Here we propose a novel organising concept to understand honest signalling. This is the concept of ‘signalling trade-off’ (Számadó et al., 2023a). Trade-offs are widely known in both economics and biology implying a constraint that prevents the maximization of several desirable traits simultaneously (Stearns, 1989, 1992; Roff, 1992; Reznick et al., 2000; Roff & Fairbairn, 2007). Traits subject to the same trade-off depend on each other or on other, common variables (potentially hidden), like resources.
Honest signalling is maintained by condition-dependent trade-offs. This claim has both empirical and theoretical support. We showed in a recent lab experiment the importance of such a condition-dependent trade-off as opposed to the (presumed) importance of equilibrium cost (Számadó et al., 2022).
Honesty was observed during the experiment in all treatments with a condition-dependent signalling trade-off, regardless of the cost of equilibrium signals (costly, cost-free or beneficial). That is, even beneficial signals can maintain honest signalling as long a condition-dependent signalling trade-off is present (Számadó et al., 2022).
We were able to show in a general model of asymmetric signalling game that honesty is maintained by signalling trade-offs (Számadó et al., 2023b). The cost of equilibrium signals can change from costly, cost-free to beneficial the signalling system will remain honest and evolutionarily stable as long as the proper condition-dependent trade-off holds (Számadó et al., 2023b).
These trade-offs might be clear in some contexts, e.g. a trade-off between present and future offspring, in some other contexts these trade-offs can be difficult to identify, such as gossip, reputation systems or modern social media. Nevertheless, signalling trade-offs seem to govern honesty both in animals and humans, both in the biological and the economical context. Identifying and understanding these signalling trade-offs is the key of understanding communication (Számadó et al., 2023a).

References
Roff, D.A. (1992). The Evolution of Life Histories: Theory and Analysis. Chapman and Hall, New York.
Roff, D.A. Fairbairn, D.J. (2007). The evolution of trade-offs: where are we? J.Evol.Biol. 20: 433-447.
Reznick, D., Nunney, L. & Tessier, A. (2000). Big houses, big cars, superfleas and the costs of reproduction. Trends Ecol. Evol. 15: 421–425.
Stearns, S.C. (1989). Trade-Offs in Life-History Evolution. Functional Ecology, Vol. 3, No. 3., pp. 259-268.
Stearns, S.C. (1992). The Evolution of Life Histories. Oxford University Press, New York.
Számadó, S., Samu, F., & Takács, K. (2022). Condition-dependent trade-offs maintain honest signalling. Royal Society Open Science, 9(10), 220335.
Számadó, S., Zachar, I. & Penn, D.J. (2023a) The trade-offs of honest and dishonest signals. EcoEvoRxiv. https://doi.org/10.32942/X2960G
Számadó, S., Zachar, I., Czégel, D., & Penn, D. J. (2023b). Honesty in signalling games is maintained by trade-offs rather than costs. BMC biology, 21(1), 4.