This preprint tests whether representation steering associated with the Big Five changes LLM-agent cooperation in three variants of the iterated Prisoner's Dilemma. An unsteered preliminary comparison reports median cooperation rates of .70 for Llama-3.1-8B-Instruct, .10 for Gemma2-9B-it, and .50 for Mistral-Nemo-Instruct-2407. The main experiments use only the 12B Mistral-Nemo-Instruct-2407: each game lasts ten rounds, and the number of games varies by opponent, but the paper never reports how many are run per condition. For each trait, the authors contrast prompts declaring personality at 100% versus 0%, compute the first principal component of activation differences at each layer, and add the vector to layers five through twenty from the end with a factor of 3.5 at every generated token. In Setup 1, Player A faces rule-based opponents, Always Cooperate, Always Defect, or Random with defection probabilities .3, .5, and .7. Positive Agreeableness, Conscientiousness, and Openness steering lowers median troublemaking to zero. Agreeableness, however, raises exploitability by .44 from a zero median baseline; it also raises forgiveness by .75 from zero and lowers retaliation by .75 from one. In Setup 2, communication is restricted to “cooperate” or “defect,” and lying rate is only the observable mismatch between message and action. From an unsteered median of .70, positive Agreeableness and Conscientiousness lower it to .00 and .10 against the altruistic opponent, and .10 and .20 against the selfish opponent. In Setup 3, both players are steered instances of the same Mistral model. The Agreeableness+/Agreeableness+ pairing accumulates 21 prison years versus 52 for baseline/baseline, about 60% fewer; pairings involving Agreeableness and Conscientiousness achieve low collective costs but often worsen the cooperative player's individual outcome. These findings document behavioral changes under one intervention, not validated psychological traits. The vectors come from a linguistic 100%/0% contrast that need not represent opposite human Big Five poles; there is no psychometric manipulation check, persona-prompting baseline, or layer/intensity ablation. The factor and layers are selected to maximize the steering effect. Although the paper calls the influence “significant,” it reports no tests, p-values, intervals, error bars, or number of repetitions. One main model, one payoff matrix, a ten-round horizon, and metrics that operationalize cooperation limit generalization. A lower message–action mismatch does not demonstrate honesty or internal intent, and prompted chain-of-thought cannot establish private cognition. The evidence supports a contextual trade-off between collective cooperation and exploitability, not a general improvement in multi-agent safety or trustworthiness.
Research question
How does the representational steering of each Big Five trait alter cooperation, vulnerability to exploitation, and the correspondence between message and action of LLM agents in variants of the iterated prisoner's dilemma?