In theory, it can be strategically advantageous for competitors to make themselves unpredictable to their opponents, for example by variably mixing hostility and friendliness. Empirically, it remains open whether and how competitors make themselves unpredictable, why they do so, and how this conditions conflict dynamics and outcomes.

We examine these questions in interactive attacker-defender contests, in which attackers invest to capture resources held and defended by their opponent.

Study 1, a re-analysis of nine (un)published experiments (total N=650), reveals significant cross-trial variability especially in pro-active attacks and less in re-active defense.

Study 2 (N=200) shows that greater variability makes both attacker’s and defender’s next move more difficult to predict, especially when variability is due to occasional rather than (in)frequent extreme investments in conflict.

Studies 3 (N=27) and 4 (N=106) show that pre-contest testosterone, a hormone associated with risk-taking and status competition, drives variability during attack which, in turn, increases sympathetic arousal in defenders and defender variability (Study 4). Rather than being motivated by wealth maximization, being unpredictable in conflict and competition emerges in function of the attacker’s desire to win ‘no matter what’ and comes with significant welfare cost to both victor and victim.

The paper has not yet been published. The manuscript will be available after publication.