<p>Exploring the motivations behind collaboratively dishonest behaviour, is often done using dice-rolling experiments, where coordinated cheating increases participant pairs’ earnings. Here we present a preregistered dice-rolling experiment that investigates how participants’ dishonest (cheating) behaviour is influenced (i) by the dishonest (cheating) behaviour of their experimental partners and (ii) by by-product altruism (or cost-free charity) that is a direct consequence of a monetary reward in this experiment. We studied a 2 × 2 factorial design of the dice-rolling game, with the presence or absence of (i) a cheating partner and (ii) by-product altruism. Following the game, participants filled out the Moral Foundations Questionnaire and the Social Dominance Orientation questionnaire. We found that without the opportunity of cost-free charity, cheating was not detectable, independent of the cheating behaviour of the partner. However, the opportunity of donating to a chosen charitable foundation (cost-free charity) significantly increased the level of cheating. We found that the relationship between the degree of dishonesty and the measured psychological traits is complex and, in some cases, contradictory. Our results do, however, confirm that participants with a stronger moral integrity were less likely to cheat. Our results showed that the level of collaborative cheating to obtain a monetary benefit is significantly increased by participants’ perception of acting in a socially beneficial way as a by-product of their unethical behaviour.</p>