October 18, 2012
by Artem Kaznatcheev

In coordination games, the players have to choose between two possible strategies, and their goal is to coordinate their choice without communication. In a classic game theory setting, coordination games are the prototypical setting for discussing the problem of equilibrium selection: if a game has more than one equilibrium then how do the players know which one to equilibrate on? The standard solution concept for this is the idea of a risk dominance. If you were playing a symmetric coordination game:

where and , then how would you chose your strategy not knowing what your opponent is going to do? Since the two pure strategy Nash equilibria are the top left and bottom right corner, you would know that you want to end up coordinating with your partner. However, given no means to do so, you could assume that your partner is going to pick one of the two strategies at random. In this case, you would want to maximize your expected payoff. Assuming that each strategy of your parner is equally probably, simple arithmetic would lead you to conclude that you should chose the first strategy (first row, call it C) given the condition:

Congratulations, through your reasoning you have arrived at the idea of a risk dominant strategy. If the above equation is satisfied then C is risk dominant over D (the second strategy choice), more likely to be selected, and the ‘better’ Nash equilibrium.

Since many view evolutionary game theory as a study of equilibrium selection, it is not surprising to see risk-dominance appear in evolution. In particular, if the risk dominance condition is met, then (for a coordination game and replicator dynamics) C will have a larger basin of attraction than D. If we pick initial levels of cooperators at random, then in your well-mixed and extremely large population, the risk-dominant strategy will dominate the population more often. If you are feeling adventurous, then I recommend as exercise to calculate the exact probability of C dominating in this setting.

From our experience with ethnocentrism and discrete populations, we know that replicator dynamics is not the end of the story. The second step is to consider finite inviscid populations where we can’t ignore dynamical stochasticity. Kandari et al. (1993) studies this setting and for a population of size concluded that C would be a more likely than D if:

Nowak et al. (2004) looked at this problem from the biological perspective of Moran processes. In a Moran process, there is no mutation, and thus the conclusion of dynamics is the absorbing state of either all C or all D. The quantity of interest becomes the fixation probability: the fixation probability of C is the probability that a single C mutant invades (leads to an all C absorbing state) a population of all D (vice-virsa for

fixation of D). Nowak et al. (2004) found that the fixation probability of C (in the weak selection limit) is higher than that of D in a population of agents if and only if the above equation is satisfied.

Antal et al. (2009) concluded this research program. They showed that the above condition was necessary for the case of arbitrary mutations, a wide range of evolutionary processes, and any two player, two strategy game. It is true for pair-comparison (Fermi rule), exponential Moran processes, and weak-selection Moral processes with arbitrary mutation rates. In general, any update process that satisfies the two requirements: (i) additive payoffs, and (ii) evolutionary dynamics depend only on the payoff differences.

Let us visualize this result. As we learnt in a previous post we know that a two strategy cooperate-defect games do not need 4 parameters to specify, and can be rewritten with just two. The symmetry arguments we applied before preserve the authors’ result, so let’s apply the transformation:

This lets us simplify the risk-dominance and finite population rules to:

Now it clear why we discussed risk-dominance before diving into finite populations. As the population size gets arbitrarily large (), our finite population rule reduces to risk-dominance and replicator dynamics. In the other extreme case is (can’t have a game with smaller populations) the rule becomes .

In the above picture of U-V space, we can see the two extreme conditions. In the green region, C is more likely than D for any population size, and in the blue it is true in the limit of infinite population. For particular you get a different division line in the blue region parallel to the two current ones. Give a specific game in the blue region, you can calculate the threshold:

For games in the blue region, if your population exceeds the threshold then C with be more likely than D.

For those interested in the mathematical details, I recommend sections 2.3 and 4 of Antal et al. (2009). In particular, I enjoy their approach in section 2.3 of showing that when the game is on the dividing line then we have a symmetric distribution around and due to the well-behaved nature of deformations of the game matrix we can extend to the non knife-edge case. The only missing study in Antal et al. (2009) is a study of the second moment of the population. In regions 5, 9, and 10 we expect a bimodal distribution, and in 2-4 and 6-8 a unimodal. Can we use the probability of mutation to bound the distance between the peaks in the former, and the variance of the peak in the latter? Another exercise for the highly enthusiastic reader.

### References

Antal, T., Nowak, M.A., & Traulsen, A. (2009). Strategy abundance in games for arbitrary mutation rates Journal of Theoretical Biology, 257 (2), 340-344 DOI: 10.1016/j.jtbi.2008.11.023

Kandori, M., Mailath, G.J., & Rob, R. (1993). Learning, mutation, and long run equilibria in games. *Econometrica* 61(1): 29-56.

Nowak, M.A., Sasak, A., Taylor, C., & Fudenberg, D. (2004). Emergence of cooperation and evolutionary stability in finite populations. *Nature* 428: 646-650.

## Constant-sum games as a way from non-cell autonomous processes to constant tumour growth rate

April 6, 2019 by Artem Kaznatcheev Leave a comment

A lot of thinking in cancer biology seems to be focused on cell-autonomous processes. This is the (overly) reductive view that key properties of cells, such as fitness, are intrinsic to the cells themselves and not a function of their interaction with other cells in the tumour. As far as starting points go, this is reasonable. But in many cases, we can start to go beyond this cell-autonomous starting point and consider non-cell-autonomous processes. This is when the key properties of a cell are not a function of just that cell but also its interaction partners. As an evolutionary game theorist, I am clearly partial to this view.

Recently, I was reading yet another preprint that has observed non-cell autonomous fitness in tumours. In this case, Johnson et al. (2019) spotted the Allee effect in the growth kinetics of cancer cells even at extremely low densities (seeding

in vitroat <200 cells in a 1 mm^3 well). This is an interesting paper, and although not explicitly game-theoretic in its approach, I think it is worth reading for evolutionary game theorists.Johnson et al.'s (2019) approach is not explicitly game-theoretic because they consider their in vitro populations as a monomorphic clonal line, and thus don't model interactions between types. Instead, they attribute non-cell autonomous processes to density dependence of the single type on itself. In this setting, they reasonably define the cell-autonomous null-model as constant exponential growth, i.e. for some constant fitness and total tumour size .

It might also be tempting to use the same model to capture cell-autonomous growth in game-theoretic models. But this would be mistaken. For this is only effectively cell-autonomous at the level of the whole tumour, but could hide non-cell-autonomous fitness at the level of the different types that make up the tumour. This apparent cell-autonomous total growth will happen whenever the type interactions are described by constant-sum games.

Given the importance of constant-sum games (more famously known as zero-sum games) to the classical game theory literature, I thought that I would write a quick introductory post about this correspondence between non-cell autonomous constant-sum games and effectively cell-autonomous growth at the level of the whole tumour.

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Filed under Analytic, Commentary, Preliminary, Technical Tagged with classic game theory, mathematical oncology, operationalization, replicator dynamics