## Abstract

Does natural selection favor veridical percepts—those that accurately (if not exhaustively) depict objective reality? Perceptual and cognitive scientists standardly claim that it does. Here we formalize this claim using the tools of evolutionary game theory and Bayesian decision theory. We state and prove the “Fitness-Beats-Truth (FBT) Theorem” which shows that the claim is false: If one starts with the assumption that perception involves inference to states of the objective world, then the FBT Theorem shows that a strategy that simply seeks to maximize expected-fitness payoff, with no attempt to estimate the “true” world state, does consistently better. More precisely, the FBT Theorem provides a quantitative measure of the extent to which the *fitness-only* strategy dominates the *truth* strategy, and of how this dominance increases with the size of the perceptual space. The FBT Theorem supports the *Interface* *Theory* *of* *Perception* (e.g. Hoffman et al. in Psychon Bull Rev https://doi.org/10.3758/s13423-015-0890-8, 2015), which proposes that our perceptual systems have evolved to provide a species-specific interface to guide adaptive behavior, and not to provide a veridical representation of objective reality.

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## Notes

- 1.
From a purely mathematical point of view, the set of monotonic fitness functions is an extremely small subset of the set of all functions on a given domain. That is to say, there are “many more” non-monotonic functions than monotonic ones; hence a random sampling of fitness functions is much more likely to yield a non-monotonic one.

- 2.
The value of

*N*at which this happens depends upon the payoff matrix, but can be arbitrarily large over the set of all payoff matrices satisfying \(a>c\)*and*\(b>d\)*.* - 3.
In this case, all the integral signs can be replaced by summations.

- 4.
An example is a closed rectangle in some

*k*-dimensional Euclidean space, such as the unit interval [0, 1] in one dimension, or the unit square in two. - 5.
In the general case, the perceptual map may have dispersion (or noise), and is mathematically expressed as a Markovian kernel \(p:W\times \mathcal{X}\to \left[\mathrm{0,1}\right].\) That is, for every element

*w*in*W*, the kernel*p*assigns a probability distribution on*X*(hence it assigns a probability value to each measurable subset of*X*). Because*X*is finite and all of its subsets are measurable, here the kernel may be viewed simply as assigning, for every element*w*in*W*, a probability value to each element of*X*. - 6.
That is, \({\mathbb{P}}(\mathrm{d}w | x)=g(w\left|x\right)\mathrm{d}w.\)

- 7.
See also the

*Invention**of**Space–Time**Theorem*in Hoffman et al. (2015).

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## Acknowledgements

We thank Federico Faggin and Robert Prentner for illuminating discussions. This work has been partially funded by the Federico and Elvia Faggin Foundation.

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## Appendix: Calculations for the Numerical Example in Table 1

### Appendix: Calculations for the Numerical Example in Table 1

In this appendix we perform the Bayesian and expected-fitness calculations using the data given in Table 1.

To compute the *Truth* estimates, we first need the probability of each stimulation \({\mathbb{P}}({x}_{1})\) and \({\mathbb{P}}({x}_{2})\). These can be computed by marginalizing over the priors in the world as follows:

By Bayes’ Theorem, the posterior probabilities of the world states, given \({x}_{1},\) are

Thus the maximum a posteriori, or *Truth* estimate for stimulus \({x}_{1}\) is \({w}_{2}\).

Posterior probabilities of the world states, given \({s}_{2},\) are:

Thus the maximum a posteriori, or *Truth* estimate for stimulus \({x}_{2}\) is \({w}_{3}.\)

Finally, the expected-fitness values of the different sensory stimulations \({x}_{1}\) and \({x}_{2}\) are, respectively:

Thus \({x}_{2}\) has a larger expected fitness than \({x}_{1}\).

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Prakash, C., Stephens, K.D., Hoffman, D.D. *et al.* Fitness Beats Truth in the Evolution of Perception.
*Acta Biotheor* **69, **319–341 (2021). https://doi.org/10.1007/s10441-020-09400-0

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### Keywords

- Perception
- Veridicality
- Evolutionary psychology
- Bayesian decision theory
- Fitness
- Evolutionary game theory
- Interface theory of perception