Aim To explore the direct and indirect effect of the personality meta-traits 'Stability' and 'Plasticity' on moral reasoning among nurse practitioners (NPs) and physician assistants (PAs). Background Moral reasoning is influenced by being prone to moral disengagement and personality traits. Moral disengagement is observed among professionals in many fields, including healthcare providers. Moral disengagement is known to be provoked by environmental stressors and influenced by certain personality traits. Design A cross-sectional approach was used including self-report questionnaires. Methods A convenience sample of Dutch NPs (N = 67) and PAs (N = 88) was surveyed via online questionnaires between January and March 2015, using (a) the Defining Issues Test; (b) the BIG five inventory; and (c) the Moral Disengagement Scale. Structural equation modelling (SEM) was employed for estimating the construct validity of two meta-traits of personality and to test unidirectional influences on moral reasoning. Results Only the Stability trait was a direct predictor of moral reasoning whereas both Stability and Plasticity were precursors of moral disengagement. Both personality meta-traits had statistically significant indirect effects on moral reasoning through a low level of moral disengagement. The influence of both personality traits on the level of moral reasoning was increased by strong self-censure on entering into morally disengaged interactions. Conclusion The personality meta-trait 'Stability' is an indicator of moral reasoning and is explained by a lower propensity to morally disengage among highly stable people. Although the meta-trait Plasticity exerts an indirect effect through moral disengagement on moral reasoning, it is not a direct indicator of moral reasoning.