get suggested as common currencies

One of the things that makes the common currency topic interesting, is the variety of candidate currencies that get suggested.

Here is a list, that I’ll add to and flesh out periodically, of things that have been proposed as currencies:


In one of the economists’ technical senses, utility is not a psychological notion, but refers to whatever an agents behaviour tends to make more probable, as revealed in the pattern of behaviour itself.

An ‘agent’ on this view is anything whose activity reveals a consistent set of preferences, although additional assumptions can be introduced so that apparently inconsistent entities still turn out to be agents.

A consistent set of preferences revealed by an agent, represents the common currency of that agent. (Any two options will stand in some relation of relative preferability.)

There are other technical notions of utility (for example from behavioural economics) that I’ll list here in the future.


This is reinforcement in the behavioural psychologists' sense, where a reinforcer is something that changes the rate at which a behaviour is emitted. (In proper terminology it is behaviours that are reinforced, while agents are rewarded.)

So the various reinforcement values of all things that are reinforcing will also define a common currency, representing the effectiveness of all the incentives to which an agent is responsive.

An exemplary statement of the argument for a common currency in this setting is the following:
"In natural settings, the goals competing for behavior are complex, multidimensional objects and outcomes. Yet, for orderly choice to be possible, the utility of all competing resources must be represented on a single, common dimension" (Shizgal & Conover 1996).
There's a discussion of Shizgal and Conover's paper here.

Pain and Pleasure

I have not read this book.

According to Bentham pain and pleasure were the “two sovereign masters” which both explained human action, and enabled good and bad consequences of actions to be identified. Contemporary economics and behavioural psychology make considerably less reference to pleasure and pain than a Benthamite might have expected. Instead they focus on largely behavioural notions of utility and reward. Among those whose primary interest is pleasure and pain, one still finds suggestions that these constitute a ‘common currency’:
“Consistent with the idea that a common currency of emotion enables the comparison of pain and pleasure in the brain, the evidence reviewed here points to there being extensive overlap in the neural circuitry and chemistry of pain and pleasure processing at the systems level.” (Leknes & Tracey, 2008, p314).


Fitness is (roughly and informally) relative propensity to have descendants. It is relative to competing variants in a population, and so typically defined by reference to genotypes.

If we focus specifically on the contribution to fitness of the behaviour of an individual, then we might go on to think that the contribution of individual behaviours (or behavioural dispositions) might be ordered with respect to how much the add or, or undermine, total fitness. Something along these lines is one goals of at least some behavioural ecologists:

“Any attempt to understand behavior in terms of the evolutionary advantage that it might confer has to find a "common currency" for comparing the costs and benefits of various alternative courses of action” (McNamara and Houston 1986: 358).


ATP is the standard abbreviation for Adenosine triphosphate. It’s the molecule used to transport energy in cells. It might seem like an odd thing to have on this list, although it is common to hear it referred to as the ‘energy currency’ of intracellular processes. Since any behavior has some energy cost (or gain), and any allocation of metabolic resources has an opportunity cost (in actions rendered unavailable, or made possible) it’s plausible to think that in a highly aggregated way, ATP represents an important overall budgetary bottom line.

Quite how to relate this to some of the other proposed currencies is another matter.


Leknes, S. and Tracey, I. (2008) A common neurobiology for pain and pleasure, Nature reviews: Neuroscience, 9,  pp314-320. [Publisher link.] [Google scholar citations.]

McNamara, J.M. and Houston, A.I. (1986) The Common Currency for Behavioral Decisions, The American Naturalist, 127(3), pp358-378. [Publisher link.] [Google scholar citations.]

Shizgal, P. & Conover, K. 1998. On the neural computation of utility, Current Directions in Psychological Science, 5(2), pp. 37-43. [Publisher’s site –may be behind a paywall] [Preprint version at CogPrints] [Google Scholar Citations]