The Indirect Genetic Effect Interaction Coefficient ψ: Theoretically Essential and Empirically Neglected

Area of Science:

• Evolutionary biology
• Quantitative genetics
• Behavioral ecology

Background:

• Indirect genetic effects (IGEs) are influenced by genes acting through the social environment.
• The interaction effect coefficient (ψ) quantifies these IGEs and their evolutionary impact.
• Theoretical emphasis on ψ contrasts with its varied empirical application.

Purpose of the Study:

• To highlight the mismatch between theoretical importance and empirical use of ψ.
• To identify conceptual and measurement issues with ψ in IGE research.
• To offer guidance for accurate empirical estimation and interpretation of ψ.

Main Methods:

• Systematic survey of published indirect genetic effect (IGE) research.
• Analysis of the conceptualization and measurement of the interaction effect coefficient (ψ).
• Review of statistical quantitative genetics approaches in IGE studies.

Main Results:

• ψ is conceptualized inconsistently across empirical studies.
• Measurement issues may lead to distorted interpretations of evolutionary consequences.
• A majority of published ψ estimates are positive, especially in cases of feedback.

Conclusions:

• Reframe ψ from a descriptive parameter to a predictive tool for evolutionary change.
• Emphasize using ψ to generate falsifiable predictions about trait evolution.
• Integrate theoretical strengths of ψ into empirical IGE research for robust evolutionary insights.