Fluctuations in a stellar system's gravitational field cause the orbits of stars to evolve. These can originate from external perturbers (collisionless framework) and from the intrinsic discreteness of the system (collisional framework). Such a distinction allows us to address the pressing question of the respective roles of nature (system's internal properties) vs. nurture (cosmic environment) in the etablishment of the observed properties of these systems. \par To account for finite-N effects, one may rely on the inhomogeneous Balescu-Lenard equation to capture the induced secular diffusion. I will present the key physical mechanisms at play in this context, as well as the formalism appropriate to capture such irreversible long-term evolutions. When applied to a tepid stellar disc, it predicts the formation of narrow ridge-like structures in action-space, in agreement with numerical simulations. In astrophysics, the inhomogeneous Balescu-Lenard equation is a rich framework, which may describe the secular diffusion of giant molecular clouds in galactic discs, the secular migration and segregation of planetesimals in proto-planetary discs, or even the long-term evolution of population of stars within the Galactic centre, as I will show.