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The interaction effect coefficient ψ has been a much-discussed, fundamental parameter of indirect genetic effect (IGE) models since its formal mathematical description in 1997. The coefficient simultaneously describes the form of changes in trait expression caused by genes in the social environment and predicts the evolutionary consequences of those IGEs. Here, we report a striking mismatch between theoretical emphasis on ψ and its usage in empirical studies. Surveying all IGE research, we find that the coefficient ψ has not been equivalently conceptualized across studies. Several issues related to its proper empirical measurement have recently been raised, and these may severely distort interpretations about the evolutionary consequences of IGEs. We provide practical advice on avoiding such pitfalls. The majority of empirical IGE studies use an alternative variance-partitioning approach rooted in well-established statistical quantitative genetics, but several hundred estimates of ψ (from 15 studies) have been published. A significant majority are positive. In addition, IGEs with feedback, that is, involving the same trait in both interacting partners, are far more likely to be positive and of greater magnitude. Although potentially challenging to measure without bias, ψ has critically-developed theoretical underpinnings that provide unique advantages for empirical work. We advocate for a shift in perspective for empirical work, from ψ as a description of IGEs, to ψ as a robust predictor of evolutionary change. Approaches that “run evolution forward” can take advantage of ψ to provide falsifiable predictions about specific trait interactions, providing much-needed insight into the evolutionary consequences of IGEs.

 

Sexual competition hinges on the ability to impress other conspecifics, to drive them away or attract them. In such cases, the selective environment may be hedonic or affective in nature, as it consists of the evaluations of the individuals making the decisions. This may contribute to the power of sexual selection because evaluations may range from positive to negative rather than simply from positive to neutral. Selection due to mate choice may therefore be stronger than currently appreciated. Further, change in preferred mate types can occur simply by changes (flips) in the evaluation of similar display features, adding to the dynamism of sexual selection as well as its strength. We tested the hypothesis of positive-to-negative behavioural responses in mate choice with a playback experiment using two treefrog species with ‘mirror image’ structures in their advertisement and aggressive calls. Female treefrog responses ranged from approach to evasion, and the presence of an aversive stimulus tainted evaluation of an attractive stimulus. Further, females in the two species showed flips in approach/evasion of stimuli with comparable signal structure. These results suggest that hedonic evaluation may have an important role in mate choice and showcase how mechanistic analysis can help understand evolutionary processes.