This paper poses a puzzle about partner choice. On one hand, an actor appears to exert causal control over its partner’s phenotype through partner choice; on the other hand, the partner’s phenotype seems largely determined by its genotype, leaving little room for the actor’s influence. I argue that this puzzle arises from adopting different causal models with different variable choices—an actor-centred model and an index-fixed model. The former is standard in social evolution theory, while the latter is my proposed alternative. I show that the puzzle has a distinctive philosophical character by interpreting it as a variant of Frege’s puzzle, rooted in ambiguity about how to represent “the partner’s phenotype” as a causal variable. I then challenge the actor-centred model on three pragmatic grounds: first, it is difficult to extend beyond a single focal individual; second, it conflates partner choice with partner control; and third, it diverges from modelling practices in economics. I conclude that the index‑fixed model offers a better representation of partner choice, and I urge reconsideration of the actor‑centred perspective in social evolution theory.
